Datasheet

MAX8520/MAX8521
Smallest TEC Power Drivers for Optical
Modules
______________________________________________________________________________________ 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
outputs with respect to GND are equal to half of V
DD
.
As the TEC current demand increases, one output will
go up and the other will go down from the initial point of
0.5V
DD
by an amount equal to 0.5  V
TEC
(V
TEC
= I
TEC
 R
TEC
). Therefore, the operating duty cycle of each
buck converter depends on the operating I
TEC
and
R
TEC
. Since inductor current calculation for heating and
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,
I
TEC(MAX)
is the maximum TEC current, and fs is the
switching frequency
As an example, for V
DD
= 3.3V, LIR = 12%, and fs =
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
R
SENSE
CS
OS1
C
COMP
R
R
0.5X
REF
1.2X
COMP
PWM
4X
LX2
3/4 V
DD
1/4 V
DD
LX1
-1.2
+1.2
10X
1
gm
Figure 3. Functional Diagram of the Current-Control Loop