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CAT4237
Dimming Control
There are several methods available to control the LED
brightness.
VIN SW
CAT4237
PWM Signal on the SHDN Pin
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
PWN
Signal
2.5 V
3.73 k W
SHDN
GND FB
V FB = 300 mV
3.1 k W 1 k W
LED
Current
R A
high, corresponds to the LEDs at nominal current. Figure 25
shows a 1 kHz signal with a 50% duty cycle applied to the
SHDN pin. The recommended PWM frequency range is
0V
C1
0.22 m F
R B
i
R 2
R 1
15 W
from 100 Hz to 2 kHz.
Figure 26. 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
5
Figure 25. Switching Waveform with 1 kHz PWM on
SHDN
0
0
10
20
30
40
50
60
70
80
90 100
Filtered PWM Signal
A filtered PWM signal used as a variable DC voltage can
PWM DUTY CYCLE (%)
Figure 27. Filtered PWM Dimming (0 V to 2.5 V)
control the LED current. Figure 26 shows the PWM control
circuitry connected to the CAT4237 FB pin. The PWM
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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