ZXSC400
Peak current definition
Program m ing LED current
In general, the IPK value m ust be chosen to ensure that Once the required output power is determ ined, the LED
the switching transistor, Q1, is in full saturation with current can be program m ed by adding a single resistor
m a xim u m o u tp u t p o w e r co n d itio n s , a s s u m in g in the LED chain. The resistor value is determ ined by
worse-case input voltage and transistor gain under all the following:
operating tem perature extrem es.
Once IPK is decided the value of RS ENS E can be
determ ined by:
VSENSE
RSENSE =
IPK
Sense Resistor
A low value sense resistor is required to set the peak
current. Power in this resistor is negligible due to the
low sense voltage threshold, VISENSE. At the bottom of
the page there is a table of recom m ended sense
resistors.
VFB
RLED
Output pow er calculation
ILED =
By m aking the above assum ptions for inductance and
peak current the output power can be determ ined by:
Where VFB = 300m V
R
selection table
P
OUT = IAV x VIN x (Watts)
LED
where
and
ILED
RLED
7.5⍀
10⍀
15⍀
20⍀
30⍀
IPK
2
(TON + TDIS)
(TON + TOFF)
IAV =
×
40m A
30m A
20m A
15m A
10m A
IPK × L
TON =
VIN
and
IPK × L
Shutdow n Control
TDIS =
(VLED − VIN)
The ZXSC400 offers a shutdown m ode that consum es
a standby current of less than 5A. When the voltage at
the STDN pin between 1V and 8V (and also open circuit),
the ZXSC400 is enabled and the driver is in norm al
operation. When the voltage at the STDN pin is 0.7V or
lower, the ZXSC400 is disabled and the driver is in
shutdown m ode. The SHDN input is a high im pedance
current source of 1A typ. The driving device can be an
open collector or an open drain or a logic output with a
“High“ voltage of 5V m ax. The device shutdown
current depends of the supply voltage, see typical
characteristics graph
and
and
TOFF 1.7s (internally set by ZXSC400)
= efficiency e.g. 100% = 1
Operating frequency can be derived by:
1
F =
TON + TOFF
Ma n u fa ct u re
Cyn te c
S e rie s
RDC
(
) Range
Size
0805
1206
Tolerance
±5%
URL
RL1220
LR1206
0.022 - 10
0.010 - 1.0
www.cyntec.com
www.irctt.com
IRC
±5%
ISSUE 1 - J ANUARY 2003
6
DIODES [ DIODES INCORPORATED ]
