CAT4137 Datasheet, PDF (11/16 Pages) Catalyst Semiconductor – CMOS Boost Converter - White LED Driver

CAT4137
DIMMING CONTROL
There are several methods available to control the
LED brightness.
1. 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. Figures 2 and 3 show 1kHz
and 4kHz signals with a 50% duty cycle applied to the
S¯¯H¯D¯N¯ pin. The PWM frequency range is from 100Hz
to 10kHz. The recommended PWM frequency range
is from 100Hz to 4kHz.
Switching waveforms PWM on S¯¯H¯D¯N¯
2. 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 4 shows the PWM control circuitry connected
to the CAT4137 FB pin. The PWM signal has a
voltage swing of 0V to 2.5V. The LED current can be
dimmed within a range from 0 to 22mA. The PWM
signal frequency can vary from very low frequency up
to 100kHz.
PWM ^2.5V
Signal _0V
VIN
SW
CAT4137
SHDN
R_A
4.02kΩ
GND
FB
V_FB = 300mV
LED
Current
V_IN
C1
0.22µF
R_B
3.3kΩ
R2
R1
1kΩ
15Ω
Figure 2. PWM at 1kHz
Figure 3. PWM at 4kHz
Figure 4. 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 100% duty cycle results in an LED current of 0mA.
LED Current vs. Duty Cycle
25
20
15
10
5
0
0
20 40
60 80 100
DUTY CYCLE [%]
© Catalyst Semiconductor, Inc.
11
Characteristics subject to change without notice
Doc. No. MD-5012Rev. B