PS1313

Dimming Control:

LED is a current driven device. Hence, current

through the LED needs to be controlled to have

dimming control. Different ways are there to control

dimming for PS1313, in the normal mode of

operation. In the first way, the feedback voltage is

controlled using an external voltage source. As

shown in the figure-8, current starts flowing down

R1, R2 and R3, as the external voltage increases.

The loop will continue to regulate the feedback

voltage to 100mV, and as a result, the current

through the LEDs has to decrease as the same

amount of that being injected from the external

source. With the external voltage from 0 to 2V, the

resistor values shown for R2 and R3 can control

LED current from 0 to 20mA.

Dimming can also be achieved using logic signal to

EN pin. As shown in Figure-9, the PWM signal can

be applied to the EN pin of PS1313. The LEDs will

switch between full load to completely shut down

state. The average current through the LEDs will be

proportional to the duty cycle of PWM. The PWM

signal in figure-9 should be of 1 KHz or less,

because of the presence of the soft-start

mechanism.

July 2007

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© 2007 Power IC. All Rights Reserved.

7

PS1313

Figure 8: Dimming control using a DC

voltage

PS1313

Figure 9: PWM dimming control using

Logic signal