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AN-53 Datasheet(PDF) 8 Page - Fairchild Semiconductor |
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AN-53 Datasheet(HTML) 8 Page - Fairchild Semiconductor |
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8 / 20 page  AN53 APPLICATION NOTE 8 Two MOSFETs in parallel. If output current is high, We recommend two MOSFETs used in parallel instead of one single MOSFET. The follow- ing significant advantages are realized using two MOSFETs in parallel: • Significant reduction of Power dissipation. Example: RC5051 with Maximum output current of 14A at 2.8V with one MOSFET on the high side: PMOSFET = (I 2 R DS,ON)(Duty Cycle) = (14A)2 (0.050 W)(2.8V / 5V) = 5.5W With two MOSFETs in parallel: PMOSFET = (I 2 RDS,ON )(Duty Cycle) = (14A/2)2 (0.037 W) (2.8V / 5V) = 1.0W/FET *Note: RDS,ON increases with temperature. Assume RDS,ON = 25m W at 25°C. RDS, ON can easily increase to 50mW at high tempera- ture when using a single MOSFET. When using two MOSFETs in par- allel, the temperature effects should not cause the RDS, ON to rise as much. • Smaller heat sink required. With power dissipation down to around one watt, considerably less heat sink is required. • Reliability. With thermal management under control, this DC-DC converter is able to deliver load currents up to 14.5A with no performance or reliability concerns. • MOSFET Gate Bias. As already discussed, the low-side MOSFET on the RC5051 needs only 5V for its gate drive supply. The high- side MOSFET can be biased by one of two methods: Charge Pump or 12V Gate Bias. • Method 1. Charge pump (or Bootstrap) method. Figure 4 displays the use of a charge pump to provide gate bias to the high-side MOSFET with the RC5051. Capacitor CP is the charge pump deployed to boost the voltage of the RC5051 output driver. When the MOSFET switches off, the source of the MOSFET is at approximately 0V. VCCQP is charged through the Schottky diode D1 to approximately 4.5V. Thus, the capacitor CP is charged to approximately 4.5V. When the MOSFET turns on, the source of the MOSFET voltage is equal to 5V. The capacitor voltage follows, and hence provides a voltage at VCCQP equal to approximately 10V. The Schottky diode D1 is required to provide the charge path when the MOSFET is off, and reverses bias when the VCCQP goes to 10V. The charge pump capacitor, CP, needs to be a high Q, high frequency capacitor. A 1 mF ceramic capacitor is recommended here. Figure 4. Charge Pump Configuration • Method 2. 12V Gate Bias. Figure 5 illustrates how a 12V source can be used to bias the VCCQP. A 47 W resistor is used to limit the transient current into the VCCQP pin and a 1 mF capacitor filter is used to filter the VCCQP supply. This method provides a higher gate bias voltage (VGS ) to the MOSFET than the charge-pump method does, and therefore reduces the RDS, ON of the MOSFET and thus reduces the power loss due to the MOSFET. Figure 6 shows how RDS, ON reduces dramatically with VGS increases. A 6.2V Zener diode (D1) is placed from VCCQP to 5V to clamp the voltage at VCCQP to a maximum of 12V and ensure that the absolute maximum voltage of the IC will not be exceeded . Figure 5. 12V Gate Bias Configuration Figure 6. RDS,ON vs. VGS for Selected MOSFETs PWM/PFM Control 65-5051-06 VO +5V D1 D2 CP Q1 Q2 L2 RS COUT VCCQP HIDRV LODRV GNDP PWM/PFM Control 65-5051-07 VO +5V +12V D2 1 µF Q1 Q2 L2 RS 47 ½ D1 VCCQP HIDRV LODRV GNDP COUT IRL2203NS NDP6030L FDP7030L NDB603AL |
Similar Part No. - AN-53 |
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Similar Description - AN-53 |
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