ACT4501
Rev 2, 14-Nov-12
APPLICATIONS INFORMATION CONT’D
Figure 3:
External High Voltage Bias Diode
for ceramic type. In the case of tantalum or
electrolytic capacitors, the ripple is dominated by
R
ESR
multiplied by the ripple current. In that case, the
output capacitor is chosen to have sufficiently low
ESR.
For ceramic output capacitor, typically choose a
capacitance of about 22µF. For tantalum or
electrolytic capacitors, choose a capacitor with less
than 50mΩ ESR.
Rectifier Diode
Use a Schottky diode as the rectifier to conduct
current when the High-Side Power Switch is off.
The Schottky diode must have current rating higher
than the maximum output current and a reverse
voltage rating higher than the maximum input
voltage.
This diode is also recommended for high duty cycle
operation and high output voltage applications.
Input Capacitor
The input capacitor needs to be carefully selected
to maintain sufficiently low ripple at the supply input
of the converter. A low ESR capacitor is highly
recommended. Since large current flows in and out
of this capacitor during switching, its ESR also
affects efficiency.
The input capacitance needs to be higher than
10µF. The best choice is the ceramic type,
however, low ESR tantalum or electrolytic types
may also be used provided that the RMS ripple
current rating is higher than 50% of the output
current. The input capacitor should be placed close
to the IN and G pins of the IC, with the shortest
traces possible. In the case of tantalum or
electrolytic types, they can be further away if a
small parallel 0.1µF ceramic capacitor is placed
right next to the IC.
Output Capacitor
The output capacitor also needs to have low ESR to
keep low output voltage ripple. The output ripple
voltage is:
V
RIPPLE
=
I
OUTMAX
K
RIPPLE
R
ESR
+
V
IN
28
×
f
SW
LC
OUT
2
(6)
where I
OUTMAX
is the maximum output current, K
RIPPLE
is the ripple factor, R
ESR
is the ESR of the output
capacitor, f
SW
is the switching frequency, L is the
inductor value, and C
OUT
is the output capacitance.
In the case of ceramic output capacitors, R
ESR
is very
small and does not contribute to the ripple.
Therefore, a lower capacitance value can be used
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TM
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