Switching Frequency

FREQ sets the switching frequency of the internal oscil-

lator. The oscillator frequency is 500kHz when FREQ =

GND. The oscillator frequency is 1MHz when FREQ =

ter-capacitor values. Efficiency is optimized with the

500kHz setting.

Voltage and Current-Limit Settings

The MAX1978 and MAX1979 provide settings to limit

the maximum differential TEC voltage. Applying a volt-

age to MAXV limits the maximum voltage across the

The MAX1978 also limits the maximum positive and

negative TEC current. The voltages applied to MAXIP

and MAXIN independently set the maximum positive

and negative output current limits. The MAX1979 con-

trols TEC current in only one direction, so the maximum

current is set only with MAXIP. MAXIN must be con-

nected to GND when using the MAX1979.

Chopper-Stabilized Instrumentation

Amplifier

The MAX1978 and MAX1979 include a chopped input

instrumentation amplifier with a fixed gain of 50. An

external thermal sensor, typically a thermistor, is con-

nected to one of the amp’s inputs. The other input is

connected to a voltage that represents the temperature

set point. This set point can be derived from a resistor-

divider network or DAC. The included instrumentation

amplifier provides low offset drift needed to prevent

temperature set-point drift with ambient temperature

changes. Temperature stability of 0.001°C can be

achieved over a 0°C to +50°C ambient temperarure

range by using the amplifier as in Figure 1. DIFOUT is

the instrumentation amplifier output and is proportional

to 50 times the difference between the set-point tem-

perature and the TEC temperature. This difference is

commonly referred to as the “error signal”. For best

temperature stability, derive the set-point voltage from

the same reference that drives the thermistor (usually

the MAX1978/MAX1979 REF output). This is called a

“ratiometric” or “bridge” connection. The bridge con-

nection optimizes stability by eliminating REF drift as an

error source. Errors at REF are nullified because they

affect the thermistor and set point equally.

The instrumentation amplifier utilizes a chopped input

scheme to minimize input offset voltage and drift. This

generates output ripple at DIFOUT that is equal to the

chop frequency. The DIFOUT peak-to-peak ripple

amplitude is typically 100mV but has no effect on tem-

perature stability. DIFOUT ripple is filtered by the inte-

grator in the following stage. The chopper frequency is

derived from, and is synchronized to, the switching fre-

quency of the power stage.

Integrator Amplifier

An on-chip integrator amplifier is provided on the

MAX1978/MAX1979. The noninverting terminal of the

amplifier is connected internally to REF. Connect an

appropriate network of resistors and capacitors between

DIFOUT and INT-, and connect INTOUT to CTLI for typi-

cal operation. CTLI directly controls the TEC current

magnitude and polarity. The thermal-control-loop dynam-

ics are set by the integrator input and feedback compo-

nents. See the Applications Information section for

details on thermal-loop compensation.

Current Monitor Output

ITEC provides a voltage output proportional to the TEC

current, ITEC (see the Functional Diagram):

Over- and Under-Temperature Alarms

The MAX1978/MAX1979 provide open-drain status out-

puts that alert a microcontroller when the TEC tempera-

ture is over or under the set-point temperature. OT and

typical thermistor connection, this translates to approxi-

mately 1.5°C error.

Reference Output

The MAX1978/MAX1979 include an on-chip 1.5V volt-

age reference accurate to 1% over temperature.

Bypass REF with 1µF to GND. REF can be used to bias

an external thermistor for temperature sensing as

shown in Figures 1 and 2. Note that the 1% accuracy of

REF does not limit the temperature stability achievable

with the MAX1978/MAX1979. This is because the ther-

mistor and set-point bridge legs are intended to be dri-

ven ratiometrically by the same reference source (REF).

Variations in the bridge-drive voltage then cancel out

and do not generate errors. Consequently, 0.001°C sta-

ble temperature control is achievable with the

MAX1978/MAX1979 reference.

An external source can be used to bias the thermistor

bridge. For best accuracy, the common-mode voltage

applied to FB+ and FB- should be kept between 0.5V

and 1V, however the input range can be extended from

(approximately -50µV/V) can be tolerated. This shift

remains constant with temperature and does not con-

tribute to set-point drift.

Integrated Temperature

Controllers for Peltier Modules

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