ZXLD1356Q

Document number: DS37077 Rev. 2 - 2

24 of 28

www.diodes.com

October 2015

© Diodes Incorporated

ZXLD1356Q

Q

Dimming Output Current Using PWM

Low Frequency PWM Mode

nom) when the ADJ pin is low. This will cause the output current to be switched on and off at the PWM frequency, resulting in an average output

Figure 4 Low Frequency PWM Operating Waveforms

The average value of output current in this mode is given by:

S

PWM

OUTavg

R

D

2

.

0

I



for DPWM > 0.001

This mode is preferable if optimum LED 'whiteness' is required. It will also provide the widest possible dimming range (approx. 1000:1) and

higher efficiency at the expense of greater output ripple.

Open and Shorted LED Protection

The ZXLD1356Q has by default open LED protection. If the LEDs should become open circuit the ZXLD1356Q will stop oscillating;

No excessive voltages will be seen by the ZXLD1356Q.

If the LEDs should become shorted together the ZXLD1356Q will continue to switch, however, the duty cycle at which it will operate will

change dramatically and the switching frequency will most likely decrease.

The on-time of the internal power MOSFET switch will be significantly reduced because almost all of the input voltage is now

developed across the inductor.

The off-time will be significantly increased because the reverse voltage across the inductor is now just the Schottky diode voltage

(see Figure 20) causing a much slower decay in inductor current.

During LED-shorted condition the inductor current will remain within its controlled levels and no excessive heat will be generated

within the ZXLD1356Q.