lt4295 (rev. b) · supply current vport, hssrc and vin supply current vvport = vhssrc = vvin = 60v...

LT4295

1Rev. B

For more information www.analog.com

TYPICAL APPLICATION

FEATURES DESCRIPTION

IEEE 802.3bt PD Interface with Forward/Flyback Controller

IEEE 802.3bt 71.3W (Class 8) PD Controller and Power Supply in Forward Mode

APPLICATIONS n High Power Wireless Data Systems n Outdoor Security Camera Equipment n Commercial and Public Information Displays n High Temperature Applications

n IEEE 802.3af/at/bt Powered Device (PD) with Forward/Flyback Controller

n Supports Up to 71.3W PDs n 5-Event Classification Sensing n Superior Surge Protection (100V Absolute Maximum) n Wide Junction Temperature Range (–40°C to 125°C) n >94% End-to-End Efficiency with LT4321 Ideal Bridge n External Hot Swap N-Channel MOSFET for Lowest

Power Dissipation and Highest System Efficiency n No-Opto Flyback Operation n Auxiliary Power Support as Low as 9V n Easy Migration of LTPoE++® PDs to IEEE 802.3bt PDs n Pin Compatible with LT4276A/B/C n 28-Lead 4mm × 5mm QFN Package

The LT®4295 is an IEEE 802.3af/at/bt-compliant powered device (PD) interface controller with a switching regulator controller. The T2P output indicates the number of clas-sification events received during IEEE 802.3bt-compliant mutual identification and negotiation of available power.

The LT4295 supports both forward and flyback power supply topologies. The flyback topology supports No-Opto feedback. Auxiliary input voltages can be accurately sensed with just a resistor divider connected to the AUX pin.

The LT4295 utilizes an external, low RDS(on) N-channel hot swap MOSFET and supports the LT4320/LT4321 ideal diode bridges, to extend the end-to-end power delivery efficiency and eliminate costly heat sinks.

The LT4295 also includes an on-chip detection signature resistor, thermal protection, slope compensation, and many user configurable settings including classification signature, inrush current, switcher frequency, gate drive delay, soft-start and load compensation.

••VPORT

VPORT

RCLASS

AUX

RCLASS++

SWVCC

VCCVINVCC

0.1µF

10µFBAV19WS(TRR ≤50ns)

22µF

HSGATE

HSSRC

FFSDLY

PG

SG

ITHB

TO MICROPROCESSOR

ISEN+

ISEN–

4295 TA01

GND FB31 ROSC T2PSS

100µH

AUX37V TO 57V

+

–

–

+

FMMT723

20mΩ

5V13A

+

–3.3k

10k0.1µF 100pF

10nF

100k

LT4295

OPTO

+

Single-Signature Power Classification

(at PD Input)CLASS POWER

0 13W

1 3.84W

2 6.49W

3 13W

4 25.5W

5 40W

6 51W

7 62W

8 71.3W

All registered trademarks and trademarks are the property of their respective owners.

Document Feedback

https://www.analog.com/LT4295?doc=LT4295.pdf

https://www.analog.com?doc=LT4295.pdf



https://form.analog.com/Form_Pages/feedback/documentfeedback.aspx?doc=LT4295.pdf&product=LT4295&Rev=B

LT4295

2Rev. B


PIN CONFIGURATIONABSOLUTE MAXIMUM RATINGS

VPORT, HSSRC, VIN Voltages .................. –0.3V to 100VHSGATE Current.................................................. ±20mAVCC Voltage .................................................. –0.3V to 8VRCLASS, RCLASS++Voltages .............................. –0.3V to 8V (and ≤ VPORT)SFST, FFSDLY, ITHB, T2P Voltages ... –0.3V to VCC+0.3VISEN+, ISEN– Voltages ...........................................±0.3VFB31 Voltage ..................................................+12V/–30VRCLASS/RCLASS++ Current .............................. –50mAAUX Current ........................................................ ±1.4mAROSC Current ..................................................... ±100µARLDCMP Current ................................................±500µAT2P Current .........................................................–2.5mAOperating Junction Temperature Range (Note 3) LT4295I ................................................–40°C to 85°C LT4295H ............................................ –40°C to 125°CStorage Temperature Range .................. –65°C to 150°C

(Notes 1, 2)

9 10

TOP VIEW

UFD PACKAGE28-LEAD (4mm × 5mm) PLASTIC QFN

TJMAX = 150°C, θJC = 3.4°C/WEXPOSED PAD (PIN 29) IS GND, MUST BE SOLDERED TO PCB

11 12 13

28 27 26 25 24

14

23

6

5

4

3

2

1GND

AUX

RCLASS++

RCLASS

T2P

VCC

VCC

VCC

DNC

VCC

PG

GND

SG

ISEN+

ISEN–

RLDCMP

VPOR

T

NC HSGA

TE

HSSR

C

V IN

SWVC

C

V CC

ROSC

SFST

FFSD

LY

ITHB

FB31

7

17

18

19

20

21

22

16

8 15

29GND

LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE

LT4295IUFD#PBF LT4295IUFD#TRPBF 4295 28-Lead (4mm × 5mm) Plastic QFN –40°C to 85°C

LT4295HUFD#PBF LT4295HUFD#TRPBF 4295 28-Lead (4mm × 5mm) Plastic QFN –40°C to 125°C

Contact the factory for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.

Tape and reel specifications. Some packages are available in 500 unit reels through designated sales channels with #TRMPBF suffix.

ORDER INFORMATION



https://www.analog.com/media/en/package-pcb-resources/package/tape-reel-rev-n.pdf?doc=LT4295.pdf

LT4295

3Rev. B


ELECTRICAL CHARACTERISTICS

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

VPORT, HSSRC, VIN Operating Voltage At VPORT Pin l 60 V

VSIG VPORT Detection Signature Range At VPORT Pin l 1.5 10 V

VCLASS VPORT Classification Signature Range At VPORT Pin l 12.5 21 V

VMARK VPORT Mark Event Range At VPORT Pin, After 1st Classification Event l 5.6 10 V

VPORT AUX Range At VPORT Pin, VAUX ≥ 6.45V l 8 60 V

Detect/Class Hysteresis Window l 1.0 V

Reset Threshold l 2.6 5.6 V

VHSON Hot Swap Turn-On Voltage l 35 37 V

VHSOFF Hot Swap Turn-Off Voltage l 30 31 V

Hot Swap On/Off Hysteresis Window l 3 V

Supply Current

VPORT, HSSRC and VIN Supply Current VVPORT = VHSSRC = VVIN = 60V l 2 mA

VPORT Supply Current During Classification VVPORT = 17.5V, RCLASS, RCLASS++ Open l 0.7 1.0 1.3 mA

VPORT Supply Current During Mark Event VVPORT = VMARK after 1st Classification Event l 0.4 2.2 mA

Detection and Classification Signature

Detection Signature Resistance VSIG (Note 4) l 23.6 24.4 25.5 kΩ

Resistance During Mark Event VMARK (Note 4) l 5.2 8.3 11.4 kΩ

RCLASS/RCLASS++ Voltage –10mA ≥ IRCLASS ≥ –36mA, VCLASS l 1.36 1.40 1.43 V

Classification Signature Stability Time VVPORT Step GND to 17.5V, 35.7Ω from RCLASS to GND

l 2 ms

Digital Interface

VAUXT AUX Threshold VPORT = 17.5V, VIN = VHSSRC = 18.5V l 6.05 6.25 6.45 V

IAUXH AUX Pin Current VAUX = 6.05V, VPORT = 17.5V, VIN = 9V, VCC = 0V l 3.3 5.3 7.3 µA

T2P Output High VVCC - VT2P, –1mA Load l 0.3 V

T2P Leakage VT2P = 0V l –1 1 µA

Hot Swap Control

IGPU HSGATE Pull Up Current VHSGATE - VHSSRC = 5V (Note 5) l –27 –22 –18 µA

HSGATE Voltage –10µA Load, with Respect to HSSRC l 10 14 V

HSGATE Pull Down Current VHSGATE - VHSSRC = 5V l 400 µA

VCC Supply

VCCREG VCC Regulation Voltage l 7.2 7.6 8.0 V

Feedback Amplifier

VFB FB31 Regulation Voltage l 3.11 3.17 3.23 V

FB31 Pin Bias Current RLDCMP Open -0.1 µA

gm Feedback Amplifier Average Trans-Conductance

Time Average, –2µA < IITHB < 2µA l –52 –40 –26 µA/V

ISINK ITHB Average Sink Current Time Average, VFB31 = 0V l 4.4 8.0 13.4 µA

Soft-Start

ISFST Charging Current VSFST = 0.5V, 3.0V l –49 –42 –36 µA

The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TJ = 25°C. VVPORT = VHSSRC = VVIN = 40V, VVCC = VCCREG, ROSC, PG, and SG Open, RFFSDLY = 5.23kΩ to GND. AUX connected to GND unless otherwise specified. (Note 2)



LT4295

4Rev. B


SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

Gate Outputs

PG, SG Output High Level I = –1mA l VCC –0.1 V

PG, SG Output Low Level I = 1mA l 1 V

PG Rise Time, Fall Time PG = 1000pF 15 ns

SG Rise Time, Fall Time SG = 400pF 15 ns

Current Sense/Overcurrent

VFAULT Overcurrent Fault Threshold VISEN+ – VISEN– l 125 140 155 mV

ΔVSENSE/ΔVITHB

Current Sense Comparator Threshold with Respect to VITHB

l –130 –111 –92 mV/V

VITHB(OS) VITHB Offset l 3.03 3.17 3.33 V

Timing

fOSC Default Switching Frequency ROSC Pin Open l 200 214 223 kHz

Switching Frequency 45.3kΩ from ROSC to GND l 280 300 320 kHz

fT2P T2P Signal Frequency fSW/256

T2P Duty Cycle in PoE Operation (Note 7) After 4-Event Classification After 5-Event Classification (RCLASS++ Has Resistor to GND)

50 25

% %

T2P Duty Cycle in Auxiliary Supply Operation (Note 7)

V(AUX) > VAUXT, and RCLASS++ Has Resistor to GND

25 %

tMIN Minimum PG On Time l 175 250 330 ns

DMAX Maximum PG Duty Cycle l 63 66 70 %

tPGDELAY PG Turn-On Delay-Flyback PG Turn-On Delay-Forward

5.23kΩ from FFSDLY to GND 52.3kΩ from FFSDLY to GND 10.5kΩ from FFSDLY to VCC 52.3kΩ from FFSDLY to VCC

45 171 92

391

ns ns ns ns

tFBDLY Feedback Amp Enable Delay Time 350 ns

tFB Feedback Amp Sense Interval 550 ns

tPGSG PG Falling to SG Rising Delay Time-Flyback PG Falling to SG Falling Delay Time- Forward

Resistor from FFSDLY to GND 10.5kΩ from FFSDLY to VCC 52.3kΩ from FFSDLY to VCC

20 67

301

ns ns ns

tSTART Start Timer (Note 6) Delay After Power Good l 80 86 93 ms

tFAULT Fault Timer (Note 6) Delay After Overcurrent Fault l 80 86 93 ms

IMPS MPS Current l 10 12 14 mA

ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TJ = 25°C. VVPORT = VHSSRC = VVIN = 40V, VVCC = VCCREG, ROSC, PG, and SG Open, RFFSDLY = 5.23kΩ to GND. AUX connected to GND unless otherwise specified. (Note 2)

Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime.Note 2. All voltages with respect to GND unless otherwise noted. Positive currents are into pins; negative currents are out of pins unless otherwise noted. Note 3. This IC includes overtemperature protection that is intended to protect the device during momentary overload conditions. Junction temperature can exceed 150°C when overtemperature protection is active. Continuous operation above the specified maximum operating junction temperature may impair device reliability.

Note 4. Detection signature resistance specifications do not include resistance added by the external diode bridge which can add as much as 1.1kΩ to the port resistance.Note 5. IGPU available in PoE powered operation. That is, available after V(VPORT) > VHSON and V(AUX) < VAUXT, over the range where V(VPORT) is between VHSOFF and 60V.Note 6. Guaranteed by design, not subject to test.Note 7. Specified as the percentage of the period which T2P is low impedance with respect to VCC.



LT4295

5Rev. B


TYPICAL PERFORMANCE CHARACTERISTICS

VFB31 vs TemperatureFeedback Amplifier Output Current vs VFB31

Switching Frequency vs Temperature

Current Sense Voltage vs Duty Cycle, ITHB

PG Delay Time vs Temperature in Flyback Mode

PG, SG Delay Time vs Temperature in Forward Mode

Input Current vs Input Voltage25k Detection Signature Range

Detection Signature Resistance vs Input Voltage VCC Current vs Temperature

VPORT VOLTAGE (V)0

0

VPOR

T CU

RREN

T (m

A)

0.4

0.3

0.2

0.1

0.5

6 8 102 4

4295 G01

125°C85°C25°C–40°C

VPORT VOLTAGE (V)1

23.75

SIGN

ATUR

E RE

SIST

ANCE

(kΩ

) 25.75

25.25

24.75

24.25

26.25

65 87 92 43

4295 G02

125°C85°C25°C–40°C

TEMPERATURE (°C)–50

0

V CC

CURR

ENT

(mA)

10

8

6

4

2

12

5025 10075 1250–25

4295 G03

214kHz

300kHz


3.162

V FB3

1 (V

)

3.176

3.174

3.172

3.170

3.168

3.166

3.164

3.178

5025 10075 1250–25

4295 G04

FB31 VOLTAGE (V)2.57

–15

ITHB

CUR

RENT

(µA)

10

5

–5

0

–10

15

3.17 3.37 3.57 3.772.77 2.97

4295 G05

125°C85°C25°C–40°C


0

DELA

Y TI

ME

(ns)

350

200

250

300

150

100

50

400

5025 10075 1250–25

4295 G09

TPGDELAY, 52.3k FROM FFSDLY TO VCC

TPGSG, 52.3k FROM FFSDLY TO VCC

TPGDELAY, 10.5k FROM FFSDLY TO VCC

TPGSG, 10.5k FROM FFSDLY TO VCC


175

FREQ

UENC

Y (k

Hz)

300

275

250

225

200

325

5025 10075 1250–25

4295 G06

45.3k FROM ROSC TO GND

ROSC OPEN

VITHB = 1.8V

VITHB = 2.3V

VITHB = 2.6V

VITHB = 2.9V

DUTY CYCLE (%)0

0

V (IS

EN+

- ISE

N–) (

mV)

140

80

100

120

60

40

20

160

4030 6050 702010

4295 G07

VITHB = 0.96V (FB31 = 0V)


0

PG D

ELAY

TIM

E (n

s)

200

150

100

50

250

5025 10075 1250–25

4295 G08

TPGDELAY, 5.23k FROM FFSDLY TO GND

TPGDELAY, 52.3k FROM FFSDLY TO GND



LT4295

6Rev. B


PIN FUNCTIONSGND(Pins 1, 19, Exposed Pad Pin 29): Device Ground. Exposed Pad must be electrically and thermally connected to pins 1, 19 and PCB GND.

AUX (Pin 2): Auxiliary Sense. Assert AUX via a resistive divider from the auxiliary power input to set the voltage at which the auxiliary supply takes over. Asserting AUX pulls down HSGATE, disconnects the detection signature resistor and disables classification signature. The AUX pin sinks IAUXH when below its threshold voltage, of VAUXT, to provide hysteresis. Connect to GND if not used.

RCLASS++ (Pin 3): Class Select Input. Connect a resistor between RCLASS++ to GND per Table 1.

RCLASS (Pin 4): Class Select Input. Connect a resistor between RCLASS and GND per Table 1.

T2P (Pin 5): PSE Type Indicator. Open drain with respect to VCC. See the T2P Output section for pin behavior.

VCC (Pins 6, 7, 8, 9, 21): Switching Regulator Controller Supply Voltage. Connect a local ceramic capacitor from VCC pin 21 to GND pin 19 as close as possible to LT4295 as shown in Table 3.

ROSC (Pin 10): Programmable Frequency Adjustment. Resistor to GND programs operating frequency. Leave open for default frequency of 214kHz.

SFST (Pin 11): Soft-Start. Capacitor to GND sets soft- start timing.

FFSDLY (Pin 12): Forward/Flyback Select and Primary Gate Delay Adjustment. Resistor to GND adjusts gate drive delay for a flyback topology. Resistor to VCC adjusts gate drive delay for a forward topology.

ITHB (Pin 13): Current Threshold Control. The voltage on this pin corresponds to the peak current of the exter-nal primary FET. Note that the voltage gain from ITHB to the input of the current sense comparator (VSENSE) is negative.

FB31 (Pin 14): Feedback Input. In flyback mode, connect external resistive divider from the third winding feedback. Reference voltage is 3.17V. Connect to GND in forward mode.

RLDCMP (Pin 15): Load Compensation Adjustment. Optional resistor to GND controls output voltage set point as a function of peak switching current. Leave RLDCMP open if load compensation is not needed.

ISEN– (Pin 16): Current Sense, Negative Input. Route as a dedicated trace to the return side of the current sense resistor.

ISEN+ (Pin 17): Current Sense, Positive Input. Route as a dedicated trace to the sense side of the current sense resistor.

SG (Pin 18): Secondary (Synchronous) Gate Drive Output.

PG (Pin 20): Primary Gate Drive Output.

DNC (Pin 22): Do Not Connect. Leave pin open.

SWVCC (Pin 23): Switch Driver for VCC’s Buck Regulator. This pin drives the base of a PNP in a buck regulator to generate VCC.

VIN (Pin 24): Buck Regulator Supply Voltage. Usually separated from HSSRC by a pi filter.

HSSRC (Pin 25): External Hot Swap MOSFET Source. Connect to source of the external MOSFET.

HSGATE (Pin 26): External Hot Swap MOSFET Gate Control Output. Capacitance to GND determines inrush time.

NC (Pin 27): No Connection. Not internally connected.

VPORT (Pin 28): PD Interface Supply Voltage and External Hot Swap MOSFET Drain Connection.



LT4295

7Rev. B


BLOCK DIAGRAM

–+

–+

–+

+–

–+

–+–+

+ – + –

SLOPECOMP

OSC

TSD

CP

SWITCHING REGULATOR

CONTROLLER

PD INTERFACECONTROLLER

START-UPREGULATOR

INTERNAL BUCK

CONTROLLER

1.4V

1.4V

HSGATE

HSSRC

11V

VPORT

VPORT SWVCCVIN

VCC

ITHB

SFST

FFSDLY

ROSC

ISEN+

ISEN–

4295 BD

T2P

GND

PG

SG

VCC

VPORTRCLASS

RCLASS++

AUX

VAUXT

IAUXH

FB31

RLDCMP

FEEDBACK AMPgm = –40µA/V

LOADCOMP

CURRENTFAULT

COMPARATOR

CURRENTSENSE

COMPARATOR

VFB

VFAULT

AV = 10

AV = 1

VCC

VSENSE

VITHB(OS)

AV =∆VSENSE∆VITHB



LT4295

8Rev. B


OVERVIEW

Power over Ethernet (PoE) continues to gain popularity as products take advantage of the combination of DC power and high speed data available from a single RJ45 con-nector. The LT4295 is IEEE 802.3bt-compliant and allows up to 71.3 Watt operation while maintaining backwards compatibility with existing PSE systems. The LT4295 combines a PoE PD interface controller and a switching regulator controller capable of either flyback or forward isolated power supply operation.

SIGNIFICANT DIFFERENCES FROM PREVIOUS PRODUCTS

The LT4295 has several significant differences from pre-vious Analog Devices products. These differences are briefly summarized below.

IEEE 802.3bt vs LTPoE++ Available PD Power

The LT4295 supports IEEE 802.3bt PD power levels up to 71.3 Watts. A PD requiring more than 71.3 Watts is beyond the allowable power levels of IEEE 802.3bt.

The LT4293, LT4275A and LT4276A are available to sup-port PD power levels up to 90W under the LTPoE++ stan-dard. See the Related Parts section for a list of LTPoE++ PSEs and PDs.

ITHB Is Inverted from the Usual ITH pin

The ITHB pin voltage has an inverse relationship to the cur-rent sense comparator threshold, VSENSE. Furthermore, the ITHB pin offset voltage, VITHB(OS), is 3.17V. See Figure 1.

APPLICATIONS INFORMATION

Figure 1. VSENSE vs. VITHB

Duty-Cycle Based Soft-Start

The LT4295 uses a duty cycle ramp soft-start that injects charge into ITHB. This allows startup without appreciable overshoot using inexpensive external components.

The Feedback Pin FB31 is 3.17V Rather Than 1.25V

The error amp feedback voltage VFB is 3.17V.

Flyback/Forward Mode Is Pin Selectable

The LT4295 operates in flyback mode if FFSDLY is pulled down by a resistor to GND. It operates in forward mode if FFSDLY is pulled up by a resistor to VCC. The value of this resistor determines the tPGDELAY and tPGSG.

T2P Pin Response

The T2P pin outputs high impedance to VCC, low imped-ance to VCC, 50% duty cycle, or 25% duty cycle, respon-sive to the number of class/mark event and responsive to

Table 1. Single-Signature Classification, Power Levels and Resistor Selection

PD REQUESTED CLASS PD REQUESTED POWER PD TYPE NOMINAL CLASS CURRENT

RESISTOR (1%)RCLS RCLS++

0 13W Type 1 2.5mA 1.00kΩ Open1 3.84W Type 1 or 3 10.5mA 150Ω Open2 6.49W Type 1 or 3 18.5mA 80.6Ω Open3 13W Type 1 or 3 28mA 52.3Ω Open4 25.5W Type 2 or 3 40mA 35.7Ω Open5 40W Type 3 40mA/2.5mA 1.00kΩ 37.4Ω6 51W Type 3 40mA/10.5mA 150Ω 47.5Ω7 62W Type 4 40mA/18.5mA 80.6Ω 64.9Ω8 71.3W Type 4 40mA/28mA 52.3Ω 118Ω

VSENSE

∆VSENSE∆VITHB

VITHBVITHB(OS)

4295 F01



LT4295

9Rev. B


Figure 2. Type 3 or Type 4 PSE, 1-Event Class Sequence

Figure 3. Type 2 PSE, 2-Event Class Sequence

APPLICATIONS INFORMATIONPoE or auxiliary power operation. See T2P Output section in the Applications Information.

VCC Is Powered by Internally Driven Buck Regulator

The LT4295 includes a buck regulator controller that must be used to generate the VCC supply voltage.

POE MODES OF OPERATION

The LT4295 has several modes of operation, depending on the input voltage sequence applied to the VPORT pin.

Detection Signature

During detection, the PSE looks for a 25kΩ detection sig-nature resistor which identifies the device as a PD. The LT4295 detection signature resistor is smaller than 25k to compensate for the additional series resistance intro-duced by the IEEE required diode bridge or the LT4321-based ideal diode bridge.

IEEE 802.3bt Single-Signature vs Dual-Signature PDs

IEEE 802.3bt defines two PD topologies: single-signature and dual-signature. The LT4295 primarily targets single-signature PD topologies, eliminating the need for a second PD control-ler. All PD descriptions and IEEE 802.3 standard references in this data sheet are limited in scope to single-signature PDs.

The LT4295 may be deployed in dual-signature PD appli-cations. For more information, contact Analog Devices Applications.

Classification Signature and Mark

The class/mark process varies depending on the PSE type. A PSE, after a successful detection, may apply a classifi-cation probe voltage of 15.5V to 20.5V and measure the PD classification signature current. Once the PSE applies a classification probe voltage, the PSE returns the PD voltage into the mark voltage range before applying another clas-sification probe voltage, or powering up the PD.

An example of 1-Event classification is shown in Figure 2. In 2-Event classification, a PSE probes for power clas-sification twice as shown in Figure 3. An IEEE 802.3bt PSE may apply as many as 5 events before powering up the PD.

4295 F02

V POR

T

VHSON

VHSOFF

VCLASSMIN

VMARKMAX

VSIGMIN

VRESET

DETECT

1ST MARK

1ST CLASS

POWER ON

4295 F03

V POR

TVHSON

VHSOFF

VCLASSMIN

VMARKMAX

VSIGMIN

VRESET

DETECT

1ST CLASS

1ST MARK 2ND MARK

2ND CLASS

POWER ON

IEEE 802.3bt Physical Classification and Demotion

IEEE 802.3bt defines physical classification to allow a PD to request a power allocation from the connected PSE and to allow the PSE to inform the PD of the PSE’s available power. Demotion is provided if the PD Requested Power level is not available at the PSE. If demoted, the PD must operate in a lower power state.

IEEE 802.3bt provides nine PD classes and four PD types, as shown in Table 1. The LT4295 class is configured by setting the RCLS and RCLS++ resistor values.

The number of class/mark events issued by the PSE directly indicates the power allocated to the PD and is summarized in Table 2.



LT4295

10Rev. B


APPLICATIONS INFORMATIONTable 2. PSE Allocated Power

PD REQUESTED CLASS

NUMBER OF PSE CLASS/MARK EVENTS

1 2 3 4 5

0 13W

1 3.84W

2 6.49W

3 13W

4 13W 25.5W

5 13W 25.5W 40W

6 13W 25.5W 51W

7 13W 25.5W 51W 62W

8 13W 25.5W 51W 71.3W

Note: Bold indicates the PD has been demoted.

IEEE 802.3bt PSEs present a single classification event (see Figure 2) to Class 0 through 3 PDs. A Class 0 through 3 PD presents its class signature to the PSE and is then powered on if sufficient power is available. Power limited IEEE 802.3bt PSEs may issue a single event to Class 4 and higher PDs in order to demote those PDs to Class 3 (13W).

IEEE 802.3bt PSEs present up to three classification events depending on type to Class 4 PDs (see Figure 4). Class 4 PDs present a class signature 4 on all events. This third event differentiates a Class 4 PD from a higher class PD. Power limited IEEE 802.3bt PSEs may issue three events to Class 5 and higher PDs in order to demote those PDs to Class 4 (25.5W).



Figure 6. Type 4 PSE, 5-Event Class Sequence

4295 F04

V POR

T

VHSON

VHSOFF

VCLASSMIN

VMARKMAX

VSIGMIN

VRESET

DETECT

1ST CLASS

1ST MARK 2ND MARK 3RD MARK

2ND CLASS 3RD CLASS

POWER ON

4295 F05

V POR

T

VHSON

VHSOFF

VCLASSMIN

VMARKMAX

VSIGMIN

VRESET

DETECT

1STCLASS

2NDCLASS

3RDCLASS

4THCLASS

POWER ON

1STMARK

2NDMARK

3RDMARK

4THMARK

4295 F06

V POR

T

VHSON

VHSOFF

VCLASSMIN

VMARKMAX

VSIGMIN

VRESET

DETECT

1STCLASS

2NDCLASS

3RDCLASS

4THCLASS

5THCLASS

POWER ON

1STMARK

2NDMARK

3RDMARK

4THMARK

5THMARK

IEEE 802.3bt PSEs present four classification events (see Figure 5) to Class 5 and 6 PDs. Class 5 and 6 PDs present a class signature 4 on the first two events. Class 5 and 6 PDs present a class signature 0 or 1, respectively, on the remaining events. Power limited IEEE 802.3bt PSEs may issue four events to Class 7 and higher PDs in order to demote those PDs to Class 6 (51W).

IEEE 802.3bt PSEs present five classification events (see Figure 6) to Class 7 and 8 PDs. Class 7 and 8 PDs present a class signature 4 on the first two events. Class 7 and 8 PDs present a class signature 2 or 3 respectively, on the remaining events.

The PD must monitor the number of classification/mark events, which is communicated through the LT4295 T2P pin.



LT4295

11Rev. B



Figure 7. Programming IINRUSH

Figure 8. VCC Buck Regulator

Classification Resistors (RCLS and RCLS++)

The RCLS and RCLS++ resistors set the classification cur-rents corresponding to the PD power classification. Select the value of RCLS and RCLS++ from Table 1 and connect the 1% resistor between the RCLASS, RCLASS++ pin and GND.

Detection Signature Corrupt During Mark Event

During the mark event, the LT4295 presents <11kΩ to the port as required by the IEEE 802.3 specification.

Inrush and Powered On

After the PSE detects and optionally classifies the PD, the PSE then powers on the PD. When the PD port voltage rises above the VHSON threshold, it begins to source IGPU out of the HSGATE pin. This current flows into an external capacitor CGATE in Figure 7 and causes a voltage to ramp up the gate of the external MOSFET. The external MOSFET acts as a source follower and ramps the voltage up on the output bulk capacitor, CPORT, thereby determining the inrush current, IINRUSH. Design IINRUSH to be ~100mA.

LT4295

HSGATE

GND4276 F07

VPORT HSSRC

CGATEIGPU

3.3k+

CPORT

VPORT

IINRUSH

IINRUSH = IGPU •CPORTCGATE

the switching regulator controller operates after a delay of tSTART.

EXTERNAL VCC SUPPLY

The external VCC supply must be configured as a buck reg-ulator shown in Figure 8. To optimize the buck regulator, use the external component values in Table 3 correspond-ing to the VIN operating range. This buck regulator runs in discontinuous mode with the inductor peak current considerably higher than average load current on VCC. Thus, the saturation current rating of the inductor must exceed the values shown in Table 3. Place the capacitor, C, as close as possible to VCC pin 21 and GND pin 19. For optimal performance, place these components as close as possible to the LT4295.

VIN

Re

VCC

VIN

VCCGND

SWVCC

LT4295

FMMT723PBSS9110T

LD

C

4276 F08

The LT4295 internal charge pump enables an N-channel MOSFET solution, replacing a larger and more costly P-channel FET. The low RDS(ON) MOSFET also maximizes power delivery and efficiency, reduces power and heat dissipation, and eases thermal design.

DELAY START

After the HSGATE charges up to approximately 7V above HSSRC, fully enhancing the external hot swap MOSFET,

Table 3. Buck Regulator Component SelectionVIN C L ISAT Re D

9V-57V PoE

22µF 10µF

22µH 100µH

≥1.2A ≥300mA

1Ω 20Ω

Schottky Ultrafast Diode

AUXILIARY SUPPLY OVERRIDE

If the AUX pin is held above VAUXT, the LT4295 enters auxiliary power operation. In this mode the detection sig-nature resistor is disconnected, classification is disabled, and HSGATE is pulled down.

The AUX pin allows for setting the auxiliary supply turn on (VAUXON) and turn off (VAUXOFF) voltage thresholds. The auxiliary supply hysteresis voltage, VAUXHYS, is set by sinking current, IAUXH, only when the AUX pin voltage is less than VAUXT. Use the following equations to set



LT4295

12Rev. B


APPLICATIONS INFORMATIONVAUXON and VAUXOFF via R1 and R2 in Figure 9. Note that an internal 6.5V Zener limits the voltage on the AUX pin.

A capacitor up to 1000pF may be placed between the AUX pin and GND to improve noise immunity. VAUXON must be lower than VHSOFF.

is determined by the AUX pin, the RCLASS++ pin, and the number of classification events. The LT4295 uses a 4-state encoding for the T2P output. T2P state and the associated PSE allocated power are shown in Table 4.

The highest priority input is the AUX pin. AUX is asserted to enter the auxiliary power state and deasserted to enter the PoE state. In the auxiliary power state, the T2P pin indicates the highest available power, based on PD Requested Class. The auxiliary power supply must be sized to provide at least the PD Requested Power.

Second, PD Requested Class and PD Requested Power are configured using the RCLASS and RCLASS++ pins. The RCLASS++ pin alone can be used to determine if the PD Class is 0−4 or 5−8, as shown in Table 1.

Last, the number of classification events determines the amount of power allocated by the PSE as described in Table 2.

Figure 9. AUX Threshold and Hysteresis Calculation

LT4295

GND4295 F09

AUX

R1

VAUX

+

–

R2

R1=VAUXON − VAUXOFF

IAUXH=

VAUXHYSIAUXH

R2 = R1VAUXOFFVAUXT

− 1

R1≥VAUX(MAX) − VAUXT

1.4mA

T2P Output

The LT4295 communicates the PSE allocated power to the PD application via the T2P pin. The T2P pin state

Table 4. T2P Response to Determine PSE Allocated Power

STATEPD REQUESTED

CLASS

NUMBER OF CLASSIFICATION

EVENTS T2P* PSE ALLOCATED POWER

Auxiliary0−4 N/A Low-Z Aux. Power5-8 N/A 25% Aux. Power

PoE

0−41 Hi-Z Min (PD Requested

Power, 13W)≥ 2 Low-Z 25.5W

5−8

1 Hi-Z 13W2 or 3 Low-Z 25.5W

4 50% Min (PD Requested Power, 51W)

5 25% Min (PD Requested Power, 71.3W)

* Specified as the percentage of the period which T2P is low impedance with respect to VCC.



LT4295

13Rev. B


Table 5. LT4295 Interoperability {T2P Response*, PSE Allocated Power, Number of Classification Events}

PD REQUESTED CLASS (PD

REQUESTED POWER)

PSE TYPE, CLASS (POWER)

AUXILIARY POWER

SOURCE**

IEEE 802.3 Type 1

IEEE 802.3 Type 2

IEEE 802.3 Type 3

IEEE 802.3 Type 4

Class 3 (13W)

Class 4 (25.5W)

Class 4 (25.5W)

Class 5 (40W)

Class 6 (51W)

Class 7 (62W)

Class 8 (71.3W)

Class 0-3 (up to 13W)

Hi-Z up to 13W

1-Event

Hi-Z up to 13W

1-Event

Hi-Z up to 13W

1-Event

Hi-Z up to 13W

1-Event

Hi-Z up to 13W

1-Event

Hi-Z up to 13W

1-Event

Hi-Z up to 13W

1-Event

Low-Z Aux. Power

N/A

Class 4 (25.5W)

Hi-Z 13W

1-Event

Low-Z 25.5W 2-Event

Low-Z 25.5W 3-Event

Low-Z 25.5W 3-Event

Low-Z 25.5W 3-Event

Low-Z 25.5W 3-Event

Low-Z 25.5W 3-Event

Low-Z Aux. Power

N/A

Class 5 (40W)

Hi-Z 13W

1-Event

Low-Z 25.5W 2-Event

Low-Z 25.5W 3-Event

50% 40W

4-Event

50% 40W

4-Event

50% 40W

4-Event

50% 40W

4-Event

25% Aux. Power

N/A

Class 6 (51W)

Hi-Z 13W

1-Event

Low-Z 25.5W 2-Event

Low-Z 25.5W 3-Event

Low-Z 25.5W 3-Event

50% 51W

4-Event

50% 51W

4-Event

50% 51W

4-Event

25% Aux. Power

N/A

Class 7 (62W)

Hi-Z 13W

1-Event

Low-Z 25.5W 2-Event

Low-Z 25.5W 3-Event

Low-Z 25.5W 3-Event

50% 51W

4-Event

25% 62W

5-Event

25% 62W

5-Event

25% Aux. Power

N/A

Class 8 (71.3W)

Hi-Z 13W

1-Event

Low-Z 25.5W 2-Event

Low-Z 25.5W 3-Event

Low-Z 25.5W 3-Event

50% 51W

4-Event

50% 51W

4-Event

25% 71.3W 5-Event

25% Aux. Power

N/ANote 1. Shade of blue indicates the PD has been demoted* Specified as the percentage of the period which T2P is low impedance with respect to VCC

** Auxiliary Power Supply must be sized to provide PD Requested Power

Interoperability Across Various PSEs and Auxiliary Power Source

Table 5 summarizes the expected T2P response, the PSE allocated power, and the number of classification events. The result is a function of PD Requested Class and power source—Auxiliary or PoE.

SWITCHING REGULATOR CONTROLLER OPERATION

The switching regulator controller portion of the LT4295 is a current mode controller capable of implementing either a flyback or a forward power supply. When used in flyback mode, no opto-isolator is required for feedback because the output voltage is sensed via the transformer’s third winding.

T2P Response* PSE Allocated Power

Number of Classification Events

25% 71.3W 5-Event



LT4295

14Rev. B


Figure 10. PG and SG Timing Relationship in Flyback Mode

Figure 11. Example PG and SG Connections in Flyback Mode

PG

SG

4295 F10

tPGDELAY

tPGon

tPGSG

•

•

••

PG

SGGND

LT4295

4295 F11

FFSDLY

RFFSDLY

ISEN+

ISEN–

+

Flyback Mode

The LT4295 is programmed into flyback mode by placing a resistor RFFSDLY from the FFSDLY pin to GND. This resis-tor must be in the range of 5.23kΩ to 52.3kΩ. If using a potentiometer to adjust RFFSDLY, ensure the adjustment of the potentiometer does not exceed 52.3kΩ.The value of RFFSDLY determines tPGDELAY according to the following equations:

tPGDELAY ≈ 2.69ns / kΩ •RFFSDLY +30ns

tPGSG ≈ 20ns

The SG pin must be connected to the secondary side MOSFET through a gate drive transformer as shown in Figure 11. Add a Schottky diode from PG to GND as shown in Figure 11 to prevent PG from going negative.

Forward Mode

The LT4295 is programmed into forward mode by placing a resistor RFFSDLY from the FFSDLY pin to VCC. The RFFSDLY resistor must be in the range of 10.5kΩ to 52.3kΩ. If using a potentiometer to adjust RFFSDLY ensure the adjust-ment of the potentiometer does not exceed 52.3kΩ.

The value of RFFSDLY determines tPGDELAY and tPGSG according to the following equations:

tPGDELAY ≈ 7.16ns/kΩ • RFFSDLY + 17ns

tPGSG ≈ 5.60ns/kΩ • RFFSDLY + 7.9ns

The PG and SG relationships in forward mode are shown in Figure 12.

In forward mode, the SG pin has the correct polarity to drive the active clamp P-channel MOSFET through a simple level shifter as shown in Figure 13. Add a Schottky diode from the PG to GND as shown in Figure 13 to pre-vent PG from going negative.

Figure 12. PG and SG Timing Relationship in Forward Mode

Figure 13. Example PG and SG Connections in Forward Mode

PG

SG

4295 F12

tPGDELAY tPGSG

••

PG

VCC

VCC

SGGND

LT4295

4295 F13

FFSDLY

RFFSDLY

ISEN+

ISEN–




LT4295

15Rev. B


Feedback Amplifier

In the flyback mode, the feedback amplifier senses the output voltage through the transformer’s third winding as shown in Figure 14. The amplifier is enabled only during the fixed interval, tFB, as shown in Figure 15. This elimi-nates the opto-isolator in isolated designs, thus greatly improving the dynamic response and stability over life-time. Since tFB is a fixed interval, the time-averaged trans-conductance, gm, varies as a function of the user-selected switching frequency.

APPLICATIONS INFORMATIONFEEDBACK AMPLIFIER OUTPUT, ITHB

As shown in the Block Diagram, VSENSE is the input of the Current Sense Comparator. VSENSE is derived from the output of a linear amplifier whose input is the voltage on the ITHB pin, VITHB.

This linear amplifier inverts its input, VITHB, with a gain, ΔVSENSE/ΔVITHB, and with an offset voltage of VITHB(OS) to yield its output, VSENSE. This relationship is shown graphically in Figure 1. Note the slope ΔVSENSE/ΔVITHB is a negative number and is provided in the electrical char-acteristics table.

VITHB = VITHB(OS) + VSENSE •

ΔVSENSEΔVITHB

⎛

⎝⎜

⎞

⎠⎟–1

The block diagram shows VSENSE is compared against the voltage across the current sense resistor, V(ISEN+)-V(ISEN–) modified by the internal slope compensation voltage discussed subsequently.

LOAD COMPENSATION

As can be seen in Figure 15, the voltage on the FB31 pin droops slightly during the flyback period. This is mostly caused by resistances of components of the secondary side such as: the secondary winding, RDS(ON) of the syn-chronous MOSFET, ESR of the output capacitor, etc. These resistances cause a feedback error that is proportional to the current in the secondary loop at the time of feedback sample window. To compensate for this error, the LT4295 places a voltage proportional to the peak current in the primary winding on the RLDCMP pin.

Determining Feedback and Load Compensation Resistors

Because the resistances of components on the secondary side are generally not well known, an empirical method must be used to determine the feedback and load com-pensation resistor values.

INITIALLY SETRFB2 = 2kΩ

RFB1 ≈RFB2VOUTVFB

NTHIRDNSECONDARY

–RFB2

Figure 14. Feedback and Load Compensation Connection

PG

FB31VOLTAGE

GND

SG4295 F15

tFBtFBDLY

VFB

Figure 15. Feedback Amplifier Timing Diagram

+–

+–

+–

•

•

•FEEDBACKFB31

LT4295THIRD

PRIMARY

4295 F14

SECONDARY

ITHB

PGISEN+

ISEN–

RLDCMP

RFB2VIN

VOUT

RSENSE

VFB

RFB1

RLDCMP

AV = 10



LT4295

16Rev. B


APPLICATIONS INFORMATIONConnect the resistor RLDCMP between the RLDCMP pin and GND. RLDCMP must be at least 10kΩ. Adjust RLDCMP for minimum change of VOUT over the full input and output load range. A potentiometer in series with 10kΩ may be initially used for RLDCMP and adjusted. The potentiometer+10kΩ may then be removed, mea-sured, and replaced with the equivalent fixed resistor. The resulting VOUT differs from the desired VOUT due to offset injected by load compensation. The change to RFB2 to correct this is predicted by:

ΔRFB2 =

ΔVOUTVFB

NTHIRDNSECONDARY

RFB22

RFB1

Where: ΔVOUT is the desired change to VOUT ΔRFB2 is the required change to RFB2

NTHIRD/NSECONDARY is the transformer third winding to secondary winding

OPTO-ISOLATOR FEEDBACK

For forward mode operation, the flyback voltage cannot be sensed across the transformer. Thus, opto-isolator feed-back must be used. When using opto-isolator feedback, connect the FB31 pin to GND and leave the RLDCMP pin open. In this condition, the feedback amplifier sinks an average current of ISINK into the ITHB pin. An example for feedback connections is shown in Figure 16. Note that since ISINK is time-averaged over the switching period, the sink current varies as a function of the user-selected switching frequency.

SOFT-START

In PoE applications, a proper soft-start design is required to prevent the PD from drawing more current than the PSE can provide.

The soft-start time, tSFST, is approximately the time in which the power supply output voltage, VOUT, is charging its output capacitance, COUT. This results in an inrush cur-rent at the port of the PD, Iport_inrush (not to be confused with IINRUSH discussed earlier in Applications Information section). Care must be taken in selecting tSFST to pre-vent the PD from drawing more current than the PSE can provide.

In the absence of an output load current, the Iport_inrush, is approximated by the following equation:

Iport_inrush ≈ COUT • VOUT2

η • tSFST • VIN

where η is the power supply efficiency,

VIN is the input voltage of the PD

Iport_inrush plus the port current due to the load current must be below the current the PSE can provide. Note that the PSE current capability depends on the PSE operating standard.

The LT4295 contains a soft-start function that controls tSFST by connecting an external capacitor, CSFST, between the SFST pin and GND. The SFST pin is pulled up with ISFST when the LT4295 begins switching. The voltage ramp on the SFST pin is proportional to the duty cycle ramp for PG.

For flyback mode, the soft-start time is:

tSFST =

600µAnF

CSFSTISFST

⎛

⎝⎜

⎞

⎠⎟ tPGon + tPGDELAY – tMIN( )

where tPGon is the time when PG is high as shown in Figure 8 once the power supply is in steady-state.

In forward mode, each of the back page applications sche-matics provides a chart with tSFST vs. CSFST. Select the application and choose a value of CSFST that corresponds to the desired soft-start time.

Figure 16. Opto-isolator Feedback Connections in the Forward Mode

LT4295ITHB

4295 F16

VCC VOUT

CXRX

FB31GND



LT4295

17Rev. B


APPLICATIONS INFORMATIONSHORT CIRCUIT RESPONSE

If the power supply output voltage is shorted, overloaded, or if the soft-start capacitor is too small, an overcurrent fault event occurs when the voltage across the sense pins exceeds VFAULT (after the blanking period of tMIN). This begins the internal fault timer tFAULT. For the duration of tFAULT, the LT4295 turns off PG and SG and pulls the SFST pin to GND. After tFAULT expires, the LT4295 initi-ates soft-start.

The fault and soft-start sequence repeats as long as the short circuit or overload conditions persist. This condition is recognized by the PG waveform shown in Figure 17 re peating at an interval of tFAULT.

CURRENT SENSE COMPARATOR

The LT4295 uses a differential current sense comparator to reduce the effects of stray resistance and inductance on the measurement of the primary current. ISEN+ and ISEN– must be Kelvin connected to the sense resistor pads.

Like most switching regulator controllers, the current sense comparator begins sensing the current tMIN after PG turns on. Then, the comparator turns PG off after the voltage across ISEN+ and ISEN– exceeds the current sense comparator threshold, VSENSE. Note that the voltage across ISEN+ and ISEN– is modified by LT4295’s internal slope compensation.

SLOPE COMPENSATION

The LT4295 incorporates current slope compensation. Slope compensation is required to ensure current loop stability when the duty cycle is greater than or near 50%. The slope compensation of the LT4295 does not reduce the maximum peak current at higher duty cycles.

CONTROL LOOP COMPENSATION

In flyback mode, loop frequency compensation is per-formed by connecting a resistor/capacitor network from the output of the feedback amplifier (ITHB pin) to GND as shown in Figure 14. In forward mode, loop compensation is performed by varying RX and CX in Figure 16.

ADJUSTABLE SWITCHING FREQUENCY

The LT4295 has a default switching frequency, fOSC, of 214 kHz when the ROSC pin is left open. If a higher switch-ing frequency, fSW, is desired (up to 300kHz), a resistor no smaller than 45.3kΩ may be added between the ROSC pin to GND. The resistor can be calculated below:

ROSC =

3900kΩ • kHzfSW – fOSC( )

kΩ( )

tFAULT

4295 F17

Figure 17. PG Waveforms with Output Shorted

OVERTEMPERATURE PROTECTION

The IEEE 802.3 specification requires a PD to withstand any applied voltage from 0V to 57V indefinitely. During classification, however, the power dissipation in the LT4295 may be as high as 1.5W. The LT4295 can easily tolerate this power for the maximum IEEE classification timing but overheats if this condition persists abnormally.

The LT4295 includes an overtemperature protection feature which is intended to protect the device during momentary overload conditions. If the junction tempera-ture exceeds the overtemperature threshold, the LT4295 pulls down HSGATE pin, disables classification, and dis-ables the switching regulator operation.



LT4295

18Rev. B


APPLICATIONS INFORMATIONA silicon diode bridge consumes up to 4% of the avail-able power. In addition, silicon diode bridges exhibit poor pairset-to-pairset unbalance performance. Each branch of a silicon diode bridge shares source/return current, and thermal runaway can cause large, non-compliant current unbalances between pairsets.

While using Schottky diodes can help reduce the power loss with a lower forward voltage, the Schottky bridge may not be suitable for high temperature PD applications. Schottky diode bridges exhibit temperature induced leak-age currents. The leakage current has a voltage depen-dency that can invalidate the measured detection signa-ture. In addition, these leakage currents can back-feed through the unpowered branch and the unused bridge, violating IEEE 802.3 specifications.

For high efficiency applications, the LT4295 supports an LT4321-based PoE ideal diode bridge that reduces the forward voltage drop from 0.7V to 20mV per diode while maintaining IEEE 802.3 compliance. The LT4321 simpli-fies thermal design, eliminates costly heatsinks, and can operate in space-constrained applications.

MAXIMUM DUTY CYCLE

The maximum duty cycle of the PG pin is modified by the chosen tPGDELAY and fSW. It is calculated below:

MAX POWER SUPPLY DUTY CYCLE = DMAX – tPGDELAY • fSW

For an appropriate margin during transient operation, the forward or flyback power supply should be designed so that its maximum steady-state duty cycle should be about 10% lower than the LT4295 Maximum Power Supply Duty Cycle calculated above.

EXTERNAL INTERFACE AND COMPONENT SELECTION

PoE Input Bridge

A PD is required to polarity-correct its input voltage. There are several different options available for bridge rectifiers; silicon diodes, Schottky diodes, and ideal diodes. When silicon or Schottky diode bridges are used, the diode for-ward voltage drops affect the voltage at the VPORT pin. The LT4295 is designed to tolerate these voltage drops. Note, the voltage parameters shown in the Electrical Characteristics section are specified at the LT4295 pack-age pins.



LT4295

19Rev. B


APPLICATIONS INFORMATIONAuxiliary Input Diode Bridge

Some PDs are required to receive AC or DC power from an auxiliary power source. A diode bridge is typically required to handle the voltage rectification and polarity correction.

In high efficiency applications, or in low auxiliary input voltage applications, the voltage drop across the rectifier cannot be tolerated. The LT4295 can be configured with an LT4320-based ideal diode bridge to recover the diode voltage drop and ease thermal design.

For applications with auxiliary input voltages below 10V, the LT4295 must be configured with an LT4320-based ideal diode bridge to recover the voltage drop and guaran-tee the minimum VPORT voltage is within the VPORT AUX range as specified in the Electrical Characteristics table.

Input Capacitor

A 0.1μF capacitor is needed from VPORT to GND to meet the input impedance requirement in IEEE 802.3 and to properly bypass the LT4295. When operating with the LT4321, locally bypass each with a 0.047μF capacitor, thus keeping the total port capacitance within specification.

Transient Voltage Suppressor

The LT4295 specifies an absolute maximum voltage of 100V and is designed to tolerate brief overvoltage events due to Ethernet cable surges. To protect the LT4295 from an overvoltage event, install a unidirectional transient volt-age suppressor (TVS) such as an SMAJ58A between the VPORT and GND pins. For PD applications that require an auxiliary power input, install a TVS between VIN and GND.

For extremely high cable discharge and surge protection, contact Analog Devices Applications.



LT4295

20Rev. B


TYPICAL APPLICATIONS13W PoE Power Supply in Flyback Mode with 5V, 2.3A Output

Efficiency vs Load Current VOUT vs Load Current

++VOUT5V AT 2.3A

–VOUT

Q1

L1: COILCRAFT, DO1813P-181HCL2: COILCRAFT, DO1608C-103L4: COILCRAFT, DO1608C-104C2: 22µF, 6.3V, MURATA GRM31CR70J226KE19C5: 47µF, 6.3V, PANASONIC 6SVP47MC7: 2.2µF, 100V, MURATA GRM32ER72A225KA35T1: WÜRTH, 750313109Q1: PSMN075-100MSET2: PCA EPA4271GE OR PULSE PE-68386NL

VPORT

GND

L210µH

L1180nH

L4100µH

10µF100V

10nF100V

3.3k

10µF10V

HSSRC SWVCC FB31PG

SGITHBROSCSFSTFFSDLYRCLASSGND

VPORT

LT4295

HSGATE

ISEN+

ISEN–

VIN VCC

C72.2µF

FDN86246

BAT54WS

BAT46WS

T2

4295 TA02a

PSMN4R2-30MLD

MMBT3906 MMBT3904

T1

1nF

1µF

6.04k

20Ω

2k

270Ω1/4W

11Ω1/4W

60mΩ1/4W

15Ω

100Ω

1µF

330pF0.1µF

107k5.23k52.3Ω

8.2Ω

PTVS58VP1UTP

4.7nF

2.2nF2KV

20k

10k

2.2nF

C222µF

C547µF6.3V

47pF630V

0.1µF100V

2.2nF2kV

BAV19WS

••

•

•

•

FMMT723

LOAD CURRENT (A)

76

EFFI

CIEN

CY (%

)

90

88

86

84

82

80

78

92

0.5 2.51 1.5 20

4295 TA02b

VPORT = 37VVPORT = 48VVPORT = 57V

LOAD CURRENT (A)

4.80

V OUT

(V)

5.15

5.10

5.05

5.00

4.95

4.90

4.85

5.20

0.5 2.51 1.5 20

4295 TA02c

VPORT = 37VVPORT = 48VVPORT = 57V



LT4295

21Rev. B


TYPICAL APPLICATIONS

40W

PoE

Pow

er S

uppl

y in

Fly

back

Mod

e w

ith 1

2V, 3

A Ou

tput

Effic

ienc

y vs

Loa

d Cu

rren

tV O

UT v

s Lo

ad C

urre

nt

++V

OUT

12V

AT 3

A

–VOU

T

Q1

L2 10µH

L118

0nH

L410

0µH

10µF

100V

10nF

100V

3.3k

24V

8.2Ω

10µF 10V

HSSR

CSW

VCC

FB31 PG SG T2

PIT

HBRO

SCSF

STFF

SDLY

RCLA

SSRC

LASS

++GN

D

VPOR

T

LT42

95

HSGA

TE

ISEN

+

ISEN

–

V IN

V CC

C7 2.2µ

F

BSZ9

0020

NS3

BAT5

4WS

BAT4

6WS

MOC

207M

V OUT

TO M

ICRO

PROC

ESSO

R

4295

TA0

3a

TPN1

600A

NH

MM

BT39

06M

MBT

3904

T1

470p

F

1µF

6.65

k

2.00

kΩ

100Ω

1/2W

11Ω

1/2W

25m

Ω1/

4W

15Ω

100Ω

1µF

220p

F

0.1µ

F10

7k

7.5k

37.4

Ω

1.00

kΩPT

VS58

VP1U

TP3.

3nF

2.2n

F2K

V26

.1k

10k

2.2n

FC2, C

310

µFC5 33

µF10

0pF

630V

47nF

100V

2.2n

F2k

V

BG36

LT43

21

BG12

TG12

TG36

TG78

TG45

BG45

BG78

OUTP

OUTNEN EN

IN12

T2

Q2Q3

Q4Q5

Q6Q7

Q8Q9

1

DATA

PAIR

S

SPAR

EPA

IRS

2 3 6 4 5 7 8

IN36

IN45

IN78

47nF

100V

L1: C

OILC

RAFT

, DO1

813P

-181

HCL2

: COI

LCRA

FT, D

O160

8C-1

03L4

: COI

LCRA

FT, D

O160

8C-1

04C2

, C3:

10µ

F, 16

V, M

URAT

A GR

M31

CR61

C106

KA88

C5: 3

3µF,

25V,

PAN

ASON

IC E

EHZA

1E33

0RC7

: 2.2

µF, 1

00V,

MUR

ATA

GRM

32ER

72A2

25KA

35T1

: WÜR

TH, 7

5031

6115

OR

PCA

EPC3

634G

Q1-Q

9: P

SMN0

75-1

00M

SET2

: PCA

EPA

4271

GE O

R PU

LSE

PE-6

8386

NLJ1

: WUR

TH 7

4995

1100

1A

BAV1

9WS

J1

5.1k

Ω10

k

20Ω

••

•

••

FMM

T723

++V

OUT

12V

AT 3

A

–VOU

T

Q1

L2 10µH

L118

0nH

L410

0µH

10µF

100V

10nF

100V

3.3k

24V

8.2Ω

10µF 10V

HSSR

CSW

VCC

FB31 PG SG T2

PIT

HBRO

SCSF

STFF

SDLY

RCLA

SSRC

LASS

++GN

D

VPOR

T

LT42

95

HSGA

TE

ISEN

+

ISEN

–

V IN

V CC

C7 2.2µ

F

BSZ9

0020

NS3

BAT5

4WS

BAT4

6WS

MOC

207M

V OUT

TO M

ICRO

PROC

ESSO

R

4295

TA0

3a

TPN1

600A

NH

MM

BT39

06M

MBT

3904

T1

470p

F

1µF

6.65

k

2.00

kΩ

100Ω

1/2W

11Ω

1/2W

25m

Ω1/

4W

15Ω

100Ω

1µF

220p

F

0.1µ

F10

7k

7.5k

37.4

Ω

1.00

kΩPT

VS58

VP1U

TP3.

3nF

2.2n

F2K

V26

.1k

10k

2.2n

FC2, C

310

µFC5 33

µF10

0pF

630V

47nF

100V

2.2n

F2k

V

BG36

LT43

21

BG12

TG12

TG36

TG78

TG45

BG45

BG78

OUTP

OUTNEN EN

IN12

T2

Q2Q3

Q4Q5

Q6Q7

Q8Q9

1

DATA

PAIR

S

SPAR

EPA

IRS

2 3 6 4 5 7 8

IN36

IN45

IN78

47nF

100V

L1: C

OILC

RAFT

, DO1

813P

-181

HCL2

: COI

LCRA

FT, D

O160

8C-1

03L4

: COI

LCRA

FT, D

O160

8C-1

04C2

, C3:

10µ

F, 16

V, M

URAT

A GR

M31

CR61

C106

KA88

C5: 3

3µF,

25V,

PAN

ASON

IC E

EHZA

1E33

0RC7

: 2.2

µF, 1

00V,

MUR

ATA

GRM

32ER

72A2

25KA

35T1

: WÜR

TH, 7

5031

6115

OR

PCA

EPC3

634G

Q1-Q

9: P

SMN0

75-1

00M

SET2

: PCA

EPA

4271

GE O

R PU

LSE

PE-6

8386

NLJ1

: WUR

TH 7

4995

1100

1A

BAV1

9WS

J1

5.1k

Ω10

k

20Ω

••

•

••

FMM

T723

VPOR

T =

44V

VPOR

T =

50V

VPOR

T =

57V

LOAD

CUR

RENT

(A)

00.

51

1.5

22.

53

8082848688909294 EFFICIENCY (%)

4295

TA0

3b

VPOR

T =

44V

VPOR

T =

50V

VPOR

T =

57V

LOAD

CUR

RENT

(A)

00.

51

1.5

22.

53

11.9

8

12.0

0

12.0

2

12.0

4

12.0

6

12.0

8

12.1

0

12.1

2

12.1

4

V(OUT) (V)

4295

TA0

3c



LT4295

22Rev. B



Effic

ienc

y vs

Loa

d Cu

rren

tV O

UT v

s Lo

ad C

urre

nt

71.3

W P

oE P

ower

Sup

ply

in F

orw

ard

Mod

e w

ith 5

V, 1

3A O

utpu

t C SFS

T (µF

)t S

FST

(ms)

0.10

1.2

0.33

3.8

1.0

12

3.3

38

Q1

10nF

100V

3.3k

L410

0µH

10µF 10V

BAV1

9WS

FMM

T723

8.2Ω

PTVS

58VP

1UTP

0.1µ

F10

0V

L12.

2µH

C5 100µ

F(×

2)

C8 100µ

F6H

VA10

0M

++

L24.

9µH

22µF

100V HS

SRC

SWVC

CFF

SDLY PG SG

ITHB

ROSC

SFST

RCLA

SS++

RCLA

SSGN

DFB

31

T2P

VPOR

T

LT42

95

HSGA

TE

ISEN

+

ISEN

–

V IN

V CC

V CC

+VOU

T

+VOU

T+5

V AT

13A

–VOU

TV C

C

C7 2.2µ

F(×

2)

BAT5

4WS

BSC1

90N1

2NS3

4295

TA0

4a

20m

1/2W

100Ω

1206

10nF

250V

5Ω

100n

F25

0V

750Ω

330Ω

240Ω

4.7n

ZR43

110

k10

.0k

10.0

k

1k

10Ω

10Ω

CMM

SH1-

40L

BSC0

54N0

4NS

BSC0

54N0

4NS

CMM

SH1-

40L

T1

CMM

SH1-

40L

8.2V

CMHZ

4694

18V

CMHZ

5248

B18

VCM

HZ52

48B

2.2n

F2k

V

33nF

0.1µ

F

0.1µ

F

10k

0.1µ

FFD

MC2

523P

CMM

SH1-

40L

M0C

207M

MM

BT39

04

VPOR

T

GND

13k

20Ω

52.3

Ω11

8Ω

0.47

µF10

0pF

100k

107k

••

L1: C

OILC

RAFT

, XAL

-101

0-22

2ME

L2: W

ÜRTH

, 744

3144

90L4

: COI

LCRA

FT, D

O160

8C-1

04C5

, 100

µF, 6

.3V,

SUN

CON

6HVA

100M

C7: 2

.2µF

, 100

V, M

URAT

A GR

M32

ER72

A225

KA35

LC8

: 100

µF, 6

.3V,

SUN

CON

6HVA

100M

T1: W

ÜRTH

, 750

3130

95Q1

: PSM

N040

-100

MSE

TO M

ICRO

PROC

ESSO

ROP

TO

10nF

VPOR

T =

41V

VPOR

T =

50V

VPOR

T =

57V

LOAD

CUR

RENT

(A)

02

46

810

1214

76788082848688909294 EFFICIENCY (%)

4295

TA0

4b

VPOR

T =

41V

VPOR

T =

50V

VPOR

T =

57V

LOAD

CUR

RENT

(A)

02

46

810

1214

4.80

4.85

4.90

4.95

5.00

5.05

5.10

5.15

5.20

V(OUT) (V)

4295

TA0

4c



LT4295

23Rev. B


Effic

ienc

y vs

Loa

d Cu

rren

tV O

UT v

s Lo

ad C

urre

nt


71.3

W P

oE P

ower

Sup

ply

in F

orw

ard

Mod

e w

ith 1

2V, 5

.5A

Outp

ut

C SFS

T (µF

)t S

FST

(ms)

0.10

1.5

0.33

4.9

1.0

15

3.3

48

+

L26.

5µH

22µF

100V

HSSR

C

SWVC

CFF

SDLY PG SG

ITHB

ROSC

SFST

RCLA

SS++

RCLA

SSGN

DFB

31

T2P

VPOR

T

LT42

95

HSGA

TE

ISEN

+

ISEN

–

V IN

V CC

V CC

+VOU

T

+VOU

T+1

2V A

T5.

5A

–VOU

T

V CC

C7 2.2µ

F(×

2)

BAT5

4WS

BSC1

90N1

2NS3

4295

TA0

5a

15m

Ω1/

2W

100Ω

1206

33nF

250V

0.22

µF25

0V

750Ω

820Ω

20k

ZR43

1

10k

10.0

k

100p

F

38.3

k

13k

10Ω

CMM

SH1-

60

BSC1

23N0

8S3

BSC1

23N0

8S3

T1

CMM

SH1-

100

CMM

SH1-

100

13V

CMHZ

4700

7.5V

CMHZ

5236

B

2.2n

F2k

V

6.8n

F

0.1µ

F

0.1µ

F

10k

0.1µ

FFD

MC2

523P

CMM

SH1-

40L

M0C

207M

MM

BT39

04

29.4

k

V CC

BG36

LT43

21

BG12

TG12

TG36

TG78

TG45

BG45

BG78

OUTP

OUTNEN EN

IN12

T2

Q2Q3

Q4Q5

Q6Q7

Q8Q9

1

DATA

PAIR

S

SPAR

EPA

IRS

2 3 6 4 5 7 8

IN36

IN45

IN78

52.3

Ω

118Ω

1µF

100p

F

100p

F

100k

107k

330p

F

20Ω

••

7.5Ω

Q1

10nF

100V

47nF

100V

3.3k

L410

0µH

10µF 10V

8.2Ω

PTVS

58VP

1UTP

47nF

100V

L18.

2µH

22µF 16V

(×2)

C8 100µ

F

L1: C

OILC

RAFT

, XAL

-101

0-82

2ME

L2: W

ÜRTH

, 744

3146

50L4

: COI

LCRA

FT, D

O160

8C-1

04C7

: 2.2

µF, 1

00V,

MUR

ATA

GRM

32ER

72A2

25KA

35L

C8: 1

00µF

, 16V

, SUN

CON

16HV

A100

MT1

: PCA

EPC

3577

G-LF

T2: W

ÜRTH

, 749

0220

16Q1

: PSM

N040

-100

MSE

Q2-Q

9: P

SMN0

75-1

00M

SE

TO M

ICRO

PROC

ESSO

ROP

TO

FMM

T723

820p

F

100p

F

+VOU

T

+VOU

T

7.5V

CMHZ

5236

B

5.1k

FMM

T624

FMM

T624

5.1k

100p

F

BAV1

9WS

VPOR

T =

41V

VPOR

T =

50V

VPOR

T =

57V

LOAD

CUR

RENT

(A)

01

23

45

6788082848688909294 EFFICIENCY (%)

4295

TA0

5b

VPOR

T =

41V

VPOR

T =

50V

VPOR

T =

57V

LOAD

CUR

RENT

(A)

01

23

45

611

.60

11.7

0

11.8

0

11.9

0

12.0

0

12.1

0

12.2

0

12.3

0

12.4

0

V(OUT) (V)

4295

TA0

5c



LT4295

24Rev. B



40W

PoE

Pow

er S

uppl

y in

Fly

back

Mod

e w

ith 5

V, 7

.3A

Outp

ut

Effic

ienc

y vs

Loa

d Cu

rren

tV O

UT v

s Lo

ad C

urre

nt

+

+VOU

T5V

AT

7.3A

–VOU

T

Q1

L1: C

OILC

RAFT

, DO1

813P

-181

HCL2

: COI

LCRA

FT, D

O160

8C-1

03L4

: COI

LCRA

FT, D

O160

8C-1

04C2

, C3:

47µ

F, 6.

3V, G

RM31

CR60

J476

ME1

9LC5

: 47µ

F, 6.

3V, P

ANAS

ONIC

6SV

P47M

C7: 2

.2µF

, 100

V, M

URAT

A GR

M32

ER72

A225

KA35

LT1

: WÜR

TH, 7

5031

4783

OR

PCA

EPC3

586G

Q1-Q

9: P

SMN0

75-1

00M

SET2

: PCA

EPA

4271

GE O

R PU

LSE

PE-6

8386

NLJ1

: WUR

TH 7

4995

1100

1AL2 10µH

L118

0nH

L410

0µH

10µF

100V

10nF

100V

47nF

100V

3.3k

24V

8.2Ω

10µF 10V

HSSR

CSW

VCC

FB31 PG SG T2

PIT

HBRO

SCSF

STFF

SDLY

RCLA

SS++

RCLA

SS

GND

VPOR

T

LT42

95

HSGA

TE

ISEN

+

ISEN

–

V IN

V CC

C7 2.2µ

F

BSZ9

00N2

0NS

3G

BAT5

4WS

BAT4

6WS

TO M

ICRO

PROC

ESSO

R42

95 T

A06a

PSM

N2R4

-30M

LD

MM

BT39

06M

MBT

3904

T1 1nF

1µF

5.90

k

2.00

kΩ

80Ω

1/4W

5.1Ω

1/4W

40m

Ω1/

4W

15Ω

100Ω

20Ω

1µF

220p

F0.

1µF

107k

7.50

k

RLDC

MP

51k

37.4

Ω

1.00

kΩPT

VS58

VP1U

TP3.

3nF

2.2n

F2K

V20

k

10k

2.2n

FC2, C

347

µF|| 4

7µF

C5 47µF

100p

F10

0V

47nF

100V

2.2n

F2k

V OPTO

BG36

LT43

21

BG12

TG12

TG36

TG78

TG45

BG45

BG78

OUTP

OUTNEN EN

IN12

T2

Q2Q3

Q4Q5

Q6Q7

Q8Q9

1

DATA

PAIR

S

SPAR

EPA

IRS

2 3 6 4 5 7 8

IN36

IN45

IN78

BAV1

9WS

J1

••

•

••

FMM

T723

VPOR

T =

50V

VPOR

T =

57V

LOAD

CUR

RENT

(A)

01

23

45

67

788082848688909294 EFFICIENCY (%)

4295

TA0

6b

VPOR

T =

50V

VPOR

T =

57V

LOAD

CUR

RENT

(A)

01

23

45

67

4.80

4.85

4.90

4.95

5.00

5.05

5.10

5.15

5.20

V(OUT) (V)

4295

TA0

6c



LT4295

25Rev. B



25.5

W P

oE a

nd 9

V to

57V

Aux

iliar

y In

put P

ower

Sup

ply

in F

lyba

ck M

ode

with

12V

, 1.9

A Ou

tput

Effic

ienc

y vs

Loa

d Cu

rren

tV O

UT v

s Lo

ad C

urre

nt

++V

OUT

12V

AT 1

.9A

–VOU

T

Q1

L1: C

OILC

RAFT

, DO1

813P

-561

ML

L2: W

ÜRTH

, 744

3330

820

L3: M

URAT

A, L

QM21

PN2R

2NGC

D L4

: COI

LCRA

FT, D

O181

3H-2

23C2

, C3:

10µ

F, 16

V, M

URAT

A GR

M31

CR61

C106

KA88

C5: 3

3µF,

20V,

KEM

ET, T

494V

336M

020A

SC7

, C8:

3.3

µF, 1

00V,

TDK

C32

25X7

S2A3

35M

T1: P

CA E

PC36

01G

OR W

ÜRTH

750

3154

22Q1

-Q9:

PSM

N075

-100

MSE

T2: P

CA E

PA42

71GE

OR

PULS

E PE

-683

86NL

J1: W

URTH

749

9511

611A

L28.

2µH

L32.

2µH

1µF

680µ

F

L156

0nH

L4 22µH

10µF

100V

100n

F10

0V0.

1µF

3.3k

158k

931k

24V

47nF

100V

8.2Ω

22µF 10V

PMEG

1001

0ELR

HSSR

CSW

VCC

FB31 PG SG T2P

ITHB

ROSC

SFST

FFSD

LYRC

LASS

GND

VPOR

T

AUX

LT42

95

HSGA

TE

ISEN

+

ISEN

–

V IN

V CC

C7, C

83.

3µF

FDM

C861

60

BAT5

4WS

BAT4

6WS

TO M

ICRO

PROC

ESSO

R42

95 T

A07a

BSZ9

00NF

20NS

3

CMLT

7820

GCM

LT38

20G

T1

220p

F

1µF

4.75

k

2.00

k

62Ω

1/4W

82Ω

||82Ω

1/4W

15m

Ω1/

2W

15Ω

100Ω

1µF

220p

F0.

1µF

107k

9.31

k

RLDC

MP

51k

35.7

Ω

PTVS

58VP

1UTP

4.7n

F

2.2n

F2K

V43

k

10k

2.2n

FC2, C

310

µF|| 1

0µF

C5 33µF

100p

F10

0V

47nF

100V

2.2n

F2k

V OPTO

BG36

LT43

21

BG12

TG12

TG36

TG78

TG45

BG45

BG78

OUTP

OUTNEN EN

IN12

TG2

TG1

OUTP

OUTN

IN1

IN2

BG2

BG1

T2

Q2Q3

Q4Q5

Q6Q7

Q8Q9

1

DATA

PAIR

S

SPAR

EPA

IRS

2 3 6 4 5 7 8

IN36

IN45

IN78

LT43

20

BSZ1

10N0

6NS3

x4

MM

SD41

48 x

3

V AUX

9V T

O 57

VDC

OR 2

4VAC

1Ω

••

•

••

+

FMM

T723

J1

V AUX

= 9V

V AUX

= 24

VV A

UX=

42.5

VV A

UX=

57V

LOAD

CUR

RENT

(A)

00.

51

1.5

2727680848892 EFFICIENCY (%)

4295

TA0

7b

V AUX

= 9V

V AUX

= 24

VV A

UX=

42.5

VV A

UX=

57V

LOAD

CUR

RENT

(A)

00.

51

1.5

211

.50

11.7

5

12.0

0

12.2

5

12.5

0

V(OUT) (V)

4295

TA0

7c



LT4295

26Rev. B



25.5

W P

oE P

ower

Sup

ply

in F

lyba

ck M

ode

with

3.3

V, 6

.8A

Outp

ut

Effic

ienc

y vs

Loa

d Cu

rren

tV O

UT v

s Lo

ad C

urre

nt

++V

OUT

3.3V

AT

6.8A

–VOU

T

Q1

L1: C

OILC

RAFT

, DO1

813P

-181

HCL2

: COI

LCRA

FT, D

O160

8C-1

03L4

: COI

LCRA

FT, D

O160

8C-1

04C2

, C3:

22µ

F, 6.

3V, M

URAT

A GR

M31

CR70

J226

KE19

C5: 6

8µF,

4V, 4

SVPA

68M

AAC7

: 2.2

µF, 1

00V,

MUR

ATA

GRM

32ER

72A2

25KA

35T1

: WÜR

TH, 7

5031

0743

OR

PCA

EPC3

408G

Q1-Q

9: P

SMN0

75-1

00M

SET2

: PCA

EPA

4271

GE O

R PU

LSE

PE-6

8386

NLJ1

: WUR

TH 7

4995

1100

1AL2 10µH

L118

0nH

L410

0µH

10µF

100V

10nF

100V

47nF

100V

3.3k

24V

8.2Ω

10µF 10V

HSSR

CSW

VCC

FB31 PG SG T2

PIT

HBRO

SCSF

STFF

SDLY

RCLA

SSGN

D

VPOR

T

LT42

95

HSGA

TE

ISEN

+

ISEN

–

V IN

V CC

C7 2.2µ

F

BSZ9

00N2

0NS3

BAT5

4WS

BAT4

6WS

TO M

ICRO

PROC

ESSO

R42

95 T

A08a

PSM

N2R4

-30M

LD

MM

BT39

06M

MBT

3904

T1 1nF

47Ω

1µF

6.49

k

2k

100Ω

1/4W

5.1Ω

1/4W

40m

Ω1/

4W

15Ω

100Ω

1µF

470p

F0.

1µF

107k

6.81

k35

.7Ω

PTVS

58VP

1UTP

4.7n

F

2.2n

F2K

V8.

25k

10k

2.2n

FC2, C

322

µF|| 2

2µF

C5 68µF

100p

F10

0V

47nF

100V

2.2n

F2k

V OPTO

BG36

LT43

21

BG12

TG12

TG36

TG78

TG45

BG45

BG78

OUTP

OUTNEN EN

IN12

T2

Q2Q3

Q4Q5

Q6Q7

Q8Q9

1

DATA

PAIR

S

SPAR

EPA

IRS

2 3 6 4 5 7 8

IN36

IN45

IN78

B054

0WS

BAV1

9WS

J1

20Ω

••

•

••

FMM

T723

VPOR

T =

42.5

VVP

ORT

= 50

VVP

ORT

= 57

V

LOAD

CUR

RENT

(A)

02

46

87880828486889092 EFFICIENCY (%)

4295

TA0

8b

VPOR

T =

42.5

VVP

ORT

= 50

VVP

ORT

= 57

V

LOAD

CUR

RENT

(A)

02

46

83.

20

3.30

3.40

3.50

V(OUT) (V)

4295

TA0

8c



LT4295

27Rev. B



25.5

W P

oE P

ower

Sup

ply

in F

lyba

ck M

ode

with

24V

, 0.9

5A O

utpu

t

Effic

ienc

y vs

Loa

d Cu

rren

tV O

UT v

s Lo

ad C

urre

nt

++V

OUT

24V

AT 0

.95A

–VOU

T

Q1

L2: C

OILC

RAFT

, DO1

608C

-103

L4: C

OILC

RAFT

, DO1

608C

-104

C2: 4

.7µF

, 50V

, MUR

ATA

GRM

31CR

71H4

75KA

12C5

: 22µ

F, 35

V, P

ANAS

ONIC

EEH

-ZA1

V220

RC7

: 2.2

µF, 1

00V,

MUR

ATA

GRM

32ER

72A2

25KA

35T1

: WÜR

TH, 7

5031

4782

OR

PCA

EPC3

603G

Q1-Q

9: P

SMN0

75-1

00M

SET2

: PCA

EPA

4271

GE O

R PU

LSE

PE-6

8386

NLJ1

: WUR

TH 7

4995

1100

1L2 10µH

L410

0µH

10µF

100V

10nF

100V

3.3k

24V

8.2Ω

10µF 10V

HSSR

CSW

VCC

FB31 PG SG T2

PIT

HBRO

SCSF

STFF

SDLY

RCLA

SSGN

D

VPOR

T

LT42

95

HSGA

TE

ISEN

+

ISEN

–

V IN

V CC

C7 2.2µ

F

BSZ5

20N1

5NS3

G

BAT5

4WS

BAT4

6WS

TO M

ICRO

PROC

ESSO

R42

95 T

A09a

BSZ1

2DN2

0NS3

MM

BT39

06M

MBT

3904

T1 150p

F

0.1µ

F

6.49

k

2.00

kΩ

20Ω

100Ω

1/4W

120Ω

||120

Ω1/

4W

40m

Ω1/

4W

15Ω

100Ω

1µF

10pF

0.47

µF

107k

5.23

k

RLDC

MP

24k

35.7

ΩPT

VS58

VP1U

TP3.

3nF

2.2n

F2K

V16

0k

10k

2.2n

FC2, C

34.

7µF

50V

C5 22µF

47pF

100V

47nF

100V

2.2n

F2k

V OPTO

BG36

LT43

21

BG12

TG12

TG36

TG78

TG45

BG45

BG78

OUTP

OUTNEN EN

IN12

T2

Q2Q3

Q4Q5

Q6Q7

Q8Q9

1

DATA

PAIR

S

SPAR

EPA

IRS

2 3 6 4 5 7 8

IN36

IN45

IN78

47nF

100V

BAV1

9WS

••

•

••

FMM

T723

J1

VPOR

T =

42.5

VVP

ORT

= 50

VVP

ORT

= 57

V

LOAD

CUR

RENT

(A)

00.

20.

40.

60.

81

80828486889092 EFFICIENCY (%)

4295

TA0

9b

VPOR

T =

42.5

VVP

ORT

= 50

VVP

ORT

= 57

V

LOAD

CUR

RENT

(A)

00.

20.

40.

60.

81

23.4

23.6

23.8

24.0

24.2

24.4

24.6

V(OUT) (V)

4295

TA0

9c



LT4295

28Rev. B



62W

PoE

Pow

er S

uppl

y in

Fly

back

Mod

e w

ith 2

4V, 2

.4A

Outp

ut

Effic

ienc

y vs

Loa

d Cu

rren

tV O

UT v

s Lo

ad C

urre

nt

++V

OUT

24V

AT 2

.4A

–VOU

T

Q1

L1: C

OILC

RAFT

, DO1

813H

-122

ML

L2: W

URTH

, 744

3144

90L4

: COI

LCRA

FT, D

O160

8C-1

04C2

: MUR

ATA

GRM

32ER

61H1

06K

C5: 4

7µF,

35V,

EEE

-FT1

V470

ARC7

, C8:

2.2

µF, 1

00V,

MUR

ATA

GRM

32ER

72A2

25KA

35Q1

: NXP

PSM

N040

-100

MSE

Q2-Q

9: P

SMN0

75-1

00M

SET1

: PCA

EPC

3630

G OR

WUR

TH 7

5031

6231

T2: P

CA E

PA42

71GE

OR

PULS

E PE

-683

86NL

J1: W

URTH

749

0220

16L2

4.9µ

H

L410

0µH

L11.

2µH

22µF

100V

10nF

100V

3.3k

24V

8.2Ω

10µF 10V

HSSR

CSW

VCC

FB31 PG SG T2

PIT

HBRO

SCSF

STFF

SDLY

RCLA

SS++

RCLA

SSGN

D

VPOR

T

LT42

95

HSGA

TE

ISEN

+

ISEN

–

V IN

V CC

C7, C

82.

2µF

BSC1

90N1

5NS3

BAT5

4WS

BAT4

6WS

TO M

ICRO

PROC

ESSO

R42

95 T

A10a

BSC3

20N2

0NS3

PBSS

5140

TPB

SS41

40T

T1 330p

F

0.1µ

F

3.74

k

2.00

kΩ

20Ω

27Ω

1/2W

18Ω 1W

15m

Ω1/

2W

15Ω

100Ω

1µF

330p

F0.

47µF

107k

5.23

k

RLDC

MP

36k

80.6

Ω

64.9

ΩPT

VS58

VP1U

TP10

nF

2.2n

F2K

V22

k

10k

2.2n

FC2 10µF

50V

C5 47µF

220p

F10

0V

47nF

100V

4.7n

F2k

V OPTO

BG36

LT43

21

BG12

TG12

TG36

TG78

TG45

BG45

BG78

OUTP

OUTNEN EN

IN12

T2

Q2Q3

Q4Q5

Q6Q7

Q8Q9

1

DATA

PAIR

S

SPAR

EPA

IRS

2 3 6 4 5 7 8

IN36

IN45

IN78

47nF

100V

BAV1

9WS

J1

••

•

••

FMM

T723

VPOR

T =

41V

VPOR

T =

50V

VPOR

T =

57V

LOAD

CUR

RENT

(A)

00.

51

1.5

22.

53

8082848688909294 EFFICIENCY (%)

4295

TA1

0b

VPOR

T =

41V

VPOR

T =

50V

VPOR

T =

57V

LOAD

CUR

RENT

(A)

00.

51

1.5

22.

53

24.0

24.1

24.2

24.3

24.4

24.5

V(OUT) (V)

4295

TA1

0c



LT4295

29Rev. B



71.3

W P

oE P

ower

Sup

ply

in F

orw

ard

Mod

e w

ith 2

4V, 2

.7A

Outp

ut

+

L26.

5µH

22µF

100V

HSSR

C

SWVC

CFF

SDLY PG SG

ITHB

ROSC

SFST

RCLA

SS++

RCLA

SSGN

DFB

31

T2P

VPOR

T

LT42

95

HSGA

TE

ISEN

+

ISEN

–

V IN

V CC

V CC

+VOU

T

+VOU

T

+VOU

T24

V AT

2.7A

–VOU

T

V CC

C7 2.2µ

F(×

2)

BAT5

4WS

BSC1

90N1

2NS3

4295

TA1

1a

15m

Ω1/

2W

82Ω

1206

10nF

250V

47nF

250V

750Ω

820Ω

1.2k

ZR43

1

10k

10.0

k

1nF

86.6

k

10k

TPH5

900C

NH

TPH5

900C

NH

T1

SMD1

200P

L-TP

SMD1

200P

L-TP

13V

CMHZ

4700

7.5V

CMHZ

5236

B

2.2n

F2k

V

6.8n

F

0.1µ

F

10k

0.1µ

FFD

MC2

523P

CMM

SH1-

40L

M0C

207M

MM

BT39

04

33k

V CC

BG36

LT43

21

BG12

TG12

TG36

TG78

TG45

BG45

BG78

OUTP

OUTNEN EN

IN12

T2

Q2Q3

Q4Q5

Q6Q7

Q8Q9

1

DATA

PAIR

S

SPAR

EPA

IRS

2 3 6 4 5 7 8

IN36

IN45

IN78

52.3

Ω

118Ω

1µF

100p

F

100k

107k

3.3n

F

20Ω

••

47Ω

Q1

10nF

100V

47nF

100V

3.3k

L410

0µH

10µF 10V

8.2Ω

PTVS

58VP

1UTP

47nF

100V

L1 22µH C2

10µF 35V

C1 47µF

35V

L1: P

ULSE

PA2

050.

223

L2: W

ÜRTH

, 744

3146

50L4

: COI

LCRA

FT, D

O160

8C-1

04C1

: PAN

ASON

IC E

EHZA

1V47

0PC2

: MUR

ATA

GRM

32ER

6YA1

06KA

12C7

: 2.2

µF, 1

00V,

MUR

ATA

GRM

32ER

72A2

25KA

35L

Q1: P

SMN0

40-1

00M

SEQ2

-Q9:

PSM

N075

-100

MSE

T1: P

CA E

PC36

36G

T2: W

ÜRTH

, 749

0220

16

TO M

ICRO

PROC

ESSO

ROP

TO

FMM

T723

220p

F

47pF

+VOU

T

+VOU

T

7.5V

CMHZ

5236

B

13k

FMM

T624

FMM

T624

13k

47pF

BAV1

9WS

C SFS

T (µF

)t S

FST

(ms)

0.10

1.4

0.22

2.4

0.47

4.4

1.0

15

3.3

46

V OUT

vs

Load

Cur

rent

Effic

ienc

y vs

Loa

d Cu

rren

t

VPOR

T =

41V

VPOR

T =

50V

VPOR

T =

57V

LOAD

CUR

RENT

(A)

00.

51

1.5

22.

53

8082848688909294 EFFICIENCY (%)

4295

TA1

1b

VPOR

T =

41V

VPOR

T =

50V

VPOR

T =

57V

LOAD

CUR

RENT

(A)

00.

51

1.5

22.

53

23.0

23.4

23.8

24.2

24.6

25.0

V(OUT) (V)

4295

TA1

1c



LT4295

30Rev. B


PACKAGE DESCRIPTION

4.00 ±0.10(2 SIDES)

2.50 REF

5.00 ±0.10(2 SIDES)

NOTE:1. DRAWING PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WGHD-3).2. DRAWING NOT TO SCALE3. ALL DIMENSIONS ARE IN MILLIMETERS4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE5. EXPOSED PAD SHALL BE SOLDER PLATED6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE

PIN 1TOP MARK(NOTE 6)

0.40 ±0.10

27 28

1

2

BOTTOM VIEW—EXPOSED PAD

3.50 REF

0.75 ±0.05 R = 0.115TYP

R = 0.05TYP

PIN 1 NOTCHR = 0.20 OR 0.35× 45° CHAMFER

0.25 ±0.05

0.50 BSC

0.200 REF

0.00 – 0.05

(UFD28) QFN 0816 REV C

RECOMMENDED SOLDER PAD PITCH AND DIMENSIONSAPPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED

0.70 ±0.05

0.25 ±0.050.50 BSC

2.50 REF

3.50 REF4.10 ±0.055.50 ±0.05

2.65 ±0.05

3.10 ±0.054.50 ±0.05

PACKAGE OUTLINE

2.65 ±0.10

3.65 ±0.10

3.65 ±0.05

UFD Package28-Lead Plastic QFN (4mm × 5mm)

(Reference LTC DWG # 05-08-1712 Rev C)



LT4295

31Rev. B


Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices.

REVISION HISTORYREV DATE DESCRIPTION PAGE NUMBER

A 09/18 Updated max input power to 71.3W per Draft 3.4Revised T2P Output, PoE Input Bridge, Input Capacitor, and Transient Voltage Supressor Applications InformationChanged RCLASS and/or RCLASS++ resistor valuesAdded J1 transformer recommendations

1-3012, 1720, 26

19, 22-26, 30

B 5/19 Removed Draft numberAdded Table 5–Interoperability

1-3013



LT4295

32Rev. B

For more information www.analog.com ANALOG DEVICES, INC. 2016–2019

05/19www.analog.com

RELATED PARTS

TYPICAL APPLICATION

PART NUMBER DESCRIPTION COMMENTSLT4293 LTPoE++/IEEE 802.3bt PD Interface Mutually identifies with LTPoE++ and IEEE 802.3bt PSEsLT4294 IEEE 802.3bt PD Controller External Switch, IEEE 802.3bt and AUX SupportLT4320/LT4320-1 Ideal Diode Bridge Controller 9V-72V, DC to 600Hz Input. Controls 4-NMOSFETs, Voltage Rectification without Diode DropsLT4321 PoE Ideal Diode Bridge Controller Controls 8-NMOSFETs for IEEE-required PD Voltage Rectification without Diode DropsLTC4292/LTC4291-1 4-Port IEEE 802.3bt PSE Controller Transformer Isolation, Supports IEEE 802.3bt PDsLTC4269-1 IEEE 802.3at PD Interface with Integrated

Flyback Switching Regulator2-Event Classification, Programmable Class, Synchronous No-Opto Flyback Controller, 50kHz to 250kHz, Aux Support

LTC4269-2 IEEE 802.3at PD Interface with Integrated Forward Switching Regulator

2-Event Classification, Programmable Class, Synchronous Forward Controller, 100kHz to 500kHz, Aux Support

LT4275A/B/C LTPoE++/PoE+/PoE PD Controller External Switch, LTPoE++ SupportLT4276A/B/C LTPoE++/PoE+/PoE PD with Forward/

Flyback Switching Regulator ControllerExternal Switch, LTPoE++ Support, User-Configurable Class, Forward or No-Opto Flyback Operation, Frequency, PG/SG Delays, Soft-Start, and Aux Support as Low as 9V, Incl Housekeeping Buck, Slope Compensation

LTC4278 IEEE 802.3at PD Interface with Integrated Flyback Switching Regulator

2-Event Classification, Programmable Class, Synchronous No-Opto Flyback Controller, 50kHz to 250kHz, 12V Aux Support

51W PoE Power Supply in Flyback Mode with 12V, 3.9A Output

Efficiency vs Load Current VOUT vs Load Current

+VOUT12V AT 3.9A

–VOUT

Q1

C5: 47µF, 35V, PANASONIC EEHZA1V470PC7, C8: 2.2µF, 100V, MURATA GRM32ER72A225KA35T1: WÜRTH, 750316116 OR PCA EPC3633GT2: PCA EPA4271GE OR PULSE PE-68386NLQ1: PSMN040-100MSEQ2-Q9: PSMN075-100MSE

L1

L4100µH

10nF100V

3.3k

24V

8.2Ω

10µF10V

HSSRCSWVCC FB31

PG

SGT2P

ITHBROSCSFSTFFSDLYRCLASSRCLASS++GND

VPORT

LT4295

HSGATE

ISEN+

ISEN–

VIN VCCTPH1500CNH

BAT54WS

BAT46WS

MOC207M

VOUT

TO MICROPROCESSOR

4295 TA12a

TPH1500CNH

PBSS514OT PBSS414OT

T1

330pF

1µF

5.62k

2.00kΩ

36Ω1/2W

20Ω1/2W

20mΩ1/2W

15Ω

100Ω

1µF

330pF0.1µF

107k5.23k47.5Ω

150Ω

PTVS58VP1UTP 3.3nF

2.2nF2KV

30k

10k

2.2nF

C2, C310µF

C547µF

220pF630V

47nF100V

2.2nF2kV

BG36

LT4321

BG12TG12 TG36

TG78TG45BG45 BG78

OUTP

OUTN

EN

EN

IN12

T2

Q2 Q3

Q4 Q5

Q6 Q7

Q8 Q9

1

DATAPAIRS

SPAREPAIRS

23

6

4

57

8

IN36IN45

IN78

47nF100V

L1: WURTH 744316022L2: WURTH 744316470L4: COILCRAFT, DO1608C-104C2, C3: 10µF, 16V, MURATA GRM32DR61C106KA01J1: WURTH 7499511001A

BAV19WS

J1

5.1kΩ

10k

20Ω

••

•

•

•L2

4.7µH

+ 10µF100V

C7, C82.2µF

FMMT723

VPORT = 42.5VVPORT = 50VVPORT = 57V

LOAD CURRENT (A)0 1 2 3 4

11.94

11.95

11.96

11.97

11.98

11.99

12.00

12.01

12.02

V (OU

T) (V

)

4295 TA12c

VPORT = 42.5VVPORT = 50VVPORT = 57V

LOAD CURRENT (A)0 1 2 3 4

70

74

78

82

86

90

94

EFFI

CIEN

CY (%

)

4295 TA12b









https://www.analog.com/LT4291-1?doc=LT4295.pdf

https://www.analog.com/LTC4269-1?doc=LT4295.pdf

https://www.analog.com/LTC4269-2?doc=LT4295.pdf



https://www.analog.com/LTC4278?doc=LT4295.pdf

lt4295 (rev. b) · supply current vport, hssrc and vin supply current vvport = vhssrc = vvin = 60v...

Documents