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 converter efficiency. For optimum performance,

required.

The Zetex FMMT618 is an ideal choice of transistor,

having a low saturation voltage. A data sheet for the

FMMT618 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 battery life solution a Schottky

rectifier diode is required. 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 and thus 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 LCD backlight reference design we

recommend an inductor value of 68uH 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

or less should be used.

A list of recommended inductors is shown in the table

below:

ZXSC310

PROVISIONAL ISSUE A - SEPTEMBER 2001

9

Ι / Ι = 50

C

Β

100 C

50 C

10 C

10A

Part No.

Manufacture

L

I

PK

R

DC

DO1608C-683

Coilcraft

68 H

0.4A

0.86

CR54-680

Sumida

68 H

0.61A

0.46

P1174.683

Pulse

68 H

0.4A

0.37

SFOP5845-R61680

Samwha

68 H

0.61A

0.46

SIS43-680

Delta

68 H

0.4

1.125