CAT4139_16 Datasheet, PDF (10/13 Pages) ON Semiconductor – 22V High Current Boost White LED Driver

CAT4139
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. 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.
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 CAT4139 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
and up to 100 kHz.
PWM
Signal
2.5 V
V_IN
0V
VIN
SW
CAT4139
SHDN
GND FB
R_A
3.73 kW
R_B ^VFB ^{= 300 mV}
C3
0.22 mF
3.1 kW
R2
1 kW
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
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)
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