CAT4201TD-GT3 Datasheet, PDF (10/15 Pages) ON Semiconductor – 350mA High Efficiency Step Down LED Driver

CAT4201
Dimming Methods
Two methods for PWM dimming control on the LEDs
are described below. The first method is to PWM on
the control pin, the other method is to turn on and off a
second resistor connected to the RSET pin and
connected in parallel with R1.
PWM on CTRL Pin
A PWM signal from a microprocessor can be used for
dimming the LEDs when tied to the CTRL pin. The
duty cycle which is the ratio between the On time and
the total cycle time sets the dimming factor. The
recommended PWM frequency on the CTRL pin is
between 100Hz and 2kHz.
VBAT
12V C1
4.7µF
R1
10kΩ
VBAT
CAT4201
RSET
CTRL
SW
GND
PWM
control
5V
R4
0V
1kΩ
R5
47kΩ
D
C2
10µF
L
22µH
R2
1kΩ
Q1
NPN
Figure 3. PWM at 1kHz on CTRL Pin
300
250
200
150
100
50
0
100 80 60 40 20 0
DUTY CYCLE [%]
Figure 4. LED Current vs. Duty Cycle
Figure 5. Circuit for PWM on CTRL
PWM on RSET Pin
Another dimming method is to place in parallel to R1
another resistor with a FET in series, as shown on
Figure 6. R1 sets the minimum LED current corres-
ponding to 0% duty cycle. The combined resistor of
R1 and Rmax sets the maximum LED current
corresponding to 100% duty cycle.
VBAT
13V C1
4.7µF
Rmax
R1
VBAT
CAT4201
RSET
CTRL
SW
GND
PWM
control
OFF ON
D
C2
10µF
L
22µH
R2
1kΩ
Q1
NPN
Figure 6. Circuit for PWM on RSET
Doc. No. MD-5025 Rev. E
10
© 2008 SCILLC. All rights reserved.
Characteristics subject to change without notice