lt3477 – 3a, dc/dc converter with dual rail-to-rail current sense · 2020. 2. 1. · 3477 g14 100...

LT3477

13477fd

For more information www.linear.com/LT3477

Typical applicaTion

DescripTion

3A, DC/DC Converter with Dual Rail-to-Rail

Current SenseFeaTures

applicaTions

n Dual 100mV Rail-to-Rail Current Sense Amplifiers n Wide Input Voltage Range: 2.5V to 25V n 3A, 42V Internal Switch n High Efficiency Power Conversion: Up to 93% n Drives LEDs in Boost, Buck-Boost or Buck Mode n Frequency Set by External Resistor: 200kHz to 3.5MHz n Programmable Soft-Start n Low VCESAT Switch: 0.3V at 2.5A n Capable of Positive and Negative Output Voltages

(Boost, Inverting, SEPIC, Flyback) n Available in Thermally Enhanced 20-Lead

(4mm × 4mm) QFN and 20-Lead TSSOP Packages

The LT®3477 is a current mode, 3A DC/DC step-up converter with dual rail-to-rail current sense amplifiers and an internal 3A, 42V switch. It combines a traditional voltage feedback loop and two unique current feedback loops to operate as a constant-current, constant-voltage source. Both cur-rent sense voltages are set at 100mV and can be adjusted independently using the IADJ1 and IADJ2 pins. Efficiency of up to 93% can be achieved in typical applications. The LT3477 features a programmable soft-start function to limit inductor current during start-up. Both inputs of the error amplifier are available externally allowing positive and negative output voltages (boost, inverting, SEPIC, Flyback). The switching frequency is programmable from 200kHz to 3.5MHz through an external resistor.

Available in thermally enhanced 20-pin (4mm × 4mm) QFN and 20-pin TSSOP packages, the LT3477 provides a complete solution for both constant-voltage and constant-current applications.

Efficiency330mA LED Driver With Open LED Protection

ISN1ISP1

VINIADJ1IADJ2

FBN

ISP2

200k3.3µF3.3µF

VIN5V

33nF4.7nF

330mA

3477 TA01a

10k

22k

1k

0.3Ω

SW

10µH

FBP SSGND

LT3477

ISN2

RT

SHDN

VC

VREF

SHDN

IOUT (A)0

EFFI

CIEN

CY (%

)

70

75

80

0.4

3477 TA01b

65

60

500.1 0.2 0.3

55

90

85

n High Power LED Driver n DSL Modems n Distributed Power n Input/Output Current Limited Boost, SEPIC,

Inverting, Flyback Converters n Constant-Voltage, Constant-Current Source

, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.All other trademarks are the property of their respective owners.

http://www.linear.com/LT3477


LT3477

23477fd


absoluTe MaxiMuM raTingsSW Pin Voltage ........................................................ 42VVIN, SHDN Pin Voltage ............................................. 25VFBP, FBN Pin Voltage ................................................. 6VVREF Pin Voltage......................................................... 6VRT, VC, SS Pin Voltage ............................................... 6VIADJ1, IADJ2 Pin Voltage ............................................ 25VISP1, ISP2, ISN1, ISN2 Pin Voltage ...............................42V

(Note 1)

Junction Temperature .......................................... 125°COperating Temperature Range (Note 2) LT3477E ...............................................– 40°C to 85°C LT3477I .............................................. –40°C to 125°CStorage Temperature Range ...................–65°C to 125°CLead Temperature (Soldering, 10 sec) TSSOP .............................................................. 300°C

FE PACKAGE20-LEAD PLASTIC TSSOP

1

2

3

4

5

6

7

8

9

10

TOP VIEW

20

19

18

17

16

15

14

13

12

11

VIN

RT

SHDN

SS

VC

FBN

FBP

VREF

IADJ2

IADJ1

NC

NC

NC

SW

SW

GND

ISN1

ISP1

ISN2

ISP2

21

TJMAX = 125°C, θJA = 40°C/W

EXPOSED PAD (PIN 21) IS PGND (MUST BE SOLDERED TO PCB)

20 19 18 17 16

6 7 8

TOP VIEW

21

UF PACKAGE20-LEAD (4mm × 4mm) PLASTIC QFN

9 10

5

4

3

2

1

11

12

13

14

15NC

NC

VIN

RT

SHDN

ISP1

ISN2

ISP2

IADJ1

IADJ2

NC SW SW GND

I SN1

SS V C FBN

FBP

V REF

TJMAX = 125°C, θJA = 37°C/W

EXPOSED PAD (PIN 21) IS PGND (MUST BE SOLDERED TO PCB)

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

LT3477EFE#PBF LT3477EFE#TRPBF 20-Lead Plastic TSSOP –40°C to 85°C

LT3477IFE#PBF LT3477IFE#TRPBF 20-Lead Plastic TSSOP –40°C to 125°C

LT3477EUF#PBF LT3477EUF#TRPBF 3477 20-Lead (4mm × 4mm) Plastic QFN –40°C to 85°C

LT3477IUF#PBF LT3477IUF#TRPBF 3477 20-Lead (4mm × 4mm) Plastic QFN –40°C to 125°C

Consult LTC Marketing for parts specified with wider operating temperature ranges.For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/. Some packages are available in 500 unit reels through designated sales channels with #TRMPBF suffix.

pin conFiguraTion

orDer inForMaTion http://www.linear.com/product/LT3477#orderinfo


http://www.linear.com/leadfree/

http://www.linear.com/tapeandreel/

http://www.linear.com/product/LT3477#orderinfo

LT3477

33477fd


elecTrical characTerisTics The l indicates specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 2.5V, VSHDN = 2.5V.PARAMETER CONDITIONS MIN TYP MAX UNITS

Minimum Input Voltage l 2.3 2.5 V

Quiescent Current VSHDN = 0V VSHDN = 2.5V, VC = 0.3V (Not Switching)

0.1 5.0

1.0 7.5

µA mA

Reference Voltage E Grade I Grade

l

l

1.216 1.210

1.235 1.235

1.250 1.260

V V

Reference Voltage Line Regulation 2.5V < VIN < 25V, VC = 0.3V 0.01 0.03 %/V

Maximum VREF Pin Current Out of Pin 100 µA

Soft-Start Pin Current SS = 0.5V, Out of Pin 9 µA

FBP Pin Bias Current 25 100 nA

FBN Pin Bias Current 25 100 nA

Feedback Amplifier Offset Voltage FBP – FBN, VC = 1V –2 2 6 mV

Feedback Amplifier Voltage Gain 500 V/V

Voltage Feedback Amplifier Transconductance 500 µS

Feedback Amplifier Sink Current VFBP = 1.25V, VFBN = 1.5V, VC = 1V 10 µA

Feedback Amplifier Source Current VFBP = 1.25V, VFBN = 1V, VC = 0.5V 10 µA

Current Sense Amplifier Sense Voltage Positive Rail, VCM = 25V, E Grade Positive Rail, VCM = 25V, I Grade Ground

l

l

97.5 97.5 88

100 100 100

102.5 103 112

mV mV mV

Switching Frequency RT = 17.2k RT = 107.4k RT = 2.44k

0.9 160 2.7

1 200 3.5

1.15 240 4.3

MHz kHz

MHz

Maximum Switch Duty Cycle RT = 17.2k l 87 93 %

Switch Current Limit (Note 3) 3 4 5 A

Switch VCESAT ISW = 1A (Note 3) 150 200 mV

Switch Leakage Current SW = 40V 0.2 5 µA

SHDN Pin Current VSHDN = 5V VSHDN = 0V

30 0.1

60 1

µA µA

SHDN Pin Threshold 0.3 1.5 2 V

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: The LT3477E is guaranteed to meet performance specifications from 0°C to 70°C. Specifications over the –40°C to 85°C operating

junction temperature range are assured by design, characterization and correlation with statistical process controls. The LT3477I is guaranteed over the full –40°C to 125°C operating junction temperature range.Note 3: Switch current limit and switch VCESAT for UF package guaranteed by design and/or correlation to static test.


LT3477

43477fd


TEMPERATURE (°C)–50

SHDN

THR

ESHO

LD (V

)

1.4

1.6

25 75

3477 G04

1.2

–25 0 50 100 1251.0

VSHDN (V)0

SHDN

PIN

CUR

RENT

(µA)

30

40

50

20

3477 G05

20

10

05 10 15

–50°C

25°C

125°C

25TEMPERATURE (°C)

–502

QUIE

SCEN

T CU

RREN

T (m

A)

3

4

5

6

–25 0 25 50

3477 G06

75 100 125 150

VC = 0.3V


0

I SS

(µA)

5

10

15

20

–25 0 25 50

3477 G07

75 100 125 150


FREQ

UENC

Y (M

Hz)

1.2

1.6

2.0

25 75 150

3477 G08

0.8

0.4

0–25 0 50 100 125

RT = 10kΩ

RT = 15kΩ

RT = 20kΩ


OFFS

ET V

OLTA

GE (m

V)

0

2

150

3477 G09

–2

–40 50 100–25 25 75 125

4

–1

1

–3

3

VC = 1V

VC = 0.5V

Typical perForMance characTerisTics

Soft-Start Pin Current Oscillator Frequency Feedback Amplifier Offset Voltage

SHDN Pin Turn-On Threshold SHDN Pin Current Quiescent Current

Switch VCE(SAT) Switch Current Limit VREF

SWITCH CURRENT (A)0

0

V CE(

SAT)

(V)

0.10

0.20

0.30

0.25

0.5 1 1.5 2

3477 G01

2.5

0.40

0.50

0.05

0.15

125°C

–50°C

0.35

0.45

3

25°C

TEMPERATURE (°C)–50 –25

0

CURR

ENT

(A)

2

5

0 50 75

3477 G02

1

4

3

25 100 125TEMPERATURE (°C)

–501.21

V REF

(V)

1.22

1.23

1.24

1.25

0 50 100 150

3477 G03

1.26

1.27

–25 25 75 125

VIN = 25V

VIN = 2.5V


LT3477

53477fd


IADJ VOLTAGE (mV)0

0

VOLT

AGE

SENS

E (m

V)

20

40

60

80

200 400 600 800

2477 G13

100

120

100 300 500 700

VCM = 10V


CURR

ENT

SENS

E VO

LTAG

E (m

V)

103

25

3477 G14

100

98

–25 0 50

97

96

104

102

101

99

75 100 125

VCM = 10V

VCM = 42V


FBP

PIN

BIAS

CUR

RENT

(nA)

30

40

50

25 75

3477 G10

20

10

–25 0 50 100 125

0

–10

“+” INDICATES THE CURRENT FLOWS OUT OF PIN


FBN

PIN

BIAS

CUR

RENT

(nA)

30

40

50

25 75

3477 G11

20

10

–25 0 50 100 125

0

–10

“+” INDICATES THE CURRENT FLOWS OUT OF PIN

Typical perForMance characTerisTics

FBP Pin Bias Current FBN Pin Bias Current

Current Sense Voltage vs IADJ

Current Sense Voltage vs Temperature


LT3477

63477fd


pin FuncTions (QFN/TSSOP)

NC (Pins 1, 2, 20/Pins 18, 19, 20): No Connect Pin. Okay to connect to ground or VIN, or to float.

VIN (Pin 3/Pin 1): Input Supply. Must be locally bypassed. Powers the internal control circuitry.

RT (Pin 4/Pin 2): Timing Resistor Pin. Adjusts the switch-ing frequency. Connect a 17.2k resistor between RT and GND for a 1MHz switching frequency. Do not leave this pin open. See Table 4 for additional RT values and switching frequencies.

SHDN (Pin 5/Pin 3): Shutdown. Tie to 2V or greater to enable the device. Tie below 0.3V to turn off the device.

SS (Pin 6/Pin 4): Soft-Start. Place a soft-start capacitor here. Leave floating if not in use.

VC (Pin 7/Pin 5): Compensation Pin for Error Amplifier. Connect a series RC from this pin to GND. Typical values are 1kΩ and 4.7nF.

FBN (Pin 8/Pin 6): The Inverting Input to the Error Ampli-fier. Connect resistive divider tap here for positive output voltage.

FBP (Pin 9/Pin 7): The Noninverting Input to the Error Amplifier. Connect resistive divider tap here for negative output voltage.

VREF (Pin 10/Pin 8): Bandgap Voltage Reference. Internally set to 1.235V. Connect this pin to FBP if generating a posi-tive output or to an external resistor divider if generating a negative voltage. This pin can provide up to 100µA of current and can be locally bypassed with a 100pF capacitor.

IADJ2 (Pin 11/Pin 9): Second Current Sense Adjustment. Setting IADJ2 to be less than 625mV leads to adjustment of the sensed voltage of the second current sense amplifier

linearly. If IADJ2 is tied to higher than 650mV, the default current sense voltage is 100mV. If current sense ampli-fier 2 is not used, always tie IADJ2 to higher than 650mV.

IADJ1 (Pin 12/Pin 10): First Current Sense Adjustment. Setting IADJ1 to be less than 625mV leads to adjustment of the sensed voltage of the first current sense amplifier linearly. If IADJ1 is tied to higher than 650mV, the default current sense voltage is 100mV. If current sense ampli-fier 1 is not used, always tie IADJ1 to higher than 650mV.

ISP2 (Pin 13/Pin 11): Second Current Sense (+) Pin. The noninverting input to the second current sense amplifier. Connect to ISN2 if not used.

ISN2 (Pin 14/Pin 12): Second Current Sense (–) Pin. The inverting input to the second current sense amplifier. Con-nect to ISP2 if not used.

ISP1 (Pin 15/Pin 13): First Current Sense (+) Pin. The noninverting input to the first current sense amplifier. Connect to ISN1 if not used.

ISN1 (Pin 16/Pin 14): First Current Sense (–) Pin. The in-verting input to the first current sense amplifier. Connect to ISP1 if not used.

GND (Pins 17/Pin 15): Ground. Tie directly to local ground plane.

SW (Pins 18, 19/Pins 16, 17): Switch Pins. Collector of the internal NPN power switch. Connect the inductor and diode here and minimize the metal trace area connected to this pin to minimize electromagnetic interference.

Exposed Pad (Pin 21/Pin 21): Power Ground. Must be soldered to PCB ground for electrical contact and rated thermal performance.


LT3477

73477fd


–

+

–

+

–

+

∑

–++

IA1

ISP1

ISN1A1

VC SWSS

A3

A4SLOPE

VADJ

IADJ1

ISP2

ISN2

FBP

FBN

SHDN

IADJ2

VREF

–

+–++

IA2

–

+

RTVIN

VA

A2VADJ

VREF1.235V

SR Q1

3477 F01

Q

OSCILLATOR

Figure 1. LT3477 Block Diagram

blocK DiagraM


LT3477

83477fd


operaTionThe LT3477 uses a fixed frequency, current mode control scheme to provide excellent line and load regulation. Op-eration can be best understood by referring to the Block Diagram in Figure 1. The start of each oscillator cycle sets the SR latch and turns on power switch Q1. The signal at the noninverting input of the PWM comparator (A4 SLOPE) is proportional to the sum of the switch current and oscillator ramp. When SLOPE exceeds VC (the output of the feedback amplifier), the PWM comparator resets the latch and turns off the power switch. In this manner, the feedback amplifier and PWM comparators set the correct peak current level to keep the output in regulation. Amplifier A3 drives A4 inverting input. A3 has three inputs, one from the voltage feedback loop and the other two from the current feedback loop. Whichever feedback input is higher takes precedence, forcing the converter into either a constant-current or a constant-voltage mode. The LT3477 is designed to transi-tion cleanly between the two modes of operation. Current sense amplifier IA1 senses the voltage between the ISP1 and ISN1 pins and provides a pre-gain to amplifier A1. When the voltage between ISP1 and ISN1 reaches 100mV, the output of IA1 provides VADJ to the inverting input of A1 and the converter is in constant-current mode. If the current sense voltage exceeds 100mV, the output of IA1 will increase causing the output of A3 to decrease, thus reducing the amount of current delivered to the output.

In this manner the current sense voltage is regulated to 100mV. The current sense level is also pin adjustable by IADJ1. Forcing IADJ1 to less than 625mV will overwrite VADJ voltage that’s set internally, thus providing current level control. The second current sense amplifier, IA2, works the same as the first current sense amplifier IA1. Both current sense amplifiers provide rail-to-rail current sense operation. Similarly, for positive output voltage operation where FBP is tied to VREF, if the FBN pin increases above VREF, the output of A3 will decrease to reduce the peak current level and regulate the output (constant-voltage mode). For negative output voltage operation where FBN is tied to GND, if the FBP pin decreases below GND level, the output of A3 will decrease to reduce the peak current level and regulate the output (constant-voltage mode).

The LT3477 also features a soft-start function. During start-up, 9µA of current charges the external soft-start capacitor. The SS pin directly limits the rate of voltage rise on the VC pin, which in turn limits the peak switch cur-rent. The switch current is constantly monitored and not allowed to exceed the nominal value of 3A. If the switch current reaches 3A, the SR latch is reset regardless of the output of the PWM comparator. Current limit protects the power switch and external components.


LT3477

93477fd


applicaTions inForMaTionCapacitor Selection

Low ESR (equivalent series resistance) ceramic capaci-tors should be used at the output to minimize the output ripple voltage. Use only X5R or X7R dielectrics, as these materials retain their capacitance over wider voltage and temperature ranges better than other dielectrics. A 4.7µF to 10µF output capacitor is sufficient for most high output current designs. Converters with lower output currents may need only a 1µF or 2.2µF output capacitor.

Table 1. Ceramic Capacitor ManufacturersMANUFACTURER PHONE WEB

Taiyo Yuden (408) 573-4150 www.t-yuden.com

AVX (803) 448-9411 www.avxcorp.com

Murata (714) 852-2001 www.murata.com

TDK (847) 803-6100 www.component.tdk.com

Inductor Selection

Several inductors that work well with the LT3477 are listed in Table 2. However, there are many other manufacturers and devices that can be used. Consult each manufacturer for more detailed information and their entire range of parts. Ferrite core inductors should be used to obtain the best efficiency. Choose an inductor that can handle the necessary peak current without saturating, and ensure that the inductor has a low DCR (copper-wire resistance) to minimize I2R power losses. A 4.7µH or 10µH inductor will suffice for most LT3477 applications.

Inductor manufacturers specify the maximum current rating as the current where the inductance falls to some

percentage of its nominal value—typically 65%. An inductor can pass a current larger than its rated value without damaging it. Aggressive designs where board space is precious will exceed the maximum current rat-ing of the inductor to save board space. Consult each manufacturer to determine how the maximum inductor current is measured and how much more current the inductor can reliably conduct.

Diode Selection

Schottky diodes, with their low forward voltage drop and fast switching speed, are ideal for LT3477 applications. Table 3 lists several Schottky diodes that work well with the LT3477. The diode’s average current rating must exceed the average output current. The diode’s maximum reverse voltage must exceed the output voltage. The diode conducts current only when the power switch is turned off (typically less than 50% duty cycle), so a 3A diode is sufficient for most designs. The companies below also offer Schottky diodes with higher voltage and current ratings.

Table 3. Suggested DiodesMANUFACTURER PART NUMBER

MAX CURRENT (A)

MAX REVERSE VOLTAGE (V)

MANUFACTURER

UPS340 UPS315

n 3 3

n 40 15

Microsemi www.microsemi.com

B220 B230 B240 B320 B330 B340 SBM340

n 2 2 2 3 3 3 3

n 20 30 40 20 30 40 40

Diodes, Inc www.diodes.com

Table 2. Suggested InductorsMANUFACTURER PART NUMBER

IDC (A)

INDUCTANCE (µH)

MAX DCR (mΩ)

L × W × H (mm)

MANUFACTURER

CDRH6D283R0 CDRH6D28100 CDRH4D284R7

3 1.7 1.32

3 10 4.7

24 65 72

6.7 × 6.7 × 3.0 6.7 × 6.7 × 3.0 5.0 × 5.0 × 3.0

Sumida www.sumida.com

LM N 05D B4R7M LM N 05D B100K

2.2 1.6

4.7 10

49 10

5.9 × 6.1 × 2.8 5.9 × 6.1 × 2.8

Taiyo Yuden www.t-yuden.com

LQH55DN4R7M01L LQH55DN100M01K

2.7 1.7

4.7 10

57 130

5.7 × 5.0 × 4.7 5.7 × 5.0 × 4.7

Murata www.murata.com

FDV0630-4R7M 4.2 4.7 49 7.0 × 7.7 × 3.0 Toko www.toko.com


LT3477

103477fd


applicaTions inForMaTionSetting Positive Output VoltagesTo set a positive output voltage, select the values of R1 and R2 (see Figure 2) according to the following equation:

VOUT = 1.235V 1+ R1

R2⎛

⎝⎜

⎞

⎠⎟

For designs needing an adjustable current level, the IADJ1 and IADJ2 pins are provided for the first and the second current sense amplifiers, respectively. With the IADJ1 and IADJ2 pins tied higher than 650mV, the nominal current sense voltage is 100mV (appearing between the ISP1 and ISN2 or ISP2 and ISN2 pins). Applying a positive DC voltage less than 600mV to the IADJ1 and IADJ2 pins will decrease the current sense voltage according to the following formula:

ISENSE =

100mVRSENSE

• VADJ618mV

For example, if 309mV is applied to the IADJ1 pin and RSENSE is 0.5Ω, the current sense will be reduced from 200mA to 100mA. The adjustability allows the regulated current to be reduced without changing the current sense resistor (e.g., to adjust brightness in an LED driver or to reduce the charge current in a battery charger).

Considerations When Sensing Input Current

In addition to regulating the DC output current for current-source applications, the constant-current loop of the LT3477 can also be used to provide an accurate input current limit. Boost converters cannot provide output short-circuit protection, but the surge turn-on current can be drastically reduced using the LT3477 current sense at the input. SEPICs, however, have an output that is DC-isolated from the input, so an input current limit not only helps soft-start the output but also provides excellent short-circuit protection.

When sensing input current, the sense resistor should be placed in front of the inductor (between the decoupling capacitor and the inductor). This will regulate the average inductor current and maintain a consistent inductor ripple current, which will, in turn, maintain a well regulated input current. Do not place the sense resistor between the input source and the input decoupling capacitor, as this may allow the inductor ripple current to vary widely (even though the average input current and the average inductor current will still be regulated). Since the inductor current is a triangular waveform (not a DC waveform like the output current) some tweaking of the compensation values (RC and CC

Figure 2. Positive Output Voltage Feedback Connections

R4

R3

3477 F03

FBP

VREF

–VOUT

LT3477

FBN

Figure 3. Negative Output Voltage Feedback Connections

R2

R1

3477 F02

FBN

VREF

VOUTLT3477

FBP

Setting Negative Output Voltages

To set a negative output voltage, select the values of R3 and R4 (see Figure 3) according to the following equation:

VOUT = 1.235V 1+R3

R4⎛

⎝⎜

⎞

⎠⎟

Selecting RSENSE/Current Sense Adjustment

Using the following formula to choose the correct cur-rent sense resistor value (for constant current or fail-safe operation).

RSENSE =

100mVISENSE


LT3477

113477fd


on the VC pin) may be required to ensure a clean inductor ripple current while the constant-current loop is in effect. For these applications, the constant-current loop response can usually be improved by reducing the RC value or by adding a capacitor (with a value of approximately CC/10) in parallel with the RC and CC compensation network.

Frequency Compensation

The LT3477 has an external compensation pin (VC), which allows the loop response to be optimized for each applica-tion. An external resistor and capacitor (or sometimes just a capacitor) are placed at the VC pin to provide a pole and a zero (or just a pole) to ensure proper loop compensation. Several other poles and zeroes are present in the closed-loop transfer function of a switching regulator, so the VC pin pole and zero are positioned to provide the best loop response. A thorough analysis of the switching regulator control loop is not within the scope of this data sheet, and will not be presented here, but values of 1k and 4.7nF will be a good choice for many designs. For those wishing to optimize the compensation, use the 1k and 4.7nF as a starting point.

Board Layout

As with all switching regulators, careful attention must be paid to the PCB board layout and component place-ment. To maximize efficiency, switch rise and fall times are made as short as possible. To prevent radiation and high frequency resonance problems, proper layout of the high frequency switching path is essential. Minimize the length and area of all traces connected to the SW pin and always use a ground plane under the switching regulator to minimize interplane coupling. The signal path including the switch, output diode D1 and output capacitor COUT, contains nanosecond rise and fall times and should be kept as short as possible.

Soft-Start

For many applications, it is necessary to minimize the inrush current at start-up. The built-in soft-start circuit significantly reduces the start-up current spike and out-put voltage overshoot. A typical value for the soft-start capacitor is 10nF.

Switching Frequency

The switching frequency of the LT3477 is set by an ex-ternal resistor attached to the RT pin. Do not leave this pin open. A resistor must always be connected for proper operation. See Table 4 and Figure 4 for resistor values and corresponding frequencies.

Increasing switching frequency reduces output voltage ripple but also reduces efficiency. The user should set the frequency for the maximum tolerable output voltage ripple.

Figure 4. Switch Frequency

RT (kΩ)

0

1.5

1.0

0.5

3.5

3.0

2.5

2.0

3477 F04

SWIT

CH F

REQU

ENCY

(MHz

)

0.1 10010

applicaTions inForMaTion

Table 4. Switching FrequencySWITCHING FREQUENCY (MHz) RT (kΩ)

n 3.5 n 2.43 n 3 n 3.65 n 2.5 n 4.87 n 2 n 6.81 n 1.5 n 10.2 n 1 n 17.4 n 0.5 n 43.2 n 0.2 n 107


LT3477

123477fd


applicaTions inForMaTionPWM Dimming

For LED applications where a wide dimming range is required, two competing methods are available: analog dimming and PWM dimming. The easiest method is to simply vary the DC current through the LED—analog dimming—but changing LED current also changes its chromaticity, undesirable in many applications. The bet-ter method is PWM dimming, which switches the LED on and off, using the duty cycle to control the average current. PWM dimming offers several advantages over analog dimming and is the method preferred by LED manufacturers. By modulating the duty cycle of the PWM signal, the average LED current changes proportionally as illustrated in Figure 5. The chromaticity of the LEDs remains unchanged in this scheme since the LED current is either zero or at programmed current. Another advantage of PWM dimming over analog dimming is that a wider dimming range is possible.

The LT3477 is a DC/DC converter that is ideally suited for LED applications. For the LT3477, analog dimming offers a dimming ratio of about 10:1; whereas, PWM dimming with the addition of a few external components results in a wider dimming range of 500:1. The technique requires a PWM logic signal applied to the gate of both NMOS (refer to Figure 7). When the PWM signal is taken high the part runs in normal operation and ILED = 100mV/RSENSE runs

through the LEDs. When the PWM input is taken low, the LEDs are disconnected and turn off. This unique external circuitry produces a fast rise time for the LED current, resulting in a wide dimming range of 500:1 at a PWM frequency of 100Hz.

The LED current can be controlled by feeding a PWM signal with a broad range of frequencies. Dimming below 80Hz is possible, but not desirable, due to perceptible flashing of LEDs at lower PWM frequencies. The LED current can be controlled at higher frequencies, but the dimming range decreases with increasing PWM frequency, as seen in Figure 6.

PWM dimming can be used in boost (shown in Figure 7), buck mode (shown in Figure 8) and buck-boost mode (shown in Figure 9). For the typical boost topology, ef-ficiency exceeds 80%. Buck mode can be used to increase the power handling capability for higher current LED applications. A buck-boost LED driver works best in ap-plications where the input voltage fluctuates to higher or lower than the total LED voltage drop.

In high temperature applications, the leakage of the Schottky diode D1 increases, which in turn, discharges the output capacitor during the PWM off time. This results in a smaller effective LED dimming ratio. Consequently, the dimming range decreases to about 200:1 at 85°C.

Figure 5. LED Current vs PWM Duty Cycle Wide Dimming Range (500:1)

Figure 6. Dimming Range vs PWM Frequency

PWM DUTY CYCLE (%)

0.1

LED

CURR

ENT

(mA)

1

10

100

1 10 100

3477 F05

0.010.1

VIN = 5VBOOST 4 LEDsPWM FREQUENCY = 100Hz

RT = 6.81k

PWM FREQUENCY (kHz)

0.11

DIM

MIN

G RA

NGE:

1 100

1000

1 10 100

3477 F06

10

RT = 6.81k


LT3477

133477fd



Figure 7c. Efficiency and LED Currentvs PWM Duty Cycle

Figure 7b. PWM Dimming Waveforms

10µs/DIV

ILED200mA/DIV

IL1A/DIV

PWM5V/DIV

3477 F07b

VIN = 5V4 LEDs300mA

PWM FREQ = 100HzBOOST

PWM DUTY CYCLE (%)0

75

80

85

80

3477 F07c

70

65

20 40 60 100

60

55

50

250

300

350

200

150

100

50

0

EFFI

CIEN

CY (%

)

VIN = 5VBOOST 4 LEDs, 300mAPWM FREQUENCY = 100Hz

EFFICIENCY

LED CURRENT

ISN1ISP1

VIN

IADJ2

IADJ1

FBN

ISP2

FBP

VREF

1MC210µF

C13.3µF

VIN5V

C1: TAIYO YUDEN EMK316BJ335MLC2: TAIYO YUDEN UDK325BJ106MML1: TOKO D53LC (PN# A915AY-2ROM)D1: ZETEX ZLLS1000D2: DIODES INC 1N4148NMOS1: ZETEX 2N7002NMOS2: FAIRCHILD FDG327NLED1 TO LED4: LUMILEDS LXHL-BW02

CC10nF

CSS33nF

300mA

LED1

LED2

LED3

LED4

3477 F07a

75k

6.81k

NMOS1

NMOS2

OUT

100k

0

5V

100Hz

PWM

RC2.4k

RSENSE0.33Ω

SW

L12.0µH D1

D2

SS

GND

LT3477

ISN2

RT

SHDN

VC

Figure 7a. 5V to 4 White LEDs: Boost With PWM Dimming


LT3477

143477fd



Figure 8a. 32V to 6 White LEDs: Buck Mode With PWM Dimming


2ms/DIV

ILED500mA/DIV

IL500mA/DIV

PWM5V/DIV

3477 F08b

PVIN = 32V6 LEDs300mA

PWM FREQUENCY = 100HzBUCK MODE

ISN1ISP1

VIN

IADJ2

IADJ1

FBN

ISP2

FBP

VREF

C33.3µF

C12.2µF

VIN3.3V

C1: NIPPON NTS40X5R1H225MC2: TAIYO YUDEN GMK316BJ105MLC3: TAIYO YUDEN LMK316BJ335KLL1: TOKO D53LC (PN# A915AY-100M)D1: ZETEX ZLLS400D2: DIODES INC 1N4148NMOS1, NM0S2: ZETEX 2N7002PMOS: SILICONIX Si2303BDSLED1 TO LED6: LUMILEDS LXHL-BW02

CC0.1µF

CSS33nF

300mA

PMOS

3477 F08a

6.81k

NMOS1

NMOS2

100k

0

5V

100Hz

PWM

280k

10k

SW

D2

SS

GND

LT3477

ISN2

RT

SHDN

VC

RSENSE0.33Ω

PVIN32V

L110µH

LED1

LED6

C21µF

D1

1k 1k

•••

PWM


LT3477

153477fd




Figure 9a. 10V to 2 White LEDs: Buck-Boost Mode With PWM Dimming

2ms/DIV

ILED500mA/DIV

IL1A/DIV

PWM10V/DIV

3477 F09b

VIN = 10V2 LEDs300mA

PWM FREQUENCY = 100HzBUCK-BOOST MODE

1M

1k

1k

C210µF

C13.3µF

VIN10V

300mA

49.9k

L14.7µH D1

ISN1ISP1

VIN

IADJ2

IADJ1

FBN

ISP2

FBP

VREF

C1: TAIYO YUDEN LMK316BJ335MLC2: TAIYO YUDEN UDK325BJ106MML1: TOKO D53LC (PN# A915AY-4R7M)D1: ZETEX ZLLS1000D2: DIODES INC 1N4148NMOS1, NMOS2: ZETEX 2N7002PMOS: SILICONIX Si2303BDSLED1, LED2: LUMILEDS LXHL-BW02

CC10nF

CSS33nF

3477 F09a

6.81k

NMOS1

100k

0

5V

100Hz

PWM

SW

D2

SS

GND

LT3477

ISN2

RT

SHDN

VC

RC1.5k

RSENSE0.33Ω

LED2 LED1

PWMNMOS2

PMOS


LT3477

163477fd


Typical applicaTions

800mA, 5V to 12V Boost Converter With Accurate Input Current Limit Efficiency

ISN1ISP1

VINIADJ1IADJ2 ISP2

ISN2

FBN

R3200k

R423.2k

C12.2µF

VIN5V

C310nF

C1: TAIYO YUDEN LMK316BJ225MDC2: AVX 1206YD106MATD1: DIODES INC. B320AL1: TOKO FDV0630-4R7M

C210µF

12V0.8A

3477 TA04a

R217.8k

SW

L14.7µH

R10.033Ω D1

FBP SSGND

LT3477

RT

VC

SHDN

VREF

SHDN

CC4.7nF

RC1k

IOUT (A)0

EFFI

CIEN

CY (%

)

70

75

80

0.4 0.5 0.6 0.7 0.8

3477 TA04b

65

60

500.1 0.2 0.3

55

90

85

5.5V SEPIC Converter With Short-Circuit Protection Efficiency

ISN1ISP1

VINIADJ1IADJ2 FBN

ISP2

R434.8k

R30.15Ω

R10.04Ω

R510k

C13.3µF

VIN3V TO

16V

C433nF

C1: TAIYO YUDEN LMK316BJ335MLC2: TAIYO YUDEN LMK325BJ106MNC3: TAIYO YUDEN LMK316BJ106ZLD1: DIODES INC. DFLS130LL1, L2: TOKO FDV0630-4R7M

C310µF

CC4.7nF

5.5V670mA

3477 TA02a

R218.2k

RC1k

SW

L14.7µH

C210µF

L24.7µH

D1

FBP SSGND

LT3477

ISN2

RT

SHDN

VC

VREF

SHDN

IOUT (A)0

EFFI

CIEN

CY (%

)

85

0.3

3477 TA02b

70

60

0.1 0.2 0.4

55

50

90

80

75

65

0.5 0.6 0.7

VIN = 3V


LT3477

173477fd


Typical applicaTions87% Efficient, 4W LED Driver

ISN1ISP1

VINIADJ1IADJ2

FBN

ISP2

R2200k

C23.3µF

C13.3µF

VIN5V

C333nF

C1: TAIYO YUDEN LMK316BJ335MLC2: TAIYO YUDEN TMK325BJ335MND1: DIODES INC. DFLS120LL1: TOKO A915AY-100M

330mA

LED1

LED2

LED3

LED4

3477 TA03a

R110k

R322k

R60.3Ω

SW

L210µH

R40.05Ω D1

FBP SSGND

LT3477

ISN2

RT

SHDN

VC

VREF

SHDN

CC4.7nF

RC1k

Efficiency

1A Buck Mode High Current LED Driver

Efficiency

IOUT (A)0

EFFI

CIEN

CY (%

)

70

75

80

0.4

3477 TA01b

65

60

500.1 0.2 0.3

55

90

85

ISN1ISP1

VINIADJ1IADJ2

FBN

ISP2

R3280k

R410k

C12.2µF

R10.1Ω

C33.3µF

PVIN32V

VIN3.3V

C433nF

C1: NIPPON UNITED CHEMICON NTS40X5R1H225MC2: TAIYO YUDEN GMK316BJ105MLC3: TAIYO YUDEN LMK316BJ475L1: TOKO A814AY-330MD1: DIODES INC DFLS140

3477 TA05a

R222k

SW

L133µH

1A LEDSTRING C2

1µF

D1

LED4

LED1

•••

FBP SSGND

LT3477

ISN2

RT

SHDN

VC

VREF

SHDN

CC4.7nF

RC1k

LED CURRENT (A)0

EFFI

CIEN

CY (%

)

50

60

70

0.6 1

3477 TA05b

40

30

200.2 0.4 0.8

80

90

100


LT3477

183477fd


Typical applicaTions

ISN1ISP1

VINIADJ1IADJ2

FBN

ISP2

R3200k

R10.1Ω

R410k

C13.3µF

LED BRIGHTNESSCONTROL

0mV TO 650mV

VIN2.7V TO 10V

C333nF

C24.7µF

CC10nF

C1: TAIYO YUDEN LMK316BJ335MLC2: MURATA GRM31CR71E475KA88LD1: DIODES, INC. B320AL1: TOKO FDV0630-4R7M

3477 TA06a

R218k

SW1

L14.7µH D1

LED1

FBP SSGND

LT3477

ISN2

RT

SHDN

VC

VREF

SHDN

LED2

Buck-Boost Mode LED Driver

Efficiency

IOUT (A)0

EFFI

CIEN

CY (%

)

65

70

75

0.6 1.0

3477 TA06b

60

55

500.2 0.4 0.8

80

85

90

VIN = 8V

VIN = 4.2V

VIN (V) IOUT (A)

2.7 0.57 3.6 0.74 4.2 0.83 5 0.93 8 1.0


LT3477

193477fd


pacKage DescripTion

4.00 ±0.10

4.00 ±0.10

NOTE:1. DRAWING IS PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WGGD-1)—TO BE APPROVED2. 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

2019

1

2

BOTTOM VIEW—EXPOSED PAD

2.00 REF2.45 ±0.10

0.75 ±0.05 R = 0.115TYP

R = 0.05TYP

0.25 ±0.05

0.50 BSC

0.200 REF

0.00 – 0.05

(UF20) QFN 01-07 REV A

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

0.70 ±0.05

0.25 ±0.050.50 BSC

2.00 REF 2.45 ±0.05

3.10 ±0.05

4.50 ±0.05

PACKAGE OUTLINE

PIN 1 NOTCHR = 0.20 TYPOR 0.35 × 45°CHAMFER

2.45 ±0.10

2.45 ±0.05

UF Package20-Lead Plastic QFN (4mm × 4mm)

(Reference LTC DWG # 05-08-1710 Rev A)

Please refer to http://www.linear.com/product/LT3477#packaging for the most recent package drawings.


http://www.linear.com/product/LT3477#packaging

LT3477

203477fd


pacKage DescripTion

FE20 (CB) TSSOP REV L 0117

0.09 – 0.20(.0035 – .0079)

0° – 8°

0.25REF

RECOMMENDED SOLDER PAD LAYOUT

0.50 – 0.75(.020 – .030)

4.30 – 4.50*(.169 – .177)

1 3 4 5 6 7 8 9 10

DETAIL A

DETAIL A IS THE PART OFTHE LEAD FRAME FEATURE

FOR REFERENCE ONLYNO MEASUREMENT PURPOSE

111214 13

6.40 – 6.60*(.252 – .260)

3.86(.152)

2.74(.108)

20 1918 17 16 15

1.20(.047)MAX

0.05 – 0.15(.002 – .006)

0.65(.0256)

BSC0.195 – 0.30

(.0077 – .0118)TYP

2

2.74(.108)

0.45 ±0.05

0.65 BSC

4.50 ±0.10

6.60 ±0.10

1.05 ±0.10

3.86(.152)

MILLIMETERS(INCHES) *DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH

SHALL NOT EXCEED 0.150mm (.006") PER SIDE

NOTE:1. CONTROLLING DIMENSION: MILLIMETERS

2. DIMENSIONS ARE IN

3. DRAWING NOT TO SCALE

SEE NOTE 4

4. RECOMMENDED MINIMUM PCB METAL SIZE FOR EXPOSED PAD ATTACHMENT

6.40(.252)BSC

FE Package20-Lead Plastic TSSOP (4.4mm)

(Reference LTC DWG # 05-08-1663 Rev L)Exposed Pad Variation CB

DETAIL A

0.60(.024)REF0.28

(.011)REF

Please refer to http://www.linear.com/product/LT3477#packaging for the most recent package drawings.


http://www.linear.com/product/LT3477#packaging

LT3477

213477fd


Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.

revision hisToryREV DATE DESCRIPTION PAGE NUMBER

D 03/17 Clarified efficiency in DescriptionClarified VREF in Block Diagram

16

(Revision history begins at Rev D)


LT3477

223477fd


Linear Technology Corporation1630 McCarthy Blvd., Milpitas, CA 95035-7417

LINEAR TECHNOLOGY CORPORATION 2005

LT 0317 REV D • PRINTED IN USA

(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com/LT3477

relaTeD parTs

Typical applicaTion

PART NUMBER DESCRIPTION COMMENTS

LT1618 Constant Current, Constant Voltage 1.4MHz, High Efficiency Boost Regulator

VIN: 1.6V to 18V, VOUT(MAX) = 5.5V, IQ = 2.5mA, ISD < 1µA, QFN16 Package

LT3436 3A (ISW), 800kHz, 34V Step-Up DC/DC Converter VIN: 3V to 25V, VOUT(MAX) = 34V, IQ = 0.9mA, ISD < 6µA, TSSOP16E Package

LTC®3453 Synchronous Buck-Boost High Power White LED Driver

VIN: 2.7V to 5.5V, VOUT(MAX) = 5.5V, IQ = 2.5mA, ISD < 1µA, QFN16 Package

LT3466 Dual Constant Current, 2MHz, High Efficiency White LED Boost Regulator With Integrated Schottky Diode

VIN: 2.7V to 24V, VOUT(MAX) = 40V, IQ = 5mA, ISD < 16µA, DFN Package

LT3479 3A, 42V Full Featured Boost/Inverter Converter With Soft-Start

VIN: 2.5V to 24V, VOUT(MAX) = 40V, IQ = 5mA, ISD < 1µA, DFN/TSSOP Packages

LTC3490 Single Cell 350mA, 1.3MHz LED Driver VIN: 1V to 3.2V, VOUT(MAX) = 4.7V, ISD < 1µA, DFN/SO8 Packages

Buck Mode High Current LED Driver

Efficiency

LED CURRENT (A)0

EFFI

CIEN

CY (%

)

50

60

70

0.6 1

3477 TA05b

40

30

200.2 0.4 0.8

80

90

100

ISN1ISP1

VINIADJ1IADJ2

FBN

ISP2

R3280k

R410k

C12.2µF

R10.1Ω

C33.3µF

PVIN32V

VIN3.3V

C433nF

C1: NIPPON UNITED CHEMICON NTS40X5R1H225MC2: TAIYO YUDEN GMK316BJ105MLC3: TAIYO YUDEN LMK316BJ475L1: TOKO A814AY-330MD1: DIODES INC DFLS140

CC4.7nF

3477 TA07

R222k

RC1k

SW

L133µH

1A LEDSTRING C2

1µF

D1

LED4

LED1

•••

FBP SSGND

LT3477

ISN2

RT

SHDN

VC

VREF

SHDN





http://www.linear.com/LTC3453



http://www.linear.com/LTC3490

lt3477 – 3a, dc/dc converter with dual rail-to-rail current sense · 2020. 2. 1. · 3477 g14 100...

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