zxld1362 Zetex Semiconductors plc., zxld1362 Datasheet - Page 21

• Manufacturer Part Number: zxld1362
• 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 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.
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 1362 and cause erratic operation
but the addition of a Schottky clamp diode (cathode to ADJ) 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 - December 2007
© Zetex Semiconductors plc 2007
V
ADJ
0V
PWM
MCU
GND
GND
P WM
3.3k
S
:
ADJ
ADJ
ADJ
ZXLD1362
ZXLD1362
ZXLD1362
GND
GND
GND
21
PWM
can be applied to the ADJ pin, as
ZXLD1362
www.zetex.com