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CAT4240
Dimming Control
There are several methods available to control the LED
brightness.
VIN
SW
PWM Signal on the SHDN Pin
CAT4240
LED brightness dimming can be done by applying a PWM
signal to the SHDN input. The LED current is repetitively
turned on and off, so that the average current is proportional
to the duty cycle. A 100% duty cycle, with SHDN always
high, corresponds to the LEDs at nominal current. Figure 23
shows a 1 kHz signal with a 50% duty cycle applied to the
SHDN pin. The recommended PWM frequency range is
from 100 Hz to 2 kHz.
PWM
Signal
2.5 V
0V
V IN
SHDN
GND
3.73 k W 3.1 k W
RA RB
C3
0.22 m F
FB
V FB = 300 mV
1 k W
R2
LED
Current
R1
15 W
Figure 24. Circuit for Filtered PWM Signal
A PWM signal at 0 V DC, or a 0% duty cycle, results in
a max LED current of about 22 mA. A PWM signal with a
93% duty cycle or more, results in an LED current of 0 mA.
25
20
15
10
Figure 23. Switching Waveform with 1 kHz PWM on
SHDN
Filtered PWM Signal
A filtered PWM signal used as a variable DC voltage can
control the LED current. Figure 24 shows the PWM control
circuitry connected to the CAT4240 FB pin. The PWM
5
0
0 10 20 30 40 50 60 70 80 90 100
PWM DUTY CYCLE (%)
Figure 25. Filtered PWM Dimming (0 V to 2.5 V)
signal has a voltage swing of 0 V to 2.5 V. The LED current
can be dimmed within a range from 0 mA to 20 mA. The
PWM signal frequency can vary from very low frequency up
to 100 kHz.
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