MAX8520 MAXIM [Maxim Integrated Products], MAX8520 Datasheet - Page 14

• Manufacturer Part Number: MAX8520
• Description: Smallest TEC Power Drivers for Optical Modules
• Manufacturer: MAXIM [Maxim Integrated Products]

The output common-mode ripple voltage can be calcu-
lated as follows:
A 1µF ceramic capacitor with ESR of 10mΩ with LIR =
12% and I
24.3mV. For size-constraint applications, the capacitor
can be made smaller at the expense of higher ripple
voltage. However, the capacitance must be high
enough so that the LC resonant frequency is less than
1/5 the switching frequency:
where f is the resonant frequency of the output filter.
The differential-mode filter capacitor (C5 in Figure 1) is
used to bypass differential ripple current through the
TEC as the result of unequal duty cycle of each output.
This happens when the TEC current is not at zero. As
TEC current increases from zero, both outputs move
away from the 50% duty-cycle point complementarily.
The common-mode ripple decreases, but the differential
ripple does not cancel perfectly, and there is a resulting
differential ripple. The maximum value happens when
one output is at 75% duty cycle and the other is at 25%
duty cycle. At this operating point, the differential ripple
is equal to 1/2 of the maximum common-mode ripple.
The TEC ripple current determines the TEC perfor-
mance, because the maximum temperature differential
that can be created between the terminals of the TEC
depends on the ratio of ripple current and DC current.
The lower the ripple current, the closer to the ideal
maximum. The differential-mode capacitor provides a
low-impedance path for the ripple current to flow, so that
the TEC ripple current is greatly reduced. The TEC ripple
current can then be calculated as follows:
where Z
frequency, R
R
Decouple each power-supply input (V
PV
pins. In applications with long distances between the
source supply and the MAX8520/MAX8521, additional
Smallest TEC Power Drivers for
Optical Modules
14
SENSE
I
V
DD
TEC(RIPPLE)
RIPPLE(P-P)
______________________________________________________________________________________
2) with a 1µF ceramic capacitor close to the supply
C5
is the current-sense resistor.
is the impedance of C5 at twice the switching
TEC(MAX)
Differential Mode Filter Capacitor
TEC
Decoupling Capacitor Selection
= (0.5 x LIR x I
= LIR x I
+ R
is the TEC equivalent resistance, and
SENSE
= 1.5A results in V
f
=
TEC(MAX)
2π
+ Z
1
LC
TEC(MAX)
C5
)
(ESR + 1/8 x C x fs)
) x (Z
RIPPLE(P-P)
DD
C5
, PV
)/(R
TEC
DD
1,
of
bypassing may be needed to stabilize the input supply.
In such cases, a low-ESR electrolytic or ceramic capaci-
tor of 100µF or more at V
A compensation capacitor is needed to ensure current-
control-loop stability (see Figure 3). Select the capacitor
so that the unity-gain bandwidth of the current-control
loop is less than or equal to 10% the resonant frequency
of the output filter:
where:
f
equal to 10% the output filter resonant frequency
g
C
R
tance value
R
Certain TEC parameters must be considered to guarantee
a robust design. These include maximum positive current,
maximum negative current, and the maximum voltage
allowed across the TEC. These limits should be used to
set the MAXIP, MAXIN, and MAXV voltages.
MAXIP and MAXIN set the maximum positive and nega-
tive TEC currents, respectively. The default current limit
is ±150mV/R
nected to REF. To set maximum limits other than the
defaults, connect a resistor-divider from REF to GND to
set V
V
where I
and I
Positive TEC current occurs when CS is less than OS1:
when I
when I
BW
m
MAXI_
TEC
SENSE
COMP
= loop transconductance, typically 100µA/V
= unity-gain bandwidth frequency, less than or
MAXI_
TECN(MAX)
Setting Max Positive and Negative TEC Current
= TEC series resistance; use the minimum resis-
TEC
TEC
C
TECP(MAX)
is related to ITEC by the following equations:
= value of the compensation capacitor
= sense resistor
COMP
V
V
Setting Voltage and Current Limits
. Use resistors in the 10kΩ to 100kΩ range.
> 0.
< 0.
MAXIN
MAXIP
SENSE
I
I
≥
TEC
TEC



is the negative maximum TEC current.
f
g
= 10(I
= 10(I
BW
is the maximum positive TEC current
m
✕
x R
when MAXIP and MAXIN are con-
R

 ×

SENSE
SENSE
Compensation Capacitor
TECP(MAX)
TECN(MAX)
DD



2π(
is sufficient.
R
= OS1 - CS
24
= CS - OS1
SENSE
×
R
✕
✕
SENSE
R
R
×
SENSE
SENSE
R
TEC
)
)
)


