ZXLD1356

Issue 2 - September 2008

21

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© Diodes Incorporated 2008

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Output current adjustment by PWM control

Directly driving ADJ input

shown below, to adjust the output current to a value above or below the nominal average value

Driving the ADJ input via open collector transistor

The recommended method of driving the ADJ pin and controlling the amplitude of the PWM

waveform is to use a small NPN switching transistor as shown below:

This scheme uses the 50k resistor between the ADJ pin and the internal voltage reference as a

pull-up resistor for the external transistor eg MMBT3904.

Driving the ADJ input from a microcontroller

Another possibility is to drive the device from the open drain output of a microcontroller. The

diagram below shows one method of doing this:

If the NMOS transistor within the microcontroller has high Gate / Drain capacitance, this

arrangement can inject a negative spike into ADJ input of the ZXLD1356 and cause erratic

operation but the addition of a Schottky clamp diode (eg Diodes Inc. SD103CWS) to ground and

inclusion of a series resistor (3.3k) will prevent this. See the section on PWM dimming for more

details of the various modes of control using high frequency and low frequency PWM signals.

PWM

GND

0V

GND

ZXLD1356

ADJ

PWM

GND

ZXLD1356

ADJ

GND

GND

ZXLD1356

ADJ

MCU

3.3k