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FAN6520BM データシート(PDF) 9 Page - Fairchild Semiconductor |
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FAN6520BM データシート(HTML) 9 Page - Fairchild Semiconductor |
9 / 14 page 9 www.fairchildsemi.com FAN6520B Rev. 1.0.3 Depending upon the whether there is a load application or a load removal, the response time to a load transient (ISTEP) is different. The following equations give the approximate response time interval for application and removal of a transient load: where TRISE is the response time to the application of a positive ISTEP, and TFALL is the response time to a load removal (negative ISTEP). The worst case response time can be either at the application or removal of load. Be sure to check both of these equations at the minimum and maximum output levels for the worst case response time. Input Capacitor Selection Use a mix of input bypass capacitors to control the volt- age overshoot across the MOSFETs. Use small ceramic capacitors for high-frequency decoupling and bulk capacitors to supply the current needed each time Q1 turns on. Place the small ceramic capacitors physically close to the MOSFETs and between the drain of Q1 and the source of Q2. The important parameters for the bulk input capacitor are the voltage rating and the RMS cur- rent rating. For reliable operation, select the bulk capaci- tor with voltage and current ratings above the maximum input voltage and the largest RMS current required by the circuit. The capacitor voltage rating should be at least 1.25 times greater than the maximum input voltage and 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; . For a through-hole design, several electrolytic capacitors may be needed. For surface-mount designs, solid tantalum capacitors can be used, but caution must be exercised 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 repu- table 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. Selection of these components should be done after the high-side MOSFET has been chosen. The required capacitance is determined using the following equation: where QG is the total gate charge of the high-side MOS- FET, and ∆V 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. If FB is < 800mV for 32 con- secutive cycles, then LDRV is turned on for ~1.6µs to charge the bootstrap capacitor. Thermal Considerations Total device dissipation: PD = PQ + PHDRV + PLDRV (4) where PQ represents quiescent power dissipation: PQ = VCC × 2.7mA (5) PHDRV represents internal power dissipation of the upper FET driver. PHDRV = PH(R) × PH(F) (6) Where PH(R) and PH(F) are internal dissipations for the rising and falling edges respectively: where: PQ1 = QG1 × VGS(Q1) × FSW (9) 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 8. Figure 8. 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 Fairchild app note AN-6003, “Shoot-through” in Synchronous Buck Con- verters. (http://www.fairchildsemi.com/an/AN/AN-6003.pdf) T RISE LI STEP × V IN V OUT – ------------------------------ = T FALL LI STEP × V OUT ------------------------ = I RMS I L DD 2 – () = (2) D V OUT V IN -------------- = C BOOT Q G ∆V BOOT ---------------------- = (3) P HR () P Q1 R HUP R HUP R E R G ++ ------------------------------------------- × = (7) P HF () P Q1 R HDN R HDN R E R G ++ -------------------------------------------- × = (8) HDRV Q1 G R G RE RHUP BOOT SW R HDN S |
同様の部品番号 - FAN6520BM |
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同様の説明 - FAN6520BM |
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