Fairchild RC5042, RC5040 MOSFET Gate Bias, Charge Pump or Bootstrap, Charge Pump Configuration

AN42 APPLICATION NOTE

MOSFET Gate Bias

The MOSFET(s) can be biased using one of two methods: Charge Pump or 12V Gate Bias.

Charge Pump (or Bootstrap)

Figure 6 employs a charge pump to provide the MOSFET gate bias. The charge pump capacitor, CP, is used as a flying capacitor to boost the voltage of the RC5040 or RC5042 out- put driver. When the MOSFET switches off, the source of the MOSFET is at -0.6V. VCCQP is charged through the Schot- tky diode to 4.5V. Thus, the capacitor CP is charged to 5V. When the MOSFET turns on, the source of the MOSFET is at approximately 5V. The capacitor voltage follows, and hence provides a voltage at VCCQP equal to 10V. The Schot- tky is required to provide the charge path when the MOSFET is off, and then reverses bias when the VCCQP goes to 10V. The capacitor CP needs to be a high Q and high frequency capacitor. A 1∝F ceramic capacitor is recommended here.

+5V
DS2
VCCQP	M1
HIDRV
CP	L1	RS
PWM/PFM		VO
Control
	DS1	CB
		65-AP42-06

Figure 6. Charge Pump Configuration

12V Gate Bias

Figure 7 illustrates how an external 12V source can be used to bias VCCQP. A 47 Ω resistor is used to limit the transient current into the VCCQP pin, and a 1∝F capacitor filter is used to filter the VCCQP supply. This method provides a higher gate bias voltage (VGS) to the MOSFET, and there- fore reduces the RDS,ON and resulting power loss within the MOSFET. Figure 8 illustrates how RDS,ON decreases dra- matically as VGS increases. A 6.2V Zener (DS2) is used to clamp the voltage at VCCQP to a maximum of 12V and ensure that the absolute maximum voltage of the IC is not exceeded.

Warning: The 12V Gate Bias method applies only to the RC5042. The RC5040 has not been designed to accept an external 12V gate bias voltage, and may be damaged if this method is used.

	+5V
+12V	47Ω
	D1
	6.2V
	VCCQP
	M1
	HIDRV
	L1	RS
PWM/PFM		VO
Control
	DS1	CB
		65-AP42-07

Figure 7. 12V Gate Bias Configuration

	0.1
	0.09									R(DS)Fuji
	0.08
										R(DS)Fuji
)	0.07
(Ω	0.06									R(DS)706A
DS,ON	0.05									R(DS)-706AEL
	0.04
R	0.03
	0.02
	0.01
	0
	1.5	2	2.5	3	3.5	4	5	6	7	8	9	10	11

Gate-Source Voltage, VGS (V)

Figure 8. RDS,ON vs. VGS for Selected MOSFETs

8