ZXSC400
Open-circuit protection
Dim m ing Control using the shutdow n pin
For applications where the LED chain m ight go The first m ethod uses the shutdown pin. By injecting a
open-circuit a Zener diode can be connected across the PWM waveform on this pin and varying the duty cycle,
LED chain preventing over-voltage and possible LED current and hence LED brightness can be adjusted.
dam age to the m ain switching transistor. The Zener
diodes should be selected by ensuring its voltage
rating is higher than the com bined forward voltage of
the LED chain. Under open circuit conditions the curren
in the Zener diode defines the output current as:
To im plem ent this m ethod of brightness control on the
ZXS C400, a p p ly PWM s ig n a l w ith a m p litu d e o f
between 0.7V and Vcc at a frequency of 120Hz or above
(to elim inate LED flicker). The LED current and hence
LED brightness is linearly proportional to the duty cycle
ratio, so for brightness control adjust duty cycle ratio as
necessary. For exam ple, a 10% duty cycle equates to
10% of full LED brightness.
VFB
IZ =
RZ
The circuit exam ple below give an open circuit output
current of 300A.
Dim m ing Control
There are four types of dim m ing control that can be
im plem ented for the ZXSC400.
ISSUE 1 - J ANUARY 2003
7
DIODES [ DIODES INCORPORATED ]
