zxld1360 Zetex Semiconductors plc., zxld1360 Datasheet - Page 16

• Manufacturer Part Number: zxld1360
• Description: 1a Led Driver With Internal Switch
• Manufacturer: Zetex Semiconductors plc.

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 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 200k resistor between the ADJ pin and the internal voltage reference as a
pull-up resistor for the external transistor.
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 Drain / Source capacitance , this
arrangement can inject a negative spike into ADJ input of the 1360 and cause erratic operation
but the addition of a Schottky clamp diode (cathode to ADJ) to ground and inclusion of a series
resistor (10K) 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 - March 2007
© Zetex Semiconductors plc 2007
V
ADJ
0V
PWM
MCU
GND
GND
PWM
10k
S
:
ADJ
ADJ
ADJ
ZXLD1360
ZXLD1360
ZXLD1360
GND
GND
GND
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ZXLD1360
www.zetex.com