ACT6357/ACT6358

Rev 1, 15-Nov-12

- 5 -

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Copyright © 2012 Active-Semi, Inc.

FUNCTIONAL BLOCK DIAGRAM

Control Scheme

The ACT6357 and ACT6358 use a minimum off-

time, current-mode control scheme to achieve ex-

cellent performance under high duty-cycle operating

conditions. This control scheme initiates a switching

cycle only when needed to maintain output voltage

regulation, resulting in very high efficiency opera-

tion.

During each switching cycle, the N-channel power

MOSFET turns on, increasing the inductor current.

The switching cycle terminates when either the in-

ductor current reaches the current limit (500mA for

the ACT6357, 1A for the ACT6358) or when the

cycle lasts longer than the maximum on-time of

4µs. Once the MOSFET turns off, it remains off for

at least the minimum off-time of 320ns, then an-

other switching begins when the error comparator

detects that the output is falling out of regulation

again.

Soft-Start

The ACT6357 and ACT6358 include a programma-

ble soft-start function, which can be used to opti-

mize an application between start-up time and

start-up inrush current. Soft start is achieved by

G. The soft start duration can be calculated from

the following equation:

typical application, use 0.1µF to generate 20ms soft

start time.

Over Voltage Protection

Both the ACT6357 and ACT6358 include internal

over-voltage protection circuitry that monitors the

OV pin voltage. Over-voltage protection is critical

when one of the LEDs in the LED string fails as an

open circuit. When this happens the feedback volt-

age drops to zero, and the control switches at maxi-

mum on time causing the output voltage to keep

rising until it exceeds the maximum voltage rating of

the power MOSFET. The ACT6357 and ACT6358's

over-voltage protection detects this condition and

switching ceases if the voltage at the OV pin

reaches 1.21V.

To set the maximum output voltage, connect a re-

sistor divider from the output node to G, with center

tap at OV, and select the two resistors with the fol-

lowing equation:

the resistor from the output to the OV pin. As a first

estimate, the OV threshold can often be set to 4V

times the number of LEDs in the string.

⎥

⎦

⎤

⎢

⎣

⎡

−

⎟

⎠

⎞

⎜

⎝

⎛

×

=

1

V

21

.

1

V

R

R

OV

1

OV

2

OV

s

F

μ

5

t

C

SS

SS

×

=