ACT111A
Rev 2, 22-Oct-12
FUNCTIONAL BLOCK DIAGRAM
FUNCTIONAL DESCRIPTION
The ACT111A is a current-mode step-down 1.5A
LED driver with no extra external compensation
components. It has wide 4.8V to 30V input voltage
range for a variety of power sources. The 100mV low
feedback voltage and an external current sense
resistor makes it deliver LED current programmable
from 20mA to 700mA with efficiency up to 96%. The
device contains an internal, low-resistance, high-
voltage power MOSFET, and operates at a high
1.4MHz operating frequency to ensure a compact,
high-efficiency design with excellent AC and DC
performance. It is in a space saving SOT23-6
package.
The ACT111A is a current mode regulator. It
controls the inductor peak current by the feedback
loop during each switching cycle. Therefore, it
improves loop dynamics. In steady state operation,
a pulse from the oscillator starts a cycle to turn on
the internal top MOSFET switch. Current in the
switch and the external inductor ramps up. As the
current level reaches the voltage level defined by
the internal error amplifier output, the internal switch
is turned off. The current in the inductor flows
through the external Schottky diode. The inductor
current is continuously adjusted by the internal error
amplifier. In the ACT111A, the voltage to the FB pin
compares to the internal accurate 100mV reference
voltage to generate error signal. Therefore, as a
current sense resistor in series with LED is
connected to the FB pin, the LED current is well
regulated.
LED dimming can be performed by directly
Innovative Power
TM
-5-
connecting a PWM signal (frequency rage from
0.1kHz to 10kHz) the DIM pin. If the DIM pin is
unconnected or pulled high, the ACT111A operates
normally.
Inductor Selection
The optimum inductor for a given application has to
be chosen with operation condition. The inductor
current waveform is a triangle with an average
value equal to the load current in continuous
conduction mode (CCM). The peak switch current is
equal to the output current plus half the peak-to-
peak inductor ripple current and is limited to around
1.8A to protect itself and power stage from overload
condition. Therefore, the maximum output current to
a load depends on the switch current limit, the
inductor value, and the input and output voltages.
The peak-to-peak inductor ripple current is usually
controlled to 20%-30% of the output current and the
inductor value is selected accordingly by:
L
=
( 1
−
D )( V
O
+
V
F
)
Δ
I
L
×
f
(1)
where f is 1.4MHz switching frequency of the
ACT111A, V
O
is the output voltage, V
F
is the
Schottky diode forward voltage drop (~0.4V), and D
is switching duty cycle given by:
D
=
V
O
+
V
F
V
IN
+
V
F
(2)
The inductor’s RMS current rating must be greater
than the maximum load current and its saturation
current should be at least 30% higher. For high
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