Application Information
Figure 10 to Figure 13 show the output ripple of a 5V to
3.3V/ 500mA regulator using 22µH inductor and various
capacitor types. At the switching frequency, the low ESR
and ESL make the ceramic capacitors still behave capaci-
tively as shown in Figure 10. Additional paralleled ceramic
capacitors will further reduce the ripple voltage, but
inevitably increase the cost. “POSCAP”, manufactured by
SANYO, is a solid electrolytic capacitor. The anode is sin-
tered tantalum and the cathode is a highly conductive
polymerized organic semiconductor. TPC series, featuring
low ESR and low profile, is used in the measurement of
Figure 11. It is shown that POSCAP presents a good bal-
ance of capacitance and ESR, compared with a ceramic
capacitor. In this application, the low ESR generates less
than 5mV of ripple and the ESL is almost unnoticeable. The
ESL of the through-hole OS-CON capacitor give rise to the
inductive impedance. It is evident from Figure 12 which
shows the step rise of the output ripple on the switch turn-
on and large spike on the switch turn-off. The ESL prevents
the output capacitor from quickly charging up the parasitic
capacitor of the inductor when the switch node is pulled
below ground through the catch diode conduction. This
results in the spike associated with the falling edge of the
switch node. The D package tantalum capacitor used in
Figure 13 has the same footprint as the POSCAP, but dou-
bles the height. The ESR of the tantalum capacitor is appar-
ently higher than the POSCAP. The electrolytic and tanta-
lum capacitors provide a low-cost solution with compro-
mised performance. The reliability of the tantalum capaci-
tor is not a serious concern for output filtering because the
output capacitor is usually free of surge current and volt-
age.
Figure 11: The output voltage ripple using one 100µF POSCAP capacitor.
Diode Selection
The diode in the buck converter provides the inductor cur-
rent path when the power switch turns off. The peak
reverse voltage is equal to the maximum input voltage. The
peak conducting current is clamped by the current limit of
the IC. The average current can be calculated from:
IO(VIN – VO)
Figure 12: The output voltage ripple using one 100µF OS-CON
ID(AVG)
=
VIN
The worse case of the diode average current occurs during
maximum load current and maximum input voltage. For
the diode to survive the short circuit condition, the current
rating of the diode should be equal to the FOLDBACK
CURRENT LIMIT. See Table 1 for schottky diodes from
ON SEMICONDUCTOR which are suggested for CS5141X
regulator.
Inductor Selection
When choosing inductors, one might have to consider
maximum load current, core and copper losses, component
height, output ripple, EMI, saturation and cost. Lower
inductor values are chosen to reduce the physical size of
the inductor. Higher value cuts down the ripple current,
core losses and allows more output current. For most
applications, the inductor value falls in the range between
2.2µH and 22µH. The saturation current ratings of the
inductor shall not exceed the IL(PK), calculated according to
Figure 13: The output voltage ripple using one 100µF tantalum capaci-
tor.
VO(VIN – VO)
I
L(PK) = IO +
2(fS) (L) (VIN)
10
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
