Diodes ZXLD1356/ ZXLD1356Q - Manual (Page 26)

ZXLD1356/ ZXLD1356Q
Document number: DS33470 Rev. 4 - 2
26 of 29
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October 2012
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ZXLD1356/ ZXLD1356Q
A Product Line of
Diodes Incorporated
Application Information
(cont.)
Dimming Output Current Using PWM
Low Frequency PWM Mode
When the ADJ pin is driven with a low frequency PWM signal (eg 100Hz), with a high level voltage VADJ and a low level of zero, the output of
the internal low pass filter will swing between 0V and VADJ, causing the input to the shutdown circuit to fall below its turn-off threshold
(200mV 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 current IOUTavg proportional to the PWM duty cycle. (See Figure 4 - Low frequency PWM operating waveforms).
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.