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Voltage References/RegulatorsPART 1

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

Texas A&M University

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ECEN 457 (ESS)

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Agenda• Motivation

• Important Definitions

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Voltage Regulator Market

Major Companies:

Texas Instruments

STMicroelectronics

Toshiba

Maxim

Linear Technologies

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Voltage References/Regulators• The function of a voltage reference/regulator:

– Provide stable dc voltage Vo starting from a

less stable voltage Vi

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Basic connection of a voltage reference/regulator

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Voltage References• Voltage reference:

– Provides an even more stable Vo to serve as a standard for other circuits.

– Application:• Digital multimeter: the full-scale accuracy is set by the internal

voltage of suitable quality.

– Other applications:• Power supplies• AD/DA converters• V-F / F-V converters

– Main requirements:• Accuracy • Stability

– To minimize errors due to self-heating:• Modest output-capabilities (few mA)

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Voltage Regulators• VI is usually a poorly specified voltage

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V

Battery (VI)

Regulated Voltage(VL)

Time

Battery DC-DC converter

Voltage

Regulator

+

-

Load

+

-

VL

VI

E.g. Ipod

Requirements:

• Similar to voltage references but less stringent

• Output current capabilities are much higher (100mA-10A)

AC-DC converter DC-DC converter

E.g. Computer

VI

t t

VI VL

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Voltage Regulators

– Two Types:

• Linear regulators

– Simple

– Low noise

– Poor Efficiency

• Switching regulators

– High efficiency

– Output voltage can be:

» Negative

» Higher than VI

– Complex

– Require inductors

– Noisier

Voltage Reference

Control Element

Error Amplifier

Feedback Network

Feedback Network

Output Filter

Load

Power Switch

Modulator

Voltage Reference

Error Amplifier

Load

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Important Definitions

• Line Regulation:

• Load Regulation:

o

o

I

VregulationLoad

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I

o

V

VregulationLine

oI

o

VV

VregulationLine

100(%)

o

o

o

I

VV

regulationLoad

100%

ILOAD

VOUT(LT)

VOUT(LR)

VIN

VOUT(LT)

VOUT(LR)

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More Definitions

• Ripple ( power supply) rejection ratio (RRR) (PSR):

• Dropout Voltage:

– The minimum difference between VI and Vo for which the circuit still operates properly.

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ro

ridB

V

VRRR 10log20

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Example 11.1

• The data sheets of the uA7805 5-V voltage regulator (Fairchild) indicate that Vo typically changes by 3mV when VI is varied from 7 V to 25 V, and by 5 mV when Io is varied from 0.25 A to 0.75 A. Moreover, RRRdB = 78dB at 120 Hz.

– Estimate the typical line and load regulation of this device. What is the output impedance of the regulator?

– Estimate the amount of output ripple Vro for every volt of Vri.

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VmV

V

V

I

o /17.0725

103 3

VxV

VmV

V

V

I

o /%0033.01005

/17.0(%)

AmV

I

V

o

o /1025.075.0

105 3

riri

ro VV

V 3

)20/78(10126.0

10

mVVro 126110126.0 3 For 1-V at 120Hz ripple at the input:

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More Definitions

• Thermal Coefficient of Vo:

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T

VVTC o

o

)(

o

oo

VT

VVTC

100)(%)(

o

oo

VT

VppminVTC

610)(

ppm = parts per million

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Example 11.2

• The data sheets of the REF101KM 10-V precision voltage reference (Burr-Brown) give a typical line regulation of 0.001%/V, a typical load regulation of 0.001%/mA , and a maximum TC of 1 ppm/C. Find the variation in Vo brought about by: (a) a change of VI from 13.5 V to 35V; (b) a 10mA change in Io ; (c) a temperature change from 0 C to 70 C.

– (a)

– (b)

– (c)

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105.1335

100/%001.0 oVV

mVVo 15.2

1010

100/%001.0 oV

mAmA

mVVo 1

1070

10/1

6

oV

CCppm

mVVo 7.0

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More Definitions

• Efficiency

)( OUTQIN

OUTOUT

IN

OUT

IIV

IV

P

P

If IQ<< ILOAD

%1.891007.3)100100(

3.3100max

AmA

mA

Example: A 3.3 V linear regulator with 3.7 V < VIN < 4.7 V, IQ = 100 μA

1 mA < Io < 100 mA. What is the minimum drop-out voltage? What is the

maximum efficiency? What is the minimum efficiency?

%8.631007.4)1001(

3.31min

AmA

mA

IN

DO

IN

DOIN

IN

OUT

V

V

V

VV

V

V 1

%2.891007.3

4.01max

V

or

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Next Class

• Bandgap voltage reference

• Voltage regulators

– Shunt voltage regulators

– Linear regulators