CAT4137_11 Datasheet, PDF (12/15 Pages) ON Semiconductor – CMOS Boost Converter - White LED Driver

CAT4137
Dimming Control
There are several methods available to control the LED
brightness.
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
high, corresponds to the LEDs at nominal current.
Figures 27 and 28 show 1 kHz and 4 kHz signals with a 50%
duty cycle applied to the SHDN pin. The PWM frequency
range is from 100 Hz to 10 kHz. The recommended PWM
frequency range is from 100 Hz to 4 kHz.
Switching Waveforms PWM on SHDN
Filtered PWM Signal
A filtered PWM signal can be used as a variable DC
voltage that can be used to control the LED current.
Figure 29 shows the PWM control circuitry connected to the
CAT4137 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 to 22 mA. The PWM signal frequency can vary from
very low frequency up to 100 kHz.
PWM
Signal
2.5 V
0V
VIN
SW
CAT4137
SHDN
GND FB
LED
Current
R_A
4.02 kW
R_B ^{VFB = 300 mV}
C1
0.22 μF
3.3 kW
R2
1 kW
R1
15 W
Figure 29. 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
100% duty cycle results in an LED current of 0 mA.
25
20
Figure 27. PWM at 1 kHz
15
10
5
0
0
20
40
60
80
100
DUTY CYCLE (%)
Figure 30. LED Current vs. Duty Cycle
Figure 28. PWM at 4 kHz
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