ZXSC300

ISSUE 4 - SEPTEMBER 2007

8

APPLICATIONS INFORMATION

The following section is a design guide for optimum

converter performance.

Switching transistor selection

The choice of switching transistor has a major impact

on the c onverter e fficiency. F or optim um

high gain is required.

The Zetex FMMT617 is an ideal choice of transistor,

having a low saturation voltage. A data sheet for the

FMMT617 is available on Zetex web site or through

your local Zetex sales office. Outline information is

included in the characteristics section of this data

sheet.

Schottky diode selection

For the maximum efficiency a Schottky rectifier diode

is required not a normal silicon diode. As with the

switching transistor the Schottky rectifier diode has a

major impact on the converter efficiency. A Schottky

diode with a low forward voltage and fast recovery

time should be used for this application.

The diode should be selected so that the maximum

forward current is greater or equal to the maximum

peak current in the inductor, and the maximum reverse

voltage is greater or equal to the output voltage.

The Zetex ZHCS1000 meets these needs. Datasheets

for the ZHCS Series are available on Zetex web site or

through your local Zetex sales office. Outline

information is included in the characteristics section of

this data sheet.

For the maximum brightness solution a pulsed current

is supplied to the LED therefore a Schottky rectifier

diode is not required.

Inductor selection

The inductor value must be chosen to satisfy

performance, cost and size requirements of the overall

solution. For the reference designs we recommend an

inductor value of 100uH with a core saturation current

rating greater than the converter peak current value

and low series resistance.

Inductor selection has a significant impact on the

converter performance. For applications where

efficiency is critical, an inductor with a series

resistance of 500m

Ω or less should be used.

Peak current definition

The peak current rating is a design parameter whose

value is dependent upon the overall application. For

the high brightness reference designs, a peak current

of 190mA was chosen to ensure that the converter

could provide the required output power to the LED.

the switching transistor, Q1, is in full saturation with

maximum output power conditions, assuming

worse-case input voltage and transistor gain under all

operating temperature extremes.

determined by:

R

V

I

SENSE

ISENSE

PK

=