zxld1360 Zetex Semiconductors plc., zxld1360 Datasheet - Page 24

• Manufacturer Part Number: zxld1360
• Description: 1a Led Driver With Internal Switch
• Manufacturer: Zetex Semiconductors plc.

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 2 - Low frequency PWM operating waveforms).
The average value of output current in this mode is given by:
I
This mode is preferable if optimum LED 'whiteness' is required. It will also provide the widest
possible dimming range (approx. 100:1) and higher efficiency at the expense of greater output
ripple.
Note that the low pass filter introduces a small error in the output duty cycle due to the difference
between the start-up and shut-down times. This time difference is a result of the 200mV shutdown
threshold and the rise and fall times at the output of the filter. To minimize this error, the PWM
frequency should be as low as possible consistent with avoiding flicker in the LED(s).
Issue 1 - March 2007
© Zetex Semiconductors plc 2007
Output Current
OUTavg
PWM Voltage
Filter Output
0.1D
PWM
IOUTnom
300mV
/R
V
V
S
0V
0V
0
ADJ
ADJ
[for D
Figure 2 Low frequency PWM operating waveforms
PWM
>0.01]
Ton
24
Toff
ZXLD1360
www.zetex.com
0.1/Rs
200mV
IOUTavg