Diodes ZXSC400 - Manual (Page 7)

Open-circuit protection
For applications where the LED chain might go
open-circuit a Zener diode can be connected across the
LED chain preventing over-voltage and possible
damage to the main switching transistor. The Zener
diodes should be selected by ensuring its voltage
rating is higher than the combined forward voltage of
the LED chain. Under open circuit conditions the curren
in the Zener diode defines the output current as:
I
Z
=
V
R
FB
Z
The circuit example below give an open circuit output
current of 300
␮A.
Dimming Control
There are four types of dimming control that can be
implemented for the ZXSC400.
Dimming Control using the shutdown pin
The first method uses the shutdown pin. By injecting a
PWM waveform on this pin and varying the duty cycle,
LED current and hence LED brightness can be adjusted.
To implement this method of brightness control on the
ZXSC400, apply PWM signal with amplitude of
between 0.7V and Vcc at a frequency of 120Hz or above
(to eliminate 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 example, a 10% duty cycle equates to
10% of full LED brightness.
ZXSC400
ISSUE 1 - JANUARY 2003
7