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MAX1968

Power Drivers for Peltier TEC Modules

Maxim Integrated 

MAX1968/MAX1969

Power Drivers for Peltier TEC Modules

Detailed Description

The MAX1968/MAX1969 TEC drivers consist of two

switching buck regulators that operate together to

directly control TEC current. This configuration creates

a differential voltage across the TEC, allowing bidirec-

tional TEC current for controlled cooling and heating.

Controlled cooling and heating allow accurate TEC

temperature control within the tight tolerances of laser

driver specifications. The voltage at CTLI directly sets

the TEC current. An external thermal-control loop is typ-

ically used to drive CTLI. Figures 1 and 2 show exam-

ples of thermal control-loop circuits.

Ripple Cancellation

Switching regulators like those used in the

MAX1968/MAX1969 inherently create ripple voltage on

the output. The regulators in the MAX1968 switch in

phase and provide complementary in-phase duty cycles

so ripple waveforms at the TEC are greatly reduced. This

feature suppresses ripple currents and electrical noise at

the TEC to prevent interference with the laser diode.

Switching Frequency

FREQ sets the switching frequency of the internal oscil-

lator. With FREQ = GND, the oscillator frequency is set

to 500kHz. The oscillator frequency is 1MHz when

FREQ = VDD.

Voltage and Current-Limit Settings

Both the MAX1968 and MAX1969 provide control of the

maximum differential TEC voltage. Applying a voltage

to MAXV limits the maximum voltage across the TEC.

The MAX1968 provides control of the maximum positive

and negative TEC current. The voltage at MAXIP and

MAXIN sets the maximum positive and negative current

through the TEC. These current limits can be indepen-

dently controlled. The MAX1969 only controls TEC cur-

rent in one direction. The maximum TEC current is

controlled by MAXIP. Connect MAXIN to GND when

using the MAX1969.

Current Monitor Output

ITEC provides a voltage output proportional to the TEC

current (ITEC). See the Functional Diagram for more

detail:

VITEC = 1.5V + 8 x (VOS1 - VCS)

Reference Output

The MAX1968/MAX1969 include an on-chip voltage ref-

erence. The 1.50V reference is accurate to 1% over

temperature. Bypass REF with 1µF to GND. REF may

be used to bias an external thermistor for temperature

sensing as shown in Figures 1 and 2.

Design Procedure

Inductor Selection

Small surface-mount inductors are ideal for use with the

MAX1968/MAX1969. 3.3µH inductors are suitable for

most applications. Select the output inductors so that

the LC resonant frequency of the inductance and the

output capacitance is less than 1/5 the selected switch-

ing frequency. For example, 3.3µH and 1µF have a res-

onance at 87.6kHz, which is adequate for 500kHz

operation

f= 1

2π LC

where:

f = resonant frequency of output filter.

Capacitor Selection

Filter Capacitors

Decouple each power-supply input (VDD, PVDD1,

PVDD2) with a 1µF ceramic capacitor close to the supply

pins. In some applications with long distances between

the source supply and the MAX1968/MAX1969, addition-

al bypassing may be needed to stabilize the input sup-

ply. In such cases, a low-ESR electrolytic capacitor of

100µF or more at VDD is usually sufficient.

Compensation Capacitor

A compensation capacitor is needed to ensure current

control-loop stability. Select the capacitor so that the

unity-gain bandwidth of the current control loop is less

than or equal to 1/12th the resonant frequency of the out-

put filter:

CCOMP

≥

⎛

⎝⎜

gm

fBW

⎞

⎠⎟

×

⎛

⎝⎜

2π

×

24 × RSENSE

(RSENSE + RTEC)

⎞

⎠⎟

(For MAX1968)

CCOMP

≥

⎛

⎝⎜

gm

fBW

⎞

⎠⎟

×

⎛

⎝⎜

2π

12 × RSENSE

× (RSENSE + RTEC)

⎞

⎠⎟

(For MAX1969)

where:

fBW = loop unity gain bandwidth

gm = loop transconductance, typically 100µA/V

CCOMP = value of the compensation capacitor

RTEC = TEC series resistance

RSENSE = sense resistor

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