Zxld1352, Dimming output current using pwm – Diodes ZXLD1352 User Manual
Page 19
ZXLD1352
ZXLD1352
Document number: DS33469 Rev. 3 - 2
19 of 21
December 2010
© Diodes Incorporated
A Product Line of
Diodes Incorporated
Dimming Output Current Using PWM
When the ADJ pin is driven with a low frequency PWM signal (eg 100Hz), with a high level voltage V
ADJ
and a low level of zero,
the output current will be switched on and off at the PWM frequency, resulting in an average output current I
OUTavg
proportional
to the PWM duty cycle. (See Figure 2)
Figure 3. Low Frequency PWM Operating Waveforms
The average value of output current in this mode is given by:
The average value of output current is given by:
I
OUTavg
= 0.1D
PWM
/R
S
for D
PWM
>0.01
PWM dimming is preferable to DC dimming 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.
V
ADJ
PWM Voltage
Ton
IOUTavg
0V
0
Toff
0.1/Rs
IOUTnom
Output Current