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.

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:

D

I

I

I

I

I

O

LAVG

L

LAVG

Lpeak

1

2

where:

OSC

in

L

f

D

L

V

I

D is the MOSFET turn on ratio and is decided by:

O

in

O

V

V

V

D

The inductor should be chosen to be able to handle this current and inductor saturation current rating

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.

The KB4312 is specially compensated to be stable with capacitors which have a worst- case minimum

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:

OSC

OUT

O

O

in

O

LPEAK

RIPPLE

f

C

I

V

V

V

ESR

I

V

For low ESR ceramic capacitors, the output ripple is dominated by the charging or discharging of the

output capacitor.

8

KB4312

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Applications Information

Boost inductor

Diode selection

Output Capacitor