cat4240 Catalyst Semiconductor, cat4240 Datasheet - Page 10

• Manufacturer Part Number: cat4240
• Description: 6 Watt Boost Led Driver
• Manufacturer: Catalyst Semiconductor

CAT4240
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-5026 Rev. C
Figure 1. Switching Waveform
¯¯¯¯¯ always high, corresponds to the
with 1kHz PWM on SHDN
¯¯¯¯¯ input. The LED current is
¯¯¯¯¯ Pin
¯¯¯¯¯
¯¯¯¯¯ pin.
10
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 CAT4240 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.
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.
PWM
Figure 3. Filtered PWM Dimming [0V to 2.5V]
2.5V
Figure 2. Circuit for Filtered PWM Signal
25
20
15
10
5
0
0V
0
10
V
20
IN
PWM DUTY CYCLE [%]
0.22µF
3.73kΩ
30
RA
C3
40
Characteristics subject to change without notice
VIN
SHDN
GND
50
CAT4240
3.1kΩ
RB
60
FB
SW
V
FB
70
© Catalyst Semiconductor, Inc.
= 300mV
1kΩ
R2
80
90 100
R1
15Ω
LED
Current