ZXLD1352 Просмотр технического описания (PDF) - Zetex => Diodes

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ZXLD1352

350mA LED driver with internal switch and enhanced PWM dimming range

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ZXLD1352

Output current adjustment by PWM control

Directly driving ADJ input

A Pulse Width Modulated (PWM) signal with duty cycle DPWM can be applied to the ADJ pin, as

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

set by resistor RS:

VADJ

PWM

0V

ADJ

ZXLD1352

GND

GND

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:

PWM

GND

ADJ ZXLD1352

GND

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:

MCU

10k

ADJ ZXLD1352

GND

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

arrangement can inject a negative spike into ADJ input of the ZXLD1352 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

Issue 1 - August 2008

14

© Diodes Incorporated 2008

www.zetex.com

www.diodes.com

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