ACT6357/ACT6358
Rev0, 03-Feb-08
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.
where t
SS
is the required soft start duration. In a
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:
⎡⎛
V
⎞ ⎤
R
OV 2
=
R
OV1
× ⎢⎜
OV
⎟ −
1
⎥
V
⎣⎝
1.21
⎠ ⎦
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
connecting a capacitor C
SS
between the SS pin and
G. The soft start duration can be calculated from
the following equation:
C
SS
=
t
SS
×
5 µF
s
where V
OV
is the over voltage detection threshold,
R
OV1
is the resistor between OV and G, and R
OV2
is
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.
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