Cirrus-logic AN152 Instrukcja Użytkownika Pobierz pdf

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Cirrus Logic, Inc. 1999

P.O. Box 17847, Austin, Texas 78760

(512) 445 7222 FAX: (512) 445 7581

http://www.cirrus.com

AN152

Application Note

USING THE CS5521/23, CS5522/24/28, AND CS5525/26

CHARGE PUMP DRIVE FOR EXTERNAL LOADS

By Bill Durbin and Jerome Johnston

INTRODUCTION

The CS5521/23, CS5522/24/28, and CS5525/26

series of A/D converters include on-chip circuitry

to drive and regulate a diode charge pump. The

purpose of this application note is to explain the

charge pump circuitry and how it can be used in a

system design.

CS552X Overview

The CS5521/23, CS5522/24/28, and CS5525/26

series of A/D converters include a chopper-stabi-

lized instrumentation amplifier for measurement of

low level dc signals (±100 mV or less). This ampli-

fier is designed to produce very low input sampling

current (I

CVF

< 300 pA over -40 to +85 C). A low

input current minimizes the errors that can occur in

thermocouple measurements when high impedance

circuitry is used for input protection as shown in

Figure 1.

The charge pump circuitry, illustrated in Figure 1,

is used to generate a negative supply (approximate-

ly -2.1 V) to power the on-chip instrumentation

amplifier. This enables the amplifier to measure

low level input signals that are negative relative to

ground while maintaining low input current. With-

in certain constraints, which are described in this

document, the charge pump can be used to power

some additional circuitry outside the converter,

such as an amplifier or a multiplexer.

1.The CS5529 is not included in this Application

Note because it does not contain a charge pump.

0.1 uF

VA+

CS5521/23,

CS5522/24/28,

& CS5525/26

NBV

CPD

1N4148

BAT85

External Load

T/C

10 K

1N4148

Charge Pump Circuit

Figure 1. Input Amplifier inside CS552x ADCs.

AUG ‘99

AN152REV1

1 2 3 4 5 6 7 8

Podsumowanie treści

Strona 1 - Application Note

Strona 2 - The CS552X’s Charge Pump

AN1522 AN152REV1Charge Pump BasicsFigure 2 illustrates a basic diode charge pump.Transistors Q1 and Q2 represent the output transis-tors of a CMOS inv

Strona 3 - AN152REV1 3

AN152AN152REV1 3es the proper magnitude, cycles of the charge pumpclock are deleted. The regulation loop maintainsthe pulse rate out of the CPD pin

Strona 4 - 4 AN152REV1

AN1524 AN152REV1plot shows that if the charge pump output has noexternal load, its average output frequency (VA+ =5 V, C =0.033 uF) is approximately 8

Strona 5 - AN152REV1 5

AN152AN152REV1 50.033 uF @ 5.5 V0.033 uF @ 5.0 V0.033 uF @ 4.5 V0.015 uF @ 5.5 V0.015 uF @ 5.0 V0.015 uF @ 4.5 V02004006008001000120014000.0 5.0 10.0

Strona 6 - CS5521/22/23/24/28

AN1526 AN152REV1VA+ = 4.5 VVA+ =5.0 VVA+ =5.5 V(µA)010020030040050060070080090010000.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0FrequencyLoad CurrentFi

Strona 7 - APPENDIX

AN152AN152REV1 7from +3.3 down to 2.7 V. The external load capa-bility of the charge pump is limited when VD+ getsto 2.7 V.Running the CS552X at Freq

Strona 8

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Podsumowanie treści

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