AT1312A/B/C

Preliminary Product Information

High Efficiency, Constant Current

White-LEDs Driver

7F, No.9,PARK AVENUE. II, Science-Based Industrial Park, Hsinchu 300,Taiwan, R.O.C.

Tel: 886-3-563-0878

Fax: 886-3-563-0879

2/23/2006 REV:1.1

13

Application Information

External components of boost converter can be designed by performing simple

calculations.

Boost inductor

Inductor selection depends on input voltage, output voltage, maximum output 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

Diode selection

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 AT1312 series are specially compensated to be stable with capacitors which have

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

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.