Smallest TEC Power Drivers for Optical

Modules

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13

Inductor Selection

The MAX8520/MAX8521 dual buck converters operate

in-phase and in complementary mode to drive the TEC

differentially in a current-mode control scheme. At zero

TEC current, the differential voltage is zero, hence the

As the TEC current demand increases, one output will

go up and the other will go down from the initial point of

cooling are identical, but reverse in polarity, the calcu-

lation only needs to be carried out for either one.

For a given inductor, and input voltage, the maximum

inductor ripple current happens when the duty cycle is

at 50%. Therefore, the inductor should be calculated at

50% duty cycle to find the maximum ripple current. The

maximum desired ripple current of a typical standard

buck converter is in the range of 20% to 40% of the

maximum load. The higher the value of the inductor, the

lower the ripple current. However, the size will be phys-

ically larger. For the TEC driver the thermal loop is

inherently slow, so the inductor can be larger for lower

ripple current for better noise and EMI performance.

Picking an inductor to yield ripple current of 10% to

20% of the maximum TEC current is a good starting

point.

Calculate the inductor value as follows:

where LIR is the selected inductor ripple-current ratio,

switching frequency

1MHz, L = 4.58µH

Even though each inductor ripple current is at its maxi-

mum at 50% duty cycle (zero TEC current), the ripple

cancels differentially because each is equal and in-

phase.

Output Filter Capacitor Selection

Common-Mode Filter Capacitors

The common-mode filter capacitors (C2 and C7 of

Figure 1) are used as filter capacitors to ground for

each output. The output ripple voltage depends on the

capacitance, the ESR of these capacitors, and the

inductor ripple current. Ceramic capacitors are recom-

mended for their low ESR and impedance at high fre-

quency.

L

V

LIR I

fs

DD

TEC MAX

=

×

()

××

025

.

()

CTLI

CS

OS1

R

R

0.5X

REF

1.2X

COMP

PWM

4X

LX2

LX1

-1.2

+1.2

10X

1

gm

Figure 3. Functional Diagram of the Current-Control Loop