Diodes ZXLD1356/ ZXLD1356Q - Manual (Page 20)

ZXLD1356/ ZXLD1356Q
Document number: DS33470 Rev. 4 - 2
20 of 29
www.diodes.com
October 2012
© Diodes Incorporated
ZXLD1356/ ZXLD1356Q
A Product Line of
Diodes Incorporated
Application Information
(cont.)
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
ZXLD1356 A DJ
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:
GND
ZXLD1356 ADJ
MCU
3.3k
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.
Shutdown Mode
Taking the ADJ pin to a voltage below 0.2V for more than approximately 100µs will turn off the output and supply current to a low standby level of
65µA nominal.
Note that the ADJ pin is not a logic input. Taking the ADJ pin to a voltage above V
REF
will increase output current above the 100% nominal
average value. (See page 17 graphs for details).
Soft-Start
An external capacitor from the ADJ pin to ground will provide a soft-start delay, by increasing the time taken for the voltage on this pin to rise to
the turn-on threshold and by slowing down the rate of rise of the control voltage at the input of the comparator. Adding capacitance increases this
delay by approximately 0.2ms/nF. The graph on the next page shows the variation of soft-start time for different values of capacitor.