ACT111A

Rev 0, 14-Feb-11

- 5 -

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F

IN

F

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V

V

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D

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V

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L

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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

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:

where f is 1.4MHz switching frequency of the

Schottky diode forward voltage drop (~0.4V), and D

is switching duty cycle given by:

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

IN

DIM

FB

G

REFERENCE

& THERMAL

SHUTDOWN

CONTROL

PWM Comparator

ILIM Comparator

REGULATOR

& UVLO

OSCILLATOR

COMPENSATION

DRIVER

CSA

Q1

BST

SW

EA

LOGIC