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,
a bipolar transistor with low V
CE(SAT)
and high gain is
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 not 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 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
ISSUE 3 - SEPTEMBER 2007
9
Part No. Manufacture L I
PK
R
DC
DO1608C-683 Coilcraft 68H 0.4A 0.86⍀
CR54-680 Sumida 68H 0.61A 0.46⍀
P1174.683 Pulse 68H 0.4A 0.37⍀
SFOP5845-R61680 Samwha 68H 0.61A 0.46⍀
SIS43-680 Delta 68H 0.4 1.125⍀
