CAT4237 datasheet(10/13 Pages) CATALYST

10 / 13 page

CAT4237

10 of 13

Doc. No. 5013, Rev. A

Characteristics subject to change without notice

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,

that

the

average

current

proportional to the duty cycle. A 100% duty

cycle, with SHDN* always high, corresponds

to the LEDs at nominal current. Figure 1

shows a 1kHz signal with a 50% duty cycle

applied

the

SHDN*

pin.

The

recommended PWM frequency range is from

100Hz to 2kHz.

Figure 1. Switching waveform

with 1kHz PWM on SHDN*

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 0 to

20mA. The PWM signal frequency can vary

from very low frequency up to 100kHz.

Figure 2. 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 93% duty cycle or

more, results in an LED current of 0mA.

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