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FAN8060 Datasheet(PDF) 6 Page - Fairchild Semiconductor |
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FAN8060 Datasheet(HTML) 6 Page - Fairchild Semiconductor |
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6 / 13 page  © 2008 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN8060 • Rev. 1.0.0 6 Operation Description The FAN8060 is a step-down converter operating in current-mode PWM architecture with a typical switching frequency of 1.2MHz. 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 1.2A, 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 4μA internal current source, causing the voltage on the capacitor to rise. When this voltage reaches 1.2V, 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: () V A nF C ms t SS 2 . 1 / 4 ) ( ) ( μ = (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. |
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Similar Description - FAN8060 |
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