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Os motores de busca de Datasheet de Componentes eletrônicos |
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Os motores de busca de Datasheet de Componentes eletrônicos |
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FAN2012I Folha de dados(PDF) 7 Page - Fairchild Semiconductor |
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FAN2012I Folha de dados(HTML) 7 Page - Fairchild Semiconductor |
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7 / 10 page  7 www.fairchildsemi.com FAN2011/FAN2012 Rev. 1.0.4 Block Diagram Figure 4. Block Diagram Detailed Operation Description The FAN2011 is a step-down pulse width modulated (PWM) current mode converter with a typical switching frequency of 1.3MHz. At the beginning of each clock cycle, the P-channel transistor is turned on. The inductor current ramps up and is monitored via an internal circuit. The P-channel switch is turned off when the sensed cur- rent causes the PWM comparator to trip when the output voltage is in regulation or when the inductor current reaches the current limit (set internally, typically 2600mA). After a minimum dead time, the N-channel transistor is turned on and the inductor current ramps down. As the clock cycle is completed, the N-channel switch is turned off and the next clock cycle starts. The duty cycle is solely given by the ratio of output voltage and input voltage. Therefore, the converter runs with a minimum duty cycle when output voltage is at minimum and input voltage is at maximum. 100% Duty Cycle Operation As the input voltage approaches the output voltage and the duty cycle exceeds the typical 95%, the converter turns the P-channel transistor continuously on. In this mode, the output voltage is equal to the input voltage minus the voltage drop across the P-channel transistor: VOUT = VIN – ILOAD × (RDS_ON + RL), where RDS_ON = P-channel switch ON resistance ILOAD = Output current RL = Inductor DC resistance UVLO and Soft Start The reference and the circuit remain reset until the VIN crosses its UVLO threshold. The FAN2011 has an internal soft-start circuit that limits the in-rush current during start-up. This prevents possi- ble voltage drops of the input voltage and eliminates the output voltage overshoot. The soft-start is implemented as a digital circuit, increasing the switch current in four steps to the P-channel current limit (2600mA). Typical start-up time for a 40µF output capacitor and a load cur- rent of 1500mA is 800µs. Short Circuit Protection The switch peak current is limited cycle by cycle to a typ- ical value of 2600mA. In the event of an output voltage short circuit, the device operates with a frequency of 400kHz and minimum-duty cycle, therefore the average input current is typically 350mA. Thermal Shutdown When the die temperature exceeds 150°C, a reset occurs and remains in effect until the die cools to 130°C, at which point, the circuit restarts. FB ERROR AMP REF FB IS CURRENT SENSE DIGITAL SOFT START UNDER VOLTAGE LOCKOUT LOGIC CONTROL MOSFET DRIVER OSC SLOPE COMPENSATION COMP OVER VOLTAGE COMP IS GND SW VIN EN GND 0.8V |
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