ADN8835ACPZ-R7 datasheet(18/27 Pages) AD

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ADN8835

Data Sheet

Rev. B | Page 18 of 27

APPLICATIONS INFORMATION

Figure 33. Signal Flow Block Diagram

SIGNAL FLOW

The ADN8835 integrates two auto-zero amplifiers, defined as

the Chopper 1 amplifier and the Chopper 2 amplifier. Both of the

amplifiers can be used as standalone amplifiers; therefore, the

implementation of temperature control can vary. Figure 33

shows the signal flow through the ADN8835, and a typical

implementation of the temperature control loop using the

Chopper 1 amplifier and the Chopper 2 amplifier.

In Figure 33, the Chopper 1 and Chopper 2 amplifiers are config-

ured as the thermistor input amplifier and the PID compensation

amplifier, respectively. The thermistor input amplifier amplifies

the thermistor voltage, and then outputs to the PID compensa-

tion amplifier. The PID compensation amplifier then compensates

a loop response over the frequency domain.

The output from the compensation loop at OUT2 is fed to the linear

MOSFET gate driver. The voltage at LDR is fed with OUT2 into

the PWM MOSFET gate driver. Including the internal transistors,

the gain of the differential output section is fixed at 5. For details

on the output drivers, see the MOSFET Driver Amplifier section.

THERMISTOR SETUP

The thermistor has a nonlinear relationship to temperature; near

optimal linearity over a specified temperature range can be achieved

with the proper value of a compensation resistor, RX, placed in

series with the thermistor.

First, the resistance of the thermistor must be known, where

•

RLOW = RTH at TLOW

•

RMID = RTH at TMID

•

RHIGH = RTH at THIGH

TLOW and THIGH are the endpoints of the temperature range and

TMID is the average. In some cases, with only the β constant

available, calculate RTH using the following equation:

























−

exp

LINEAR

AMPLIFIER

LDR

TEC CURRENT SENSE

PGNDS

PVINS

PWM POWER

STAGE

PWM

MOSFET

DRIVER

CONTROL

SFB

PWM

MODULATOR

PGNDS

LDR

TEC

PGNDL

PVINL

PGNDL

–

OSCILLATOR

IN2P

OUT2

IN2N

TEC DRIVER

LINEAR POWER

STAGE

–

IN1P

OUT1

IN1N

RFB

RTH

VREF/2

VREF

VTEMPSET

VOUT2

VOUT1

VIN

TEMPERATURE ERROR

AMPLIFIER

AV = RFB/(RTH + RX) – RFB/R

CHOPPER 1

PID COMPENSATION

AMPLIFIER

AV = Z2/Z1

CHOPPER 2

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