Linear Regulatormade by JACKY CHEN

Technical Review of Linear Regulator Operation & Performance1.Dropout Volta

ge4.Efficiency 7.Transient Resp

onse

2.Quiescent current & Ground cu

rrent

5.Load Regulator

8.Frequency

Response

3.LDO Topology

6.Line Regulation

9.Range of ESR

1.Dropout Voltage

• Dropout voltage is the input-to-output voltage difference at which the circuit cease to “regulate” against further reductions in input voltage.

Pinch off

RegulationUn-regulation

2.Quiescent Current & Ground Current

• Quiescent current or ground current is the difference between input and output currents.

• Quiescent consists of bias current & drive current of pass element.– Bias current

• Band-gap reference.• Sample resistor.• Error amplifier.

oinq III

2

2

TGSD

TGSD

VVI

VVL

WkI

BFC II

3.LDO Topology

• 4 functional blocks.

Darlington

• The Darlington needs at least 1.6V dropout to regulate.

NPN

• The base volt of the NPN must always higher than the emitter volt.

PNP

• The big advantage of PNP is its very little dropout voltage.

PMOS & NMOS

• MOSFET is voltage controlled device ,so,very low quiescent current is obtained.

Summary

• PNP bipolar good for low dropout application,but it got high quiescent current and low efficiency.

• NMOS most advantage because of its low on resistance.gate drive difficulties make it less than ideal.

• PMOS has been highly developed,and now the performance level exceeding most bipolar devices.

4.Efficiency

• Power dissipation & efficiency :

inqo

oo

ooin

VII

VI

IVVP

5.Load Regulation

• Load regulation is a measure of the circuit ability to maintain the o/p under varying load condition.

Load Regulation

• Since load regulation is steady-state parameter,all frequency components are neglected.

• The load regulation is limited by the open loop current gain of the system.

6.Line Regulation

• Line regulation is a measure of the circuit ability to maintain the o/p under varying line input voltage.

in

o

V

VegulationLine

R_

eqrsaeqds

eqino

eqoeqds

eqino

oeqds

eqino

RVVgRR

RVV

RIRR

RVV

VRR

RVV

rin

eqdso

s

eq

eqrineqds

eq

o

os

VR

RRV

RRGR

RRV

GV

RGRRR

RGVRRVRR

RRR

V

VRR

RV

2

21

2

21

221

2121

21

2

)(

1

2

21

2

21

2

21)(

1

1

Rg

RR

RRV

V

GR

RR

RRV

V

VR

RRV

aLdsin

o

Ldsin

o

rstatesteadyo

Line Regulation

• Since line regulation is steady-state parameter,all frequency components are neglected.

• Increase dc open loop gain increase line regulation.

2

211

R

RR

RRV

V

Ldsin

o

2

211

Rg

RR

RRV

V

aLdsin

o

7.Transient Response

• Transient response is the maximum allowable o/p volt variation for a load current step change.

• The transient response is a function of output Cap value(Co),the ESR ,the bypass Cap(Cb),and maximum load current Io(max).

8.Frequency Response

• The essential elements of a linear regulator

The first Pole:Po is

The Second Pole:Pb is

In addition,another pole exist from the input impedanceOf the pass element,and the o/p impedance of the errorAmplifier.Rpar,Cpar.

Summary

9.Range of ESR

• A LDO needs would require an o/p cap and an o/p ESR to stabilize the control loop.

The linear regulator is unstableBecause the phase shift at unit gain is –180,to make it stable A zero must be added.

With External Compensation

• The zero produced by the ESR locates before the UGF,so,the phase shift at UGF will be around –90.