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FAN5182QSCX_NL Arkusz danych(PDF) 11 Page - Fairchild Semiconductor |
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FAN5182QSCX_NL Arkusz danych(HTML) 11 Page - Fairchild Semiconductor |
11 / 18 page 11 www.fairchildsemi.com FAN5182 Rev. 1.0.1 Power Good Monitoring The power good comparator monitors the output voltage via the FB pin. The PWRGD pin is an open-drain output whose high level (when connected to a pull-up resistor) indicates that the output voltage is within the nominal limits specified in the electri- cal characteristic table. PWRGD goes low if the output voltage is outside of this specified range or whenever the EN pin is pulled low. Figure 8. shows the PWRGD response when the input power supply is switched off. Figure 8. Shutdown Waveforms As part of the protection for the load and output components of the supply, the PWM outputs are driven low (turning on the low- side MOSFETs) when the output voltage exceeds the crowbar trip point. This crowbar action stops once the output voltage falls below the reset threshold of approximately 650mV. Turning on the low-side MOSFETs pulls down the output as the reverse current builds up in the inductors. If the output over volt- age is due to a short in the high-side MOSFET, this crowbar action can trip the input supply over current protection or blow the input fuse, protecting the load from being damaged. Enable and UVLO For the FAN5182 to begin switching, the input supply (VCC) to the controller must be higher than the UVLO threshold, and the EN pin must be higher than its logic threshold. If UVLO is less than the threshold or the EN pin is logic low, the FAN5182 is dis- abled. This holds the PWM outputs at ground, shorts the DELAY capacitor to ground, and holds the ILIMIT pin at ground. In the application circuit, the ILIMIT pin should be connected to the OD pins of the FAN5009 drivers. Grounding the ILIMIT pin will disable the drivers such that both HDRV and LDRV are hold- ing low. This feature is important in preventing fast discharge of the output capacitors when the controller shuts off. If the driver outputs are not being disabled, a negative output voltage can be generated due to high current being discharged from the output capacitors through the inductors. |
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