CAT4139

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10

Dimming Control

There are several methods available to control the LED

brightness.

PWM Signal on the SHDN Pin

LED brightness dimming can be done by applying a PWM

signal to the SHDN 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 SHDN always

high, corresponds to the LEDs at nominal current. Figure 23

shows a 1 kHz signal with a 50% duty cycle applied to the

SHDN pin. The recommended PWM frequency range is from

100 Hz to 2 kHz.

Figure 23. Switching Waveform with 1 kHz

PWM on SHDN

Filtered PWM Signal

A filtered PWM signal used as a variable DC voltage can

control the LED current. Figure 24 shows the PWM control

circuitry connected to the CAT4139 FB pin. The PWM

signal has a voltage swing of 0 V to 2.5 V. The LED current

can be dimmed within a range from 0 mA to 20 mA. The

PWM signal frequency can vary from very low frequency

and up to 100 kHz.

Figure 24. Circuit for Filtered PWM Signal

VIN

SW

CAT4139

0 V

2.5 V

R1

15

W

R2

1 k

W

3.1 k

W

3.73 k

W

C3

0.22

mF

LED

Current

PWM

Signal

FB

GND

SHDN

A PWM signal at 0 V DC, or a 0% duty cycle, results in

a max LED current of about 22 mA. A PWM signal with a

93% duty cycle or more, results in an LED current of 0 mA.

Figure 25. Filtered PWM Dimming (0 V to 2.5 V)

25

20

15

10

5

0

0

102030

4050

60

70

8090 100

PWM DUTY CYCLE (%)