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 goes

up and the other goes down from the initial point of

and cooling are identical, but reverse in polarity, the

calculations only need 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 is physically

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 recommended for their

low ESR and impedance at high frequency.

L

V

LIR

I

fs

DD

TEC MAX

=

×

()

××

025

.

()

CTLI

CS

REF

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