ACT6359

Rev 1, 14-Feb-11

- 5 -

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Copyright © 2011 Active-Semi, Inc.

FUNCTIONAL BLOCK DIAGRAM

Control Scheme

The ACT6359 uses a minimum off-time, current-

mode control scheme to achieve excellent perform-

ance 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 operation.

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) 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

another switching begins when the error compara-

tor detects that the output is falling out of regulation

again.

Over Voltage Protection

The ACT6359 includes internal over-voltage pro-

tection 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 voltage drops to

zero, and the control switches at maximum on time

causing the output voltage to keep rising until it ex-

ceeds the maximum voltage rating of the power

MOSFET. The over-voltage protection circuit de-

tects this condition and switching ceases if the volt-

age 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.

Setting the LED Current

The LED current is programmed by appropriate

between FB and G. To set the LED current, choose

the resistor according to the equation:

mum LED current.

DRIVER

ENABLE

CIRCUIT

UVLO

CONTROL

LOGIC

THERMAL

SHUTDOWN

BANDGAP

REFERENCE

MAXIMUM

ON TIME

MINIMUM

OFF TIME

1.21V

OV COMPARATOR

+

-

ERROR

COMPARATOR

+

-

EN

G

FB

IN

OV

SW

ACT6359

0.29V

⎥

⎦

⎤

⎢

⎣

⎡

−

⎟

⎠

⎞

⎜

⎝

⎛

×

=

1

V

21

.

1

V

R

R

OV

1

OV

2

OV

LED

FB

FB

I

V

R

=