CAT4139 Datasheet, PDF (9/13 Pages) Catalyst 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 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 CAT4139 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 and up to 100kHz.
PWM ^2.5V
Signal _0V
VIN
SW
CAT4139
SHDN
R_A
3.73kΩ
GND
FB
V_FB = 300mV
LED
Current
V_IN
C3
0.22µF
R_B
3.1kΩ
R2
R1
1kΩ
15Ω
Figure 2. Circuit for Filtered PWM Signal
Figure 1. Switching Waveform
with 1kHz PWM on S¯¯H¯D¯N¯
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.
25
20
15
10
5
0
0 10 20 30 40 50 60 70 80 90 100
PWM DUTY CYCLE [%]
Figure 3. Filtered PWM Dimming [0V to 2.5V]
© Catalyst Semiconductor, Inc.
9
Characteristics subject to change without notice
Doc. No. MD-5032 Rev. A