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APPLICATIONS INFORMATION CONT’D
The selected inductor should not saturate at I
LPK.
The maximum output current is calculated as:
I
OUTMAX
=
I
LIM
_
1
I
_
2
LPK PK
ACT4303
Rev 0, 05-Dec-11
Output Capacitor
(5)
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
+
L
LIM
is the internal current limit, which is typically
3.2A, as shown in Electrical Characteristics Table.
V
IN
28
×
f
SW
LC
OUT
2
(6)
External High Voltage Bias Diode
It is recommended that an external High Voltage
Bias diode be added when the system has a 5V
fixed input or the power supply generates a 5V
output. This helps improve the efficiency of the
regulator. The High Voltage Bias diode can be a
low cost one such as IN4148 or BAT54.
Figure 4:
External High Voltage Bias Diode
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 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
This diode is also recommended for high duty cycle
operation and high output voltage applications.
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.
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.
Innovative Power
TM
-7-
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