CAT4238_07 Datasheet, PDF (10/14 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 S^¯¯H^¯D^¯N^¯ Pin
LED brightness dimming can be done by applying a
PWM signal to the S¯¯H¯D¯N¯ 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 S¯¯H¯D¯N¯ always high, corresponds to the
LEDs at nominal current. Figure 1 shows a 1kHz
signal with a 50% duty cycle applied to the S¯¯H¯D¯N¯ pin.
The recommended PWM frequency range is from
100Hz to 2kHz.
Filtered PWM Signal
A filtered PWM signal used as a variable DC voltage
can control the LED current. Figure 2 shows the
PWM control circuitry connected to the CAT4238 FB
pin. The PWM signal has a voltage swing of 0V to
2.5V. The LED current can be dimmed within a range
from 0mA to 20mA. The PWM signal frequency can
vary from very low frequency up to 100kHz.
PWM ^2.5V
Signal _0V
3.73kΩ
R_{A C1}
0.22µF
VIN
SW
CAT4238
SHDN
GND
FB
V_FB = 300mV
3.1kΩ
1kΩ
R_B
i
R₂
LED
Current
R1
15Ω
Figure 1. Switching Waveform
with 1kHz PWM on S¯¯H¯D¯N¯
Figure 2. Circuit for Filtered PWM Signal
A PWM signal at 0V DC, or a 0% duty cycle, results
in a max LED current of about 22mA. A PWM signal
with a 93% duty cycle or more, results in an LED
current of 0mA.
FILTERED PWM DIMMING [0V to 2.5V]
25
20
15
10
5
0
0 10 20 30 40 50 60 70 80 90 100
PWM DUTY CYCLE [%]
Doc. No. MD-5019 Rev. C
10
© Catalyst Semiconductor, Inc.
Characteristics subject to change without notice