Kingbor Technology Co.,Ltd
KB4312
TEL:(86)0755-26508846 FAX:(86)0755-26509052
Applications Information
External components of main boost converter can be designed by performing simple calculations. It
need to follow regulation by the output voltage and the maximum load current, as well as maximum
and minimum input voltages. Begin by selecting an inductor value. Once L is know, choose the diode
and capacitors.
Boost inductor
Inductor selection depends on input voltage, output voltage, maximum current , switching frequency
and availability of inductor values. The following boost circuit equations are useful in choosing the
inductor values based on the application. They allow the trading of peak current and inductor value
while allowing for consideration of component availability and cost.
The peak inductor current is given by:
'IL
2
ILpeak ILAVG
ꢁ
IO
ILAVG
1ꢀ D
where:
ϦIL is the inductor peak-to-peak current ripple and is decided by:
Vin
L
D
'IL
u
fOSC
D is the MOSFET turn on ratio and is decided by:
VO ꢀV in
VO
D
fOSC is the switching frequency.
The inductor should be chosen to be able to handle this current and inductor saturation current rating
should be greater than IPEAK
.
Diode selection
The output diode has average current of IO, and peak current the same as the inductor’s peak current
and a voltage rating at least 1.5 times the output voltage. Schottky diode is recommended and it should
be able to handle those current.
Output Capacitor
The KB4312 is specially compensated to be stable with capacitors which have a worst- case minimum
value of 1uF at the particular VOUT being set. Output ripple voltage requirements also determine the
minimum value and type of capacitors. Output ripple voltage consists of two components the voltage
drop caused by the switching current through the ESR of the output capacitor and the charging and
discharging of the output capacitor:
VO ꢀVin
VO
IO
VRIPPLE ILPEAK u ESR ꢁ
u
COUT u fOSC
For low ESR ceramic capacitors, the output ripple is dominated by the charging or discharging of the
output capacitor.
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KINGBOR [ KINGBOR TECHNOLOGY CO ]
