ACT6359
Rev PrA, 20-Nov-07
FUNCTIONAL BLOCK DIAGRAM
IN
SW
EN
ENABLE
CIRCUIT
UVLO
DRIVER
THERMAL
SHUTDOWN
OV COMPARATOR
OV
1.21V
+
-
CONTROL
LOGIC
MAXIMUM
ON TIME
MINIMUM
OFF TIME
G
BANDGAP
REFERENCE
-
+
ERROR
COMPARATOR
0.207V
FB
ACT6359
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.
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:
⎡⎛
V
⎞ ⎤
R
OV 2
=
R
OV1
× ⎢⎜
OV
⎟ −
1
⎥
⎣⎝
1.21V
⎠ ⎦
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.
Setting the LED Current
The LED current is programmed by appropriate
selection of the feedback resistor R
FB
connected
between FB and G. To set the LED current, choose
the resistor according to the equation:
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
Innovative Products.
Active Solutions.
-5-
R
CS
=
V
FB
I
LED
where V
FB
is the FB feedback voltage (typically
290mV at V
EN
= 3.3V) and I
LED
is the desired maxi-
mum LED current.
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