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FAN5308MPX датащи(PDF) 8 Page - Fairchild Semiconductor |
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FAN5308MPX датащи(HTML) 8 Page - Fairchild Semiconductor |
8 / 11 page ©2005 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN5308 Rev. 1.0.2 8 The N-channel transistor is turned off before the inductor current becomes negative. At this time, the P-channel is switched on again, starting the next pulse. The converter continues these pulses until the high threshold (typical 1.6% above nominal value) is reached. A higher output voltage in PFM mode gives additional headroom for the voltage drop during a load transient from light to full load. The voltage overshoot during this load transient is also minimized due to active regulation during turn on of the N-channel rectifier switch. The device stays in sleep mode until the output voltage falls below the low thresh- old. The FAN5308 enters the PWM mode as soon as the output voltage can no longer be regulated in PFM with constant peak current. 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: where: RdsON = 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 FAN5308 has an internal soft-start circuit that limits the inrush current during start-up. This prevents possible voltage drops of the input voltage and eliminates the out- put voltage overshoot. The soft-start is implemented as a digital circuit, increasing the switch current in four steps to the P-channel current limit (1500mA). Typical start-up time for a 20µF output capacitor and a load current of 800mA is 800µs. Short-Circuit Protection The switch peak current is limited cycle-by-cycle to a typ- ical value of 1500mA. 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 200mA. Thermal Shutdown When the die temperature exceeds 150°C, a reset occurs and remains in effect until the die cools to 130°C. At that time, the circuit is allowed to restart. VOUT = VIN – ILOAD × (RdsON + RL) EQ. 2 |
Аналогичный номер детали - FAN5308MPX |
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Аналогичное описание - FAN5308MPX |
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