Diodes ZXSC400 - Manual (Page 5)

APPLICATIONS INFORMATION
Switching transistor selection
The choice of switching transistor has a major impact
on the converter efficiency. For optimum performance,
a bipolar transistor with low V
CE(SAT)
and high gain is
required. The V
CEO
of the switching transistor is also an
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
P K
(A) R
D C
( )
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