30/05/2012 - 1 ATLCE - G2 - © 2011 DDC

Politecnico di Torino - ICT School

Analog and Telecommunication Electronics

G2 - Linear voltage regulators

» Shunt regulators» Series regulator» Integrated regulators» LDO

30/05/2012 - 2 ATLCE - G2 - © 2011 DDC

Lesson G2: Linear voltage regulators

• Shunt regulator– Zener diode– Voltage reference

• Series regulator– Transistor and Op Amp circuits– Current limit

• Integrated regulators– Low DropOut (LDO) regulators

• References: book1 (Sedra): Ch 3.5, 3.4

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Unregulated output voltage

• With the transf.-diode-C scheme, the output voltage– Has ripple, related with output current– Changes when load current changes– Changes when the mains voltage changes

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Output voltage regulation

• A voltage regulator can deliver constant VO

– When load current changes: load regulation SL = ΔVOΔIL» Equivalent output resistance Ro = SI

– When input voltage changes: in/out regulation Si = ΔVO/ΔVi– Reduce the ripple (is considered as a change of Vi)

ViVo

+

LVOLTAGE

REGULATOR

IL

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Shunt and series regulation

• Goal of regulator constant Vo, for changes in Vi , L

• Two basic techniques

– Build a current divider, change the partition ratiousing the parallel branch Rp

shunt regulators

– Build a voltage divider, change the partition ratiousing the series branch Rs

series regulators

RPVi

Vo

+L

RS

Vi

Vo+

L

RS

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Shunt regulator

• Basic shunt regulator: the zener diode– Very simple– Low efficiency, suitable for low power– Voltage reference circuits are shunt regulators

(low current capability)

• Current in the zener diode– Higher than Izmin (5 mA)– Less that Izmax

(to limit power dissipation)

• Constant input current– A benefit for some applications V

z

Vdc

Rpol

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Example 1: zener regulator

• Design a shunt regulator from these specs:– Vi 10 20 V– Vo = 5 V (as close as possible); – Io = 0 100 mA

• Available zener diode:– Vzo = 5 V, Rz = 10 ohm, Izmin = 5 mA– Pdmax = 2 W

• Evaluate– Drop resistance R (min/max)– Vo min/max (any combination of Vi, Io, R)– Pdmax on R and Zener– Discuss selection Rmin/max

VoVi

R

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Example 1: results

• Drop resistance R (min/max)– Rmin = Rmax =

• Vo min/max (any combination of Vi, Io, R)– Vomin = (Vi = Vimin, Io = Iomax, R = Rmax)– Vomax = (Vi = Vimax, Io = Iomin, R = Rmin)

• Pdmax on R and Zener– PdRmax = PdZmax =

• Discuss selection Rmin/max– Low values for R increase the current in the R-Dz, causing high

power dissipation– High values give lower dissipation, but could limit the max out

current capability

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Series regulators

• Need a “controlled variable resistor”– Can use BJT or MOS– Current amplifier from a reference or shunt regulator– Feedback circuits (voltage reference and Op Amp)

• Always Vo < Vi– All regulators require a minimum drop voltage (Vdrop)

» losses on regulator; low efficiency– LDO (Low Drop Out) regulators for better efficiency

• Available as commodity ICs– Standard voltages (5, 6, 9, 12, …)– Variable voltage

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• The series regulator operates as a controlled variable resistor

– The variable R is a BJT or MOS transistor– The controller CNT compares output Vo with a reference Vr

RLVi Vo

+

Series regulator – basic scheme

R

VoCNTRLVi

Vr

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Example of series voltage regulator

• Basic R-Zener regulator– Regulation by current steering between Zener and load– Limited current capability (Izmax)

• Add Emitter follower (CC) as current amplifier

– Output voltageVo = Vz – Vbe same Sv

– Allows large load current change ΔIo = ΔIz β lower Ro

– Lower bound on Ro from gm (hie)

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Feedback voltage regulator

• Control circuitry includes a reference and an amplifier

• Compare (a fraction β) the output voltage Vo with the reference Vr

• Drive the control element to keep

β Vo = Vr

• Needs a voltage reference

Vr

VS1

VO

IO

βVU

β

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Voltage reference circuits

• Provide a known and stable voltage– Independent from Power Supply, temperature, aging, load, …– Required for voltage regulators and measurement circuits– Use Zener diode or Band-gap reference– The amplifier isolates the load from the reference

VREF

Vdc

Rpol

Vz

Vdc

Rpol

REF+ +

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Example of band-gap voltage reference

• Zener diode– Zener voltage changes with temperature (inversion at about 6V)– Zener pair to compensate temperature changes– Need voltages higher than 9 V– Not the best choice for high precision and stability

• Band-gap reference– Uses combination of Vbe and VT + Op Amp– Can operate from low voltages – A 3-pin regulator with low current but precise and stable– Integrated within 3-pin regulators

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Op Amp and BJT

• The BJT is connected as a CC power stage– The feedback loop makes β VO = Vref– Drop from Vi to Vo: Vao + Vbe

VI VO

R2

VREF

βVU

R1

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Op Amp and BJT – Vo < Vref

• Voltage divider on Vref– The feedback loop makes VO = β Vref– Drop from Vi to Vo: Vao + Vbe

VI VO

R2βVREF

R1

VREF

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Op Amp and Darlington pair

• A darlington pair has higher current gain– Drop from Vi to Vo: Vao + 2 Vbe

VIR2

R1

VREF

VO

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Current limit

• Current sense resistor Rs– As Vs rises above 0,7V,

Q3 steers current away from Q1, Q2.

• External current sense – at least 4 pins

• Variable current limit– Current sense

differential amplifier– Variable threshold

• External V sense/divider– One additional pin (tot 5)

R2

R1

RS

RL

Vref

Vs

Vo

Vi Q1

Q2

Q3

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Load current sense

• High side– Sense inside the regulator– Needs differential amplifier (matched components)

» Input common mode close to supply

• Low side– Referred to GND, no need for differential amplifier– Load not directly to GND– Additional pins on regulator

• High side with INA circuits– Differential amplifier for high side current sense– No need for matched components

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INA differential amplifier

• Precise differential amplifiers need matched resistors

• INA circuit: used to sense low differential voltage close to supply

• No need for matched components

• Can handle common mode voltage near +Vi

VS(R2/R1)

RS R1

VI

VS

R2

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Efficiency of series linear regulators

• Losses caused by voltage drop and leakage current.– Usually leakage current much less than output current

• To improve efficiency

– Reduce Voltage drop Low Drop-Out (LDO)

– Reduce leakage current IQ Low power Vr and control

– Move to switching regulators

IN

OUT

ININ

LEAKINOUT

IN

OUTV

VIV

IIVP

P

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Drop-out voltage on regulator

• Transistors in a series regulator requires Vce margin to operate in active region

– Vi > Vo+VBE1+VBE2

– A power BJT has VBE = 1V, therefore Vi > Vo + 1,7V

• The Op Amp adds further internal drop VAO from positive supply to output (from 1V to a few mV):

– Vi > Vo + 1,7V +VAO = Vo + VHR, – the headroom voltage VHR represents the minimum drop-out

voltage on the series regulator– Usually VHR is about 2-3V

• The power dissipated on regulator depends on headroom voltage: Pd = VHR x Iomax

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Standard regulator

• Drop-out is related with– Op Amp output voltage range:

rail-to-rail Op Amps provide Base drive Vb = Vs

– Power transistor VBE.A minimum about 1V.

• Using MOS does not reduce drop-out

– VGSON > VBE

– charge pump for gate driving (Ig = 0)

Vi

R2

R1

Vref

Vu

Vi

R2

R1

Vref

Vu

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Low DrOpout (LDO) with pnp BJT

• Output stage becomes CE – No longer a voltage follower, amplifier with gain– Keep negative feedback (switch +/- Op Amp pins)– Gain depends on load, critical stability– Analyze the loop:

» Op Amp, CE stage, feedback network

• Using LDO– Manufacturers specify type and

value of output capacitor– Keep value and type of capacitor,

to get the required ESR!

Vi

R2

R1

Vref

ZLVu

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Issues with LDO regulators

• PNP have lower current gain (β)– The Op-Amp must sink a high current, related with the load– High leakage, decreased regulator efficiency– Not best for variable output voltage regulators– Darlington to increase the current gain, no longer LDO

• Good design rules– Input capacitor C1

to compensate wire inductance

– Diode to protect fromfast switch of at input (may cause Vo > Vi)

VoVi C1 C2

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Remote sense: four-wire

• Output current causes voltage drop on power wires– Use separate sense wires connected directly to the load– In sense wires no current, therefore no voltage drop

VO

VsVi

RL

ILVD1

VD2

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EMI sensitivity

• Remote sense wires can pick EM interference and noise (e.g. from 50Hz mains).

• Use differential signals and twisted pair for remote sense

VoViRL

Vs

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Variable Vo from 3-pin regulators

• The 3-pin regulator keeps a constant preset voltage Vpbetween OUT and COMMON pins.

– If voltage of common pin is Vc = βVo (rather than GND), we get Vo = Vp + Vc

– Vc comes from Vo through a voltage divider R1/R2– Vc = βVo = Vo R2/(R1 + R2)– Vo = Vp R1/(R1 + R2)

• The quiescent current IQadds a (small) drop on R2

– IR2 =IQ +Vreg /R1– Vo =Vp + R2·IR2

– Vo =Vp R1/(R1+R2) + IQ·R2

VoViIQ

R1

R2

Vp

Vc

IN

COMMON

OUT

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Commercial voltage regulator

• Power supply regulators are available as standard ICs

• Linear regulators: the 78xx family (positive)– Xx = output voltage– High power package– Max current related with case

» 50 mA … 5 A

• Other devices– 79xx family: negative– 317 family: variable output– LM9076 LDO + shutdown + delayed reset

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78xx data sheet

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78xx data sheet

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LDO regulator with commands

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Numeric example

• Goal: 5V, 1A PSU with linear regulator– Draw complete block diagram without and with regulator– Define specs for unregulated voltage (e.g: 8Vdc, 1Vr)– Evaluate parameters for transformer, diodes, capacitor– Evaluate Sv and Ro

• Add zener regulator; – Define required zener parameters– Evaluate Sv and Ro

• Add power transistor (CC)– Evaluate Sv and Ro

• Select suitable 3-pin regulator– Compare Sv and Ro

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Lesson G2 – final test

• Describe the difference between parallel and series regulators.

• Which are the benefits of feedback regulators?

• Explain how to get output voltage lower than reference voltage with a feedback regulator.

• Describe a current limiter circuit.

• Describe operation of foldback current limiters

• Motivate and describe the 4-wire technique for remote voltage sensing.

• Describe high-side current sensing with INA.

• Discuss benefits and problems of LDO regulators.