CAT4238 Datasheet, PDF (9/12 Pages) Catalyst Semiconductor – High Efficiency 10 LED Boost Converter

CAT4238
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 22
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
VIN
SW
CAT4238
SHDN
GND FB
V_FB = 300 mV
3.73 kW 3.1 kW
1 kW
R_{A C1} R_{B i}
R₂
0.22 mF
LED
Current
R₁
15 W
Figure 23. 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 22. 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 23 shows the PWM control
circuitry connected to the CAT4238 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.
5
0
0 10 20 30 40 50 60 70 80 90 100
PWM DUTY CYCLE (%)
Figure 24. Filtered PWM Dimming (0 V to 2.5 V)
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