zxld1352 Zetex Semiconductors plc., zxld1352 Datasheet - Page 20

• Manufacturer Part Number: zxld1352
• Description: 350ma Led Driver With Internal Switch And Enhanced Pwm Dimming Range
• Manufacturer: Zetex Semiconductors plc.

ZXLD1352
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
and a low level of zero, the output current will be switched on and off at the PWM frequency,
ADJ
resulting in an average output current I
proportional to the PWM duty cycle. (See Figure 2
OUTavg
V
ADJ
Ton
Toff
PWM Voltage
0V
IOUTnom
0.1/Rs
Output Current
IOUTavg
0
Figure 2 Low frequency PWM operating waveforms
The average value of output current is given by:
I
= 0.1D
/R
[for D
>0 01]
OUTavg
PWM
S
PWM
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.
Issue 1 - August 2008
20
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