ACT111A datasheet(7/12 Pages) ACTIVE-SEMI

7 / 12 page

ACT111A

Rev 0, 14-Feb-11

Innovative PowerTM

- 7 -

www.active-semi.com

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VDIM(MAX) (V)

R1 (kΩ)

RDIM (kΩ)

3.3

976

576

1470

The following equation determines resistor values:

According to the equation, Table 1 shows the

respective resister values with different DC dimming

voltage. VTB is 100mV, R1 is chosen to be 30kΩ.

Table 1:

Open LED Protection

In case of LED failure, the ACT111A will operate at

maximum duty cycle due to the feedback voltage

drops to zero. This will results in the output voltage

moving up. To prevent over voltage on the output, a

Zener and a series resistor are used as shown in

Figure 3 and Figure 6 (ZD1 and R4).

Thermal Shutdown

The ACT111A automatically turns off when the IC

junction temperature exceeds 160°C, and reenables

when the IC junction temperature drops by 10°C (typ).

PC Board Layout

To achieve good performance, it is extremely

important to have optimized component placement

and layout on PCB for a high switching frequency

and

high

efficiency

regulator.

Here

are

recommendations for the layout: Place input

capacitor to IN pin, inductor and diode to SW pin as

close as possible to reduce the voltage ringing at

these pins. Place the current sense resistor close to

FB pin. Minimize ground noise by connecting high

current ground returns, the input capacitor ground

lead, and the output filter ground lead to a single

point (star ground configuration). There are two

power loops in normal operation, one is formed

when the SW is high and the high current flows

through input capacitor, internal MOSFET, inductor,

LEDs, RSENSE to ground. The other loop is through

inductor, LEDs, RSENSE, ground to diode. Make

these loop areas as small as possible to minimize

noise interaction. SW pad is a noisy node switching

from VIN to GND. It should be isolated away from

the rest of circuit for good EMI and low noise

operation.

()

MAX

DIM

−

(7)

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