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:

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