zxld1352 Zetex Semiconductors plc., zxld1352 Datasheet - Page 14

• Manufacturer Part Number: zxld1352
• Description: 350ma Led Driver With Internal Switch And Enhanced Pwm Dimming Range
• Manufacturer: Zetex Semiconductors plc.

Output current adjustment by PWM control
Directly driving ADJ input
A Pulse Width Modulated (PWM) signal with duty cycle D
shown below, to adjust the output current to a value above or below the nominal average value
set by resistor R
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 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
© Diodes Incorporated 2008
V
ADJ
0V
PWM
MCU
GND
GND
PWM
S
10k
:
ADJ
ADJ
ADJ
ZXLD1352
ZXLD1352
ZXLD1352
GND
GND
GND
14
PWM
can be applied to the ADJ pin, as
ZXLD1352
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