Datasheet

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SLVS519A − MAY 2004 − REVISED OCTOBER 2004
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Figure 29. 3.3-V Power Supply with Panasonic SP Output Filter Capacitors
In Figure 29, the TPS54356 shown with two 180-µF 4-V
special polymer dielectric output filter capacitors(C2 and
C5). C10 is a high frequency bypass capacitor and does
not enter into the design equations. In the previous
example, the output LC corner frequency is to be set at the
second zero frequency f
Z2
of the internal compensation
network, approximately 5 kHz, while the first zero is used
to provide phase boost prior to the LC corner frequency.
The special polymer electrolytic capacitors used in this
design require that the closed loop crossover frequency be
lowered due to the significantly lower ESR of this type of
capacitor.
Inductor Selection
The inductor is the same 10-µH value as the previous
example.
Capacitor Selection
To lower the closed loop crossover it is necessary to
reduce the LC corner frequency below 5 kHz. Using a
target value of 2500 Hz, the output capacitor value is given
by:
C
2 
1
4p
2
ƒ
Z2
2
L
out

1
4  p
2
 2500
2
 10
5
 405 m
F
Use 2 x 180 µF = 360 µF as a combination of standard
values that is close to 405 µF.
The RMS ripple current in the output capacitor is the same
as before. The selected capacitors are each 3.3 A. See the
closed loop response curve for this design in Figure 21.
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Figure 30. 3.3-V Power Supply with Sanyo OSCON Output Filter Capacitor
In Figure 30, the TPS54356 shown with a Sanyo OSCON
output filter capacitor(C2). C10 is a high frequency bypass
capacitor and does not enter into the design equations.
This design is identical to the previous example except that
a single OSCON capacitor of 330 µF is used for the
calculated value of 405 µF. Compare the closed loop
response for this design in Figure 22 to the closed loop
response in Figure 21. Note that there is only a slight
difference in the response and the general similarity in both
the gain and phase plots. This is the expected result for
these two similar output filters.
Many other additional output filter designs are possible.
Use the SWIFT Designer Software to generate other
designs or follow the general design procedures illustrated
in this application section.