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FAN6520AIMX датащи(PDF) 10 Page - Fairchild Semiconductor |
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FAN6520AIMX датащи(HTML) 10 Page - Fairchild Semiconductor |
10 / 15 page ©2005 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN6520A Rev. 1.0.5 10 1.25 times greater than the maximum input voltage. A voltage rating of 1.5 times is a conservative guideline. The RMS current rating requirement (IRMS) for the input capacitor of a buck regulator is: where the converter duty cycle is . For a through-hole design, several electrolytic capacitors may be needed. For surface-mount designs, solid tanta- lum capacitors can be used, but caution must be exer- cised with regard to the capacitor’s surge current rating. The capacitors must be capable of handling the surge current at power-up. Some capacitor series available from reputable manufacturers are surge current tested. Bootstrap Circuit The bootstrap circuit uses a charge storage capacitor (CBOOT) and the internal diode, as shown in Figure 1. Select these components after the high-side MOSFET has been chosen. The required capacitance is deter- mined using the following equation: where QG is the total gate charge of the high-side MOS- FET and ΔV BOOT is the voltage droop allowed on the high-side MOSFET drive. To prevent loss of gate drive, the bootstrap capacitance should be at least 50 times greater than the CISS of Q1. Thermal Considerations Total device dissipation: PD = PQ + PHDRV + PLDRV (14) where PQ represents quiescent power dissipation. PQ = VCC × [4mA + 0.036 (FSW – 100)] (15) where FSW is switching frequency (in kHz). PHDRV represents internal power dissipation of the upper FET driver. PHDRV = PH(R) × PH(F) (16) where PH(R) and PH(F) are internal dissipations for the rising and falling edges respectively. where: PQ1 = QG1 × VGS(Q1) × FSW (19) where QG1 is total gate charge of Q1 for its applied VGS. As described in the equations above, the total power consumed in driving the gate is divided in proportion to the resistances in series with the MOSFET's internal gate node, as shown in Figure 9. Figure 9. Driver Dissipation Model RG is the polysilicon gate resistance internal to the FET. RE is the external gate drive resistor implemented in many designs. Note that the introduction of RE can reduce driver power dissipation, but excess RE may cause errors in the “adaptive gate drive” circuitry. For more information, please refer to Application Note AN- 6003, “Shoot-through” in Synchronous Buck Converters at http://www.fairchildsemi.com/an/AN/AN-6006.pdf. PLDRV is dissipation of the lower FET driver. PLDRV = PL(R) × PL(F) (20) where PH(R) and PH(F) are internal dissipations for the rising and falling edges, respectively: where: PQ2 = QG2 × VGS(Q2) × FSW. (23) Power MOSFET Selection For more information on MOSFET selection for synchro- nous buck regulators, refer to: AN-6005: Synchronous Buck MOSFET Loss Calculations at http://www.fairchild- semi.com/an/AN/AN-6005.pdf. Losses in a MOSFET are the sum of its switching (PSW) and conduction (PCOND) losses. In typical applications, the FAN6520A converter's output voltage is low with respect to its input voltage; therefore the lower MOSFET (Q2) is conducting the full load cur- rent for most of the cycle. Choose a MOSFET for Q2 that has low RDS(ON) to minimize conduction losses. In contrast, the high-side MOSFET (Q1) has a much shorter duty cycle and its conduction loss has less impact. Q1, however, sees most of the switching losses, so Q1’s primary selection criteria should be gate charge. I RMS I L DD 2 – () = (12) D V OUT V IN -------------- = C BOOT Q G ΔV BOOT -------------------------- = (13) P HR () P Q1 R HUP R HUP R E R G ++ -------------------------------------------- × = (17) P HF () P Q1 R HDN R HDN R E R G ++ -------------------------------------------- × = (18) HDRV Q1 G R G RE RHUP BOOT SW R HDN S P LR () P Q2 R LUP R LUP R E R G ++ ------------------------------------------- × = (21) P LF () P Q2 R LDN R HDN R E R G ++ -------------------------------------------- × = (22) |
Аналогичный номер детали - FAN6520AIMX |
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Аналогичное описание - FAN6520AIMX |
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