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

space critical applications.

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 space critical applications.

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 500m

or less should be used.

A list of recommended inductors is listed in the table

below:

Peak current definition

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

SENSE

V

SENSE

I

PK

=

ZXSC410

ZXSC420

SEMICO NDUC TORS

ISSUE 2 - May 2003

5

Pin No.

Name

Description

Supply voltage, 1.8V to 8V.

2

GND

Ground

3

STDN/EOR

Shutdown ZXSC410 / End of regulation ZXSC420

4

SENSE

Inductor current sense input. Internal threshold voltage set to

28mV. Connect external sense resistor.

Reference voltage. Internal threshold set to 300mV. Connect

external resistor network to set output voltage.

6

DRIVE

Drive output for external switching transistor. Connect to base or

gate of external switching transistor.

Part No.

Manufacture

L

(A)

( )

CMD4D11-100MC Sumida

10µH 0.5 0.457

CMD4D11-220MC Sumida

22µH 0.4 0.676

LPO2506OB-103

Coilcraft

10µH 1.0

0.24

ST2006103

Standex

Electronics Inc

10µH 0.6

0.1

PIN DESCRIPTIONS