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Lecture 2: Voltage References/Regulators

ECEN 457(ESS): Op-Amps and ApplicationsAnalog & Mixed-Signal Center

Texas A&M University

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2

Agenda• Last lecture

– Motivation

– Important Definitions

• Today– Voltage References

• Bandgap Voltage Reference

– Voltage Regulators

• Shunt Regulator

• Linear Regulators

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Voltage References Thermal stability is very important in voltage references

IC components are strongly influenced by temperature

Silicon pn junction, which forms the basis for diodes and BJTs.

Its forward-bias voltage VD and current ID are related:

Their expressions are:

The TC of the thermal voltage is:

The TC of the junction voltage VD at a given bias ID is TC(VD) =

3

qkTVT / TGs VVBTI 0

3 exp

CmVqkVTC T /0862.0)(

q

k

T

VVVTC DG

D

3)( 0

SDTD IIVV ln

where VT is the thermal voltage and IS is the saturation voltage.

where k = 1.381x10-23 is Boltzmann’s constant, q = 1.602x10-19 C is the electron charge,

T is the absolute temperature, B is a proportionality constant, and VG0 = 1.205 V is the

bandgap voltage for silicon.

T

VVTVTV

T

IIVII

T

VVTC TG

TDsD

TsDT

D

0ln3ln

ln)(

T

VD

Assuming VD = 650mV at 25C, we get TC(VD) -2.1mV/C.

Bandgap Voltage Reference

• Advantage:

– Low voltage (VDD< 5V)

• Based on the idea of adding the voltage drop VBE of a base emitter junction, which has negative TC, to a voltage KVT proportional to the thermal voltage VT , which has a positive TC.

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BETBG VVKV

)()()( BETBG VTCVTCKVTC

Bandgap voltage reference

0)( BGVTCSo

Thus, we need T

BE

VTC

VTCK

30

T

BEG

V

VVK

At 25C we have VBG= 1.205V + 3 x 25.7mV = 1.282V!

TGBET

T

BEGBG VVVV

V

VVV 33 0

0

Bandgap Voltage Reference (Brokaw)

• Based on two BJTs of different emitter areas.

• The emitter area of Q1 is n times as large as the emitter area AE of Q2.

• Thus, the saturation currents satisfy Is1/Is2 = n.

5

33

2112

3

1122

3

121

lnlnlnln

R

nV

R

IIIIV

R

IIIIV

R

VVI TscscTscscTBEBE

c

TBEcBEccBEBG VnR

RVRIVRIIVV

ln22

3

424124212

nR

Rk ln2

3

4where

TBEBG VkVV 2

BGREF VRRV 121

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Shunt Voltage Regulator

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Shunt regulator

Equivalent circuit of the shunt regulatori

v-vz0

vz

Iz 1/rz

Breakdown Zener diode characteristics

To function as a regulator, the diode must operate well within the breakdown region under all

possible line and load regulation. In particular, IZ must never be allowed to drop below some

safety value Iz(min) .

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Shunt Voltage Regulator (Load Regulation)

Vo=151.98/(151.98+470)*15V=3.66V

At this point the circuit is a voltage divider!

Design Approach

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zzZZ IrVV 01)

2)

3)

(min)0(min)(max)(min) zzZIozs IrVVIIR

4

(max)

(min)

o

Z

II

Compromise between the need to ensure proper worst-case

operation and avoid excessive power wastage.

Applying superposition principle, we find:

OUTszZ

zs

sIN

zs

zOUT IRrV

rR

RV

rR

rV ||0

zs

z

rR

rregulationLine

sz RrregulationLoad ||

Equivalent circuit of the shunt regulator

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Example 11.3• A raw voltage 10V VIN 20V is to be stabilized by a 6.8-V,0.5-W,10-

Zener diode and is to feed a load with 0 IOUT 10mA. (a) Find a suitable value for Rs , and estimate the line and load regulation. (b) Estimate the effect of the full-scale changes of VIN and IOUT on VOUT.

(a) Let

(b) Changing VIN from 10V to 20V gives:

Changing IOUT from 0 to 10mA gives:

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mAI

IOUT

Z 5.24

(max)

(min)

kmAmAmARs 284.0105.2/5.21043.610

270sR

VmVregulationLine /7.3510270

10

mAmVregulationLoad /64.9270||10

VVVmVVOUT 357.010/7.35

VmAmAmVVOUT 096.010/64.9

Self-Regulated Voltage Reference

Zo VR

RV

1

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Shifts the burden of line and load

regulation from the diode to the op amp!

Vo is adjustable, for instance, via R2.

R3 can be raised to avoid unnecessary

power wastage and self-heating effects.

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111RR

Ra

z

a

zregulationLoad oo

Appearing in series with VZ.

CMRRPSRRR

RregulationLine

5.011

1

2

CMRRPSRRVV Ios

5.01

where a and zo are the open-loop gain and

impedance of the op amp.

1

21R

RVV oso

Self-Regulated Voltage Reference (load regulation)

Basic Series Voltage Regulator

1) VBE = VBE(on)

2) VCE VCE(sat)

Forward-active region: IC=IB

Power Transistor (Q1):

= 20

VBE(on) 1V

VCE(sat) 0.5V

Typical Transistor (Q2):

= 100

VBE(on) 0.7V

VCE(sat) 0.1V

Darlington Pair or series pass element

REFo VR

RV

1

21

The regulator can be seen as a non-inverting

amplifier with a Darlington current booster!

Q1 x Q2

Example 11.9• Let RB = 510 and RE=3.3k in the regulator . Assuming a reference

voltage of 1.282V and typical BJT parameters, find (a) R2/R1 for Vo=5.0V, (b) the error amplifier output drive needed to provide, Io=1A, (c)the dropout voltage VDO if the error amplifier saturates at VOH =VI-0.5V, and (d) the maximum efficiency attainable for the given Io

a) 282.1151

2

R

Rgives .9.2

1

2 R

R

b) AIo 1 we have ,482111111 mAII EB

.48)(112 mARVII EonBEBE

;47.0101481222 mAIII EBOA

VVVVVV oRonBEonBEOA B7517.047.051.0)(1)(2

For

and

The error amplifier must source:

in addition, :

Continue Example 11.9…

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c) OHOA VV )(satCECE VV

.5.7 VVI

VVVVVV oIDO 5.20.55.7

d) %671005.7/5(%),5.7 VVI

For this circuit to work properly we need and

for both BJTs. It is readily seen that these conditions are met if

Hence,