zxld1362 Zetex Semiconductors plc., zxld1362 Datasheet - Page 29

• Manufacturer Part Number: zxld1362
• Description: 1a Led Driver With Internal Switch
• Manufacturer: Zetex Semiconductors plc.

ZXLD1362
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:
I
0.1D
/R
[for D
>0.001]
OUTavg
PWM
S
PWM
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.
Issue 1 - December 2007
29
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© Zetex Semiconductors plc 2007