FAN8060 データシートの表示（PDF） - ON Semiconductor
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FAN8060 Datasheet PDF : 14 Pages
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Operation Description
The FAN8060 is a step-down converter operating in
current-mode PWM architecture with a typical switching
frequency of 1.2 MHz. At the beginning of each clock
cycle, the P-channel transistor is turned on. The current
in the inductor ramps up and is sensed via an internal
circuit. The P-channel switch is turned off when the
sensed current causes the PWM comparator to trip,
which is when the output voltage is in regulation or
when the inductor current reaches the current limit (set
internally to 1.2 A, typically). After a minimum dead time
to prevent shoot-through current, the N-channel
transistor is turned on and the current ramps down. As
the clock cycle is completed, the N-channel switch is
turned off and the next clock cycle starts.
Light Load Operation
As the output load reduces, the current in the inductor
during off time is sensed across the low side MOSFET.
When the current reverses direction, the low-side
MOSFET is turned off and the high-side MOSFET is not
turned on until the output is out of regulation.
100% Duty Cycle Operation
As the input voltage approaches the output voltage, the
controller starts to increase the duty cycle to maintain
output regulation until duty cycle reaches 85%. The
controller then transitions to a 100% duty cycle mode
over several cycles to support the load. When the
dropout condition is met, the converter turns the P-
channel high side continuously on. In this mode, the
output voltage is equal to the input voltage, minus the
voltage drop across the P-channel MOSFET.
Soft Start
When the input voltage on AVIN exceeds the UVLO
threshold and EN is high, the circuit releases SS and
enables the PWM regulator. A capacitor connected to
the SS pin and AGND is charged by a 4 μA internal
current source, causing the voltage on the capacitor to
rise. When this voltage reaches 1.2 V, the output is in
regulation. The SS voltage continues to rise to AVIN.
The time for the output to reach regulation is given by
the following equation:
t(ms)
=
CSS (nF )
(4µA /1.2V
)
(1)
Output overload and short-circuit protection is active
during soft-start. When the part is disabled, SS pin is
pulled low internally.
Overload & Short-Circuit Protection
FAN8060 employs cycle-by-cycle current limiting, which
limits current by reducing duty cycle during overload. As
the load increases beyond the limit, the output voltage
starts to reduce, thereby reducing the FB voltage. When
the FB node is half the reference voltage and the
COMP node has reached maximum value, short-circuit
protection is detected. At that time, both the SS pin and
the COMP pin are pulled to ground until the inductor
current crosses zero. At that point, both SS and COMP
are released for the current to ramp up again. This
continues until the short-circuit condition is released.
© 2013 Fairchild Semiconductor Corporation
FAN8060 • Rev. 1.0.1
6
www.fairchildsemi.com
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