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FAN6520A Fiches technique(PDF) 7 Page - Fairchild Semiconductor |
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FAN6520A Fiches technique(HTML) 7 Page - Fairchild Semiconductor |
7 / 15 page ©2005 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN6520A Rev. 1.0.5 7 Figure 4. Soft-Start Interval The FAN6520A incorporates a MOSFET shoot-through protection method that allows a converter to both sink and source current. Care should be exercised when designing a converter with the FAN6520A when it is known that the converter may sink current. When the converter is sinking current, it is behaving as a boost converter regulating its input voltage. This means that the converter is boosting current into the VCC rail, which supplies the bias voltage to the FAN6520A. If this current has nowhere to go—such as to other distributed loads on the VCC rail, through a voltage limiting protec- tion device, or other methods—the capacitance on the VCC bus absorbs the current. This allows the voltage level of the VCC rail to increase. If the voltage level of the rail is boosted to a level that exceeds the maximum volt- age rating of the FAN6520A, the IC experiences an irre- versible failure and the converter is no longer operational. Ensure that there is a path for the current to follow, other than the capacitance on the rail, to prevent this failure mode. Application Information Layout Considerations In any high-frequency switching converter, layout is very important. Switching current from one power device to another can generate voltage transients across the impedances of the interconnecting bond wires and circuit traces. Use wide, short-printed traces to minimize inter- connecting impedances. The critical components should be located as close together as possible, using ground plane construction or single-point grounding. Figure 5. Printed Circuit Board Power and Ground Planes or Islands Figure 5 shows the critical power components of the con- verter. To minimize voltage overshoot, the interconnect- ing wires (indicated by heavy lines) should be part of a ground or power plane in a printed circuit board. The components shown in Figure 5 should be located as close together as possible. Note that the capacitors CIN and COUT may each represent numerous physical capac- itors. Locate the FAN6520A within two inches of the Q1 and Q2 MOSFETs. The circuit traces for the MOSFETs’ gate and source connections from the FAN6520A must be sized to handle up to 1A peak current. Figure 5 shows the circuit traces that require additional layout consideration. Use single point and ground plane construction for the circuits shown. Minimize any leakage current paths on the COMP/OCSET pin and locate the resistor, ROSCET, close to the COMP/OCSET pin because the internal current source is only 20µA. Pro- vide local VCC decoupling between the VCC and GND pins. Locate the capacitor, CBOOT, as close as practical to the BOOT and PHASE pins. All components used for feedback compensation should be located as close to the IC as practical. Figure 6. PCB Small Signal Layout Guidelines +VOUT Q2 LDRV SW HDRV Q1 CIN LOUT COUT Vin +VOUT Q2 VCC SW BOOT LOUT COUT GND CBOOT CVCC +5V DBOOT FAN6520A Q1 Vin COMP/OCSET +5V |
Numéro de pièce similaire - FAN6520A_06 |
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Description similaire - FAN6520A_06 |
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