MAX8520ETP-T Maxim Integrated Products, MAX8520ETP-T Datasheet - Page 13

• Manufacturer Part Number: MAX8520ETP-T
• Description: Other Power Management TEC Power Driver for Optical Modules
• Manufacturer: Maxim Integrated Products

Specifications of MAX8520ETP-T

• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

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
As the TEC current demand increases, one output will
go up and the other will go down from the initial point of
0.5V

buck converter depends on the operating I
R
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.
Figure 3. Functional Diagram of the Current-Control Loop
TEC
R
TEC
DD
. Since inductor current calculation for heating and
). Therefore, the operating duty cycle of each
by an amount equal to 0.5
CTLI
1
______________________________________________________________________________________
Smallest TEC Power Drivers for Optical
0.5X
R
Inductor Selection
R
REF

V
TEC
gm
C
(V
COMP
TEC
1/4 V
TEC
COMP
DD
= I
and
TEC
DD
-1.2
+1.2
.
1.2X
Calculate the inductor value as follows:
where LIR is the selected inductor ripple-current ratio,
I
switching frequency
As an example, for V
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.
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.
TEC(MAX)
Output Filter Capacitor Selection
10X
CS
OS1
PWM
is the maximum TEC current, and fs is the
4X
LX2
LX1
Common-Mode Filter Capacitors
3/4 V
L
=
DD
LIR I
DD
(
×
0 25
.
TEC MAX
= 3.3V, LIR = 12%, and fs =
×
(
V
R
DD
SENSE
)
)
×
Modules
fs
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