MAX8520/MAX8521
Smallest TEC Power Drivers for Optical
Modules
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Detailed Description
The MAX8520/MAX8521 TEC drivers consist of two
switching buck regulators that operate together to
directly control the TEC current. This configuration cre-
ates a differential voltage across the TEC, allowing bi-
directional TEC current for controlled cooling and
heating. Controlled cooling and heating allow accurate
TEC temperature control to within 0.01°C. The voltage at
CTLI directly sets the TEC current. An external thermal-
control loop is typically used to drive CTLI. Figures 1
and 2 show examples of the thermal-control-loop circuit.
Ripple Cancellation
Switching regulators like those used in the MAX8520/
MAX8521 inherently create ripple voltage on the output.
The dual regulators in the MAX8520/MAX8521 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
For the MAX8521, FREQ sets the switching frequency
of the internal oscillator. With FREQ = GND, the oscilla-
tor frequency is set to 500kHz. The oscillator frequency
is 1MHz when FREQ = V
DD
.
For the MAX8520, connect a resistor (R
EXT
in Figure 2)
from FREQ to GND. Choose R
EXT
= 60kΩ for 1MHz
operation, and R
EXT
= 150kΩ for 500KHz operation. For
any intermediary frequency between 500kHz and
1MHz, use the following equation to find the value of
R
EXT
value needed for V
DD
= 5V:
where R
EXT
is the resistance given in kΩ, and fs is the
desired frequency given in MHz. Note that for V
DD
<
5V, the frequency is reduced slightly, to the extent of
about 7% when V
DD
reaches 3V. This should be taken
into consideration when selecting the value for R
EXT
at
known supply voltage.
Voltage and Current-Limit Setting
Both the MAX8520 and MAX8521 provide control of the
maximum differential TEC voltage. Applying a voltage
to MAXV limits the maximum voltage across the TEC.
The voltage at MAXIP and MAXIN sets the maximum
positive and negative current through the TEC. These
current limits can be independently controlled.
Current Monitor Output
ITEC provides a voltage output proportional to the TEC
current (I
TEC
). See the
Functional Diagram
for more
detail:
V
ITEC
= 1.5V +(8 (VOS1-VCS))
Reference Output
The MAX8520/MAX8521 include an on-chip voltage ref-
erence. The 1.50V reference is accurate to 1% over
temperature. Bypass REF with 0.1µF to GND. REF can
be used to bias an external thermistor for temperature
sensing as shown in Figures 1 and 2.
Thermal and Fault-Current Protection
The MAX8520/MAX8521 provide fault-current protec-
tion in either FETs by turning off both high-side and
low-side FETs when the peak current exceeds 3A in
either FETs. In addition, thermal-overload protection
limits the total power dissipation in the chip. When the
device’s die junction temperature exceeds +165°C, an
on-chip thermal sensor shuts down the device. The
thermal sensor turns the device on again after the junc-
tion temperature cools down by +15°C.
Design Procedures
Duty-Cycle Range Selection
By design, the MAX8520/MAX8521 are capable of
operating from 0% to 100% duty cycle, allowing both
LX outputs to enter dropout. However, as the LX pulse
width narrows, accurate duty-cycle control becomes
difficult. This can result in a low-frequency noise
appearing at the TEC output (typically in the 20kHz to
50kHz range). While this noise is typically filtered out by
the low thermal-loop bandwidth, for best result, operate
the PWM with a pulse width greater than 200ns. For
500kHz application, the recommended duty-cycle
range is from 10% to 90%. For 1MHz application, it is
from 20% to 80%.
