Datasheet

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. V
MAXV
can vary from 0V
to V
REF
. The voltage across the TEC is four times
V
MAXV
and can be positive or negative:
|V
OS1
- V
OS2
| = 4 x V
MAXV
or V
DD
, whichever is lower
Set V
MAXV
with a resistor-divider between REF and
GND using resistors from 10kΩ to 100kΩ. V
MAXV
can
vary from 0V to V
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. The transfer function relating current through
the TEC (I
TEC
) and V
CTLI
is given by:
I
TEC
= (V
CTLI
- V
REF
)/(10  R
SENSE
)
where V
REF
is 1.50V and:
ITEC = (V
OS1
- V
CS
)/R
SENSE
CTLI is centered around REF (1.50V). I
TEC
is zero when
CTLI = 1.50V. When V
CTLI
> 1.50V the current flow is from
OS2 to OS1. The voltages on the pins relate as follows:
V
OS2
> V
OS1
> V
CS
The opposite applies when V
CTLI
< 1.50V current flows
from OS1 to OS2:
V
OS2
< V
OS1
< V
CS
Shutdown Control
The MAX8520/MAX8521 can be placed in a power-saving
shutdown mode by driving SHDN low. When the
MAX8520/MAX8521 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 proportional
to the actual TEC current. V
ITEC
= V
REF
when TEC current
is zero. The transfer function for the ITEC output is:
V
ITEC
= 1.50V + 8  (V
OS1
– V
CS
)
Use ITEC to monitor the cooling or heating current
through the TEC. For stability keep the load capaci-
tance on ITEC to less than 150pF.
Applications Information
The MAX8520/MAX8521 typically drive a thermo-elec-
tric cooler inside a thermal-control loop. TEC drive
polarity and power are regulated based on temperature
information read from a thermistor or other temperature-
measuring device to maintain a stable control tempera-
ture. Temperature stability of +0.01°C can be achieved
with carefully selected external components.
There are numerous ways to implement the thermal loop.
Figures 1 and 2 show designs that employ precision op
amps, along with a DAC or potentiometer to set the con-
trol temperature. The loop can also be implemented digi-
tally, using a precision A/D to read the thermistor or other
temperature sensor, a microcontroller to implement the
control algorithm, and a DAC (or filtered-PWM signal) to
send the appropriate signal to the MAX8520/MAX8521
CTLI input. Regardless of the form taken by the thermal-
control circuitry, all designs are similar in that they read
temperature, compare it to a set-point signal, and then
send an error-correcting signal to the MAX8520/
MAX8521 that moves the temperature in the appropriate
direction.
PCB Layout and Routing
High switching frequencies and large peak currents
make PCB layout a very important part of design. Good
design minimizes excessive EMI and voltage gradients
in the ground plane, both of which can result in instabil-
ity or regulation errors. Follow these guidelines for good
PCB layout:
1) Place decoupling capacitors as close to the IC pins
as possible.
2) Keep a separate power ground plane, which is con-
nected to PGND1 and PGND2. PVDD1, PVDD2,
PGND1 and PGND2 are noisy points. Connect decou-
pling capacitors from PVDD_ to PGND_ as direct as
possible. Output capacitors C2, C7 returns are con-
nected to PGND plane.
3) Connect a decoupling capacitor from V
DD
to GND.
Connect GND to a signal ground plane (separate from
the power ground plane above). Other V
DD
decoupling
capacitors (such as the input capacitor) need to be
connected to the PGND plane.
4) Connect GND and PGND_ pins together at a single
point, as close as possible to the chip.
5) Keep the power loop, which consists of input capaci-
tors, output inductors and capacitors, as compact and
small as possible.
MAX8520/MAX8521
Smallest TEC Power Drivers for Optical
Modules
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