ACT6311

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www.active-semi.com

Copyright © 2011 Active-Semi, Inc.

Rev 3, 22-Feb-11

Start-up and Inrush Current

In order to facilitate quick startup, a soft-start circuit

is not incorporated into the ACT6311. When the IC

is first turned on with no external soft-start circuit,

the peak inrush current is about 400mA. Figure 8

shows an implementation for soft-start. When

soft-start

and

dimming

controls

are

used

simultaneously, a low frequency PWM signal (less

than 10kHz) or the methods in Figures 5, 6 and 7

should be used.

Open-Circuit Protection (White LEDs)

If one of the LEDs is disconnected, the FB voltage

drops to zero and the IC switches at maximum duty

cycle. This results in a high voltage that may

exceed the SW voltage rating. To limit this voltage,

use a Zener diode as shown in Figure 9. The Zener

voltage must be large than the total forward voltage

of the LEDs and the current rating should be higher

than 0.1mA.

Board Layout

To reduce EMI, minimize the area and path length

of all traces connected to SW. Use a ground plane

under the switching regulator and connect R1

directly to the G pin of the IC.

ACT6311

FB

R2

56k

LED1

LED2

LED3

R1

62Ω

R3

33.4k

VDC

ACT6311

FB

R2

56k

LED1

LED2

LED3

R1

62Ω

R3

33.4k

C1

0.1µF

R4

10k

PWM

Figure 4. Current Setting for White LED Application

Figure 5. DC Voltage Controlled Dimming

Figure 6. Filtered PWM Controlled Dimming

Figure 7. Logic Controlled Dimming

ACT6311

FB

LED1

LED2

LED3

R1

R2

LOGIC