ACT111A
Rev 0, 14-Feb-11
The following equation determines resistor values:
VDIM ) −VFB
(
MAX
RDIM = R1
(7)
VFB
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
VDIM(MAX) (V)
R1 (kΩ)
RDIM (kΩ)
1470
5
3.3
2
30
30
30
976
576
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.
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