cat4237 Catalyst Semiconductor, cat4237 Datasheet - Page 10

• Manufacturer Part Number: cat4237
• Description: High Voltage Cmos Boost White Led Driver
• Manufacturer: Catalyst Semiconductor

CAT4237
DIMMING CONTROL
There are several methods available to control the
LED brightness.
PWM signal on the SHDN
LED brightness dimming can be done by applying a
PWM signal to the SHDN
repetitively turned on and off, so that the average
current is proportional to the duty cycle. A 100% duty
cycle, with SHDN
LEDs at nominal current. Figure 1 shows a 1kHz
signal with a 50% duty cycle applied to the SHDN
The recommended PWM frequency range is from
100Hz to 2kHz.
Doc. No. MD-5013, Rev. B
Figure 1. Switching Waveform
¯¯¯¯¯ always high, corresponds to the
with 1kHz PWM on SHDN
¯¯¯¯¯ Pin
¯¯¯¯¯ input. The LED current is
¯¯¯¯¯
¯¯¯¯¯ pin.
10
Signal
PWN
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 CAT4237 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.
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.
2.5V
0V
Figure 2. Circuit for Filtered PWM Signal
25
20
15
10
5
0
0
VIN
10
FILTERED PWM DIMMING [0V to 2.5V]
20
3.73kΩ
R
0.22µF
PWM DUTY CYCLE [%]
A
30
C1
40
Characteristics subject to change without notice
3.1kΩ
SHDN
VIN
GND
R
50
B
CAT4237
60
FB
© Catalyst Semiconductor, Inc.
i
V
SW
FB
70
= 300mV
1kΩ
80
R
2
90 100
R 1
15Ω
LED
Current