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FAN4855 Datasheet(PDF) 8 Page - Fairchild Semiconductor |
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FAN4855 Datasheet(HTML) 8 Page - Fairchild Semiconductor |
8 / 12 page 8 www.fairchildsemi.com FAN4855 Rev. 1.0.1 Block Diagram Functional Description Boost Regulator FAN4855 is an adjustable boost regulator that combines variable ON and minimum OFF architecture with syn- chronous rectification. Unique control circuitry provides high-efficiency power conversion for both light and heavy loads by transitioning between discontinuous and contin- uous conduction mode based on load conditions. There is no oscillator; a constant-peak-current limit of 0.8A in the inductor allows the inductor current to vary between this peak limit and some lesser value. The switching fre- quency depends upon the load, the input and output volt- age ranging up to 430kHz. The input voltage VIN comes to VIN pin and through the external inductor to the VL pin of the device. The loop from VOUT closes through the external resistive voltage divider to the feedback pin VFB. The transfer ratio of this divider determines the output voltage. When VFB voltage drops below the VREF = 1.24V the error amplifier A1 sig- nals to regulator to deliver charge to the output by trig- gering the Variable On-Time One Shot. One Shot generates a pulse at the gate of the Power NMOS tran- sistor Q1. This transistor will charge the Inductor L1 for the time interval TON resulting in a peak current given by: When the one–shot times out, the Q1 transistor releases the VL pin, allowing the inductor to fly-back and momen- tarly charge the output through the body diode of the transistor Q2. But, as the voltage across the Q2 changes polarity, its gate will be driven low by the Synchronous Rectifier Control Circuit (SRC), causing Q2 to short out its body diode. The inductor then delivers the charge to the load by discharging into it through Q2. Under light load conditions, the amount of energy deliv- ered in this single pulse satisfies the voltage-control loop, and the converter does not command any more energy pulses until the output drops again below the lower-volt- age threshold. Under medium and heavy loads, a single energy pulse is not sufficient to force the output voltage above its upper threshold before the minimum off time has expired and a second charge cycle is commanded. Since the inductor current has not reached zero in this case, the peak current is greater than the previous value at the end of the second cycle. The result is a ratcheting of inductor current until either the output voltage is satis- fied, or the converter reaches its set current limit. After a period of time TOFF > 1µS, determined by Mini- mum Off–Time Logic and if VOUT is low (VFB < VREF), the Variable On-Time One Shot will be turned ON again and the process repeats. The output capacitor of the converter filters the variable component, limiting the output voltage ripple to a value determined by its capacitance and its ESR. LBO SHDN Start-Up Minimum Off-Time Logic Variable On-Time One Shot Current Limit Control Synchronous Rectifier Control 47 2 6 5 8 3 1 VIN VL VOUT ILIMIT ILIMIT VREF VOUT GND VFB ILIMIT SHDN N1 LBI Q1 Q2 0.39V A3 Control Logic A2 A1 – + – + – + SHDN I L PEAK () T ON V IN × L 1 -------------------------- = |
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