MAX1968EUI+ Datasheet MAX1968EUI+T, MAX1968EUI+, MAX1969EUI+ | P10-P12

MAX1968/MAX1969

Power Drivers for Peltier TEC Modules

Maxim Integrated | 10www.maximintegrated.com

Functional Diagram

MAX V

TEC

MAXV

GND

COMP

CTLI

ITEC

MAXIP

FREQ

PWM CONTROL

AND

GATE CONTROL

MAXIN

SENSE

PGND1

PGND2

MAXV

REF

MAX1968

SHDN

MAX I

TEC

MAXIP

/ V

REF

)

(0.15V / R

SENSE

)

MAX I

TEC

-(V

MAXIN

/ V

REF

)

(0.15V / R

SENSE

)

REF

OS1

OS2

LX1

LX2

OFF

3V TO

5.5V

OS1

REF

MAX1968/MAX1969

Power Drivers for Peltier TEC Modules

Maxim Integrated | 11www.maximintegrated.com

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 = V

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 (I

TEC

). See the

Functional Diagram

for more

detail:

ITEC

= 1.5V + 8 x (V

OS1

- V

)

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

where:

f = resonant frequency of output filter.

Capacitor Selection

Filter Capacitors

Decouple each power-supply input (V

, PV

2) 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 V

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:

where:

= loop unity gain bandwidth

= loop transconductance, typically 100µA/V

COMP

= value of the compensation capacitor

TEC

= TEC series resistance

SENSE

= sense resistor

For MAX

COMP

SENSE

SENSE TEC

COMP

SENSE

SENSE TEC

≥

⎛

⎝

⎜

⎞

⎠

⎟

×+

⎛

⎝

⎜

⎞

⎠

⎟

≥

⎛

⎝

⎜

⎞

⎠

⎟

×+

⎛

⎝

⎜

⎞

⎠

⎟

1968

1969

()

2π

MAX1968/MAX1969

Power Drivers for Peltier TEC Modules

Maxim Integrated | 12www.maximintegrated.com

Setting Voltage and Current Limits

Certain TEC parameters must be considered to guaran-

tee a robust design. These include maximum positive

current, maximum negative current, and the maximum

voltage allowed across the TEC. These limits should be

used to set the MAXIP, MAXIN, and MAXV voltages.

Setting Max Positive and

Negative TEC Current

MAXIP and MAXIN set the maximum positive and nega-

tive TEC currents, respectively. The default current limit

is ±150mV / R

SENSE

when MAXIP and MAXIN are con-

nected to REF. To set maximum limits other than the

defaults, connect a resistor-divider from REF to GND to

set V

MAXI_

. Use resistors in the 10kΩ to 100kΩ range.

MAXI_

is related to I

TEC

by the following equations:

MAXIP

= 10(I

TECP(MAX

) x R

SENSE

)

MAXIN

= 10(I

TECN(MAX

) x R

SENSE

)

where I

TECP(MAX)

is the maximum positive TEC current

and I

TECN(MAX)

is the negative maximum TEC current.

Positive TEC current occurs when CS is less than OS1:

TEC

x R

SENSE

= V

OS1

-V

when I

TEC

> 0.

TEC

x R

SENSE

= V

-V

OS1

when I

TEC

< 0.

The MAX1969 controls the TEC current in one direction

(unipolar current flow from OS1 to CS). Set the maxi-

mum unipolar TEC current by applying a voltage to

MAXIN. Connect MAXIP to MAXIN. The equation for

setting MAXIN is the same for the MAX1968 and

MAX1969.

Take care not to exceed the positive or negative cur-

rent limit on the TEC. Refer to the manufacturer’s data

sheet for these limits.

Setting MAX TEC Voltage

Apply a voltage to the MAXV pin to control the maxi-

mum differential TEC voltage. MAXV can vary from 0 to

REF. The voltage across the TEC is four times V

MAXV

and can be positive or negative:

OS1

- V

OS2

| = 4 x V

MAXV

Set V

MAXV

with a resistor-divider between REF and

GND using resistors from 10kΩ to 100kΩ. V

MAXV

can

vary from 0 to REF.

Control Inputs/Outputs

Output Current Control

The voltage at CTLI directly sets the TEC current. CTLI

is typically driven from the output of a temperature con-

trol loop. For the purposes of the following equations, it

is assumed that positive TEC current is cooling (see

Figure 1). The transfer function relating current through

the TEC (I

TEC

) and V

CTLI

is given by:

TEC

= (V

CTLI

- V

REF

)/(10 x R

SENSE

)

where V

REF

is 1.50V and:

TEC

= (V

OS1

- V

)/R

SENSE

CTLI is centered around REF (1.50V). I

TEC

is zero when

CTLI = 1.50V. When VCTLI > 1.50V the MAX1968 is

cooling. Current flow is from OS2 to OS1. The voltages

on the pins relate as follows:

OS2

> V

OS1

> V

The opposite applies when heating. When V

CTLI

1.50V current flows from OS1 to OS2:

OS2

< V

OS1

< V

Shutdown Control

The MAX1968/MAX1969 can be placed in a power-sav-

ing shutdown mode by driving SHDN low. When the

MAX1968/MAX1969 are shut down, the TEC is off (OS1

and OS2 decay to GND) and supply current is reduced

to 2mA (typ).

ITEC Output

ITEC is a status output that provides a voltage proportion-

al to the actual TEC current. ITEC = REF when TEC cur-

rent is zero. The transfer function for the ITEC output is:

ITEC

= 1.50 + 8 x (V

OS1

- V

)

Use ITEC to monitor the cooling or heating current

through the TEC. The maximum capacitance that ITEC

can drive is 100pF.

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Gate Drivers Power Driver for Peltier TEC Modules

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