APPLICATIONS INFORMATION

Switching transistor selection

The choice of switching transistor has a major impact

on the converter efficiency. For optimum performance,

important parameter as this sees the full output

voltage when the transistor is switched off. Zetex

SuperSOT™ transistors are an ideal choice for this

application.

Schottky diode selection

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 rating 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 ZHCS Series meet these

needs.

Combination devices

To minimise the external component count Zetex

recommends the ZX3CDBS1M832 combination of

NPN transistor and Schottky diode in a 3mm x 2mm

MLP package. This device is recommended for use in

applications using 1 to 4 white LEDs.

The IC is also capable of driving MOSFETs. Zetex

recommends the ZXMNS3BM832 combination of low

threshold voltage N-Channel MOSFET and Schottky

diode in a 3mm x 2mm MLP package. This device is

recommended for use in applications using 1 to 8 white

LEDs.

Capacitor Selection

A small value, low ESR ceramic capacitor is required to

filter the output, typically 1 F to 4.7 F.

Generally an input capacitor is not required, but a small

ceramic capacitor may be added to aid EMC, typically

1 F to 4.7 F.

Inductor Selection

The inductor value must be chosen to satisfy

performance, cost and size requirements of the overall

solution.

Inductor selection has a significant impact on the

converter performance. For applications where

efficiency is critical, an inductor with a series resistance

of 250m

or less should be used.

A list of recommended inductors is listed in the table

below:

ZXSC400

ISSUE 1 - JANUARY 2003

5

Part No.

Manufacture

L

I

R

)

CMD4D11-100MC

Sumida

10 H

0.5

0.457

CMD4D11-220MC

Sumida

22 H

0.4

0.676

LPO2506OB-103

Coilcraft

10 H

1.0

0.24

ST2006103

Standex Electronics Inc

10 H

0.6

0.1