T0

Body Diode Voltage Drop

VOUT

FET is OFF

FET is ON

0 V

LM74610-Q1

SNOSCZ1A – JULY 2015 – REVISED OCTOBER 2015

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Feature Description (continued)

7.3.2 During T1

Once the voltage on the capacitor reaches a higher voltage level of 6.3V (typical), the charge pump is disabled

and the MOSFET turns ON. The energy stored in the capacitor is used to provide the gate drive for the MOSFET

(T1 in Figure 8). When the MOSFET is ON, it provides a low resistive path for the drain current to flow and

minimizes the power dissipation associated with forward conduction. The power losses during the MOSFET ON

then begin to flow through the body diode of the MOSFET, causing the MOSFET body diode voltage drop to

appear across Anode and Cathode pins. The charge pump circuitry is re-activated and begins charging the Vcap.

The LM74610-Q1 operation keeps the MOSFET ON at approximately 98% duty cycle (typical) regardless of the

external charge pump capacitor value. This is the key factor to minimizing the power losses. The forward voltage

7.3.3 Pin Operation

7.3.3.1 Anode and Cathode Pins

The LM74610-Q1 Anode and Cathode pins are connected to the source and drain of the external MOSFET. The

current into the Anode pin is 30 µA (typical). When power is initially applied, the load current flows through the

body diode of the external MOSFET, the voltage across Anode and Cathode pins is equal to the forward diode

turned ON, the Anode and Cathode pins constantly sense the voltage difference across the MOSFET to

determine the magnitude and polarity of the voltage across it. When the MOSFET is on, the voltage difference

drain of the external MOSFET becomes negative, this is sensed as a fault condition by Anode and Cathode pins

and gate is turned off by Gate Pull Down pin as shown in Figure 1. The reverse voltage threshold across Anode

and Cathode to detect the fault condition is -20 mV. The consistent sensing of voltage polarity across the

MOSFET enables the LM74610-Q1 to provide a fast response to the power source failure and limit the amount

and duration of the reverse current flow.

7.3.3.2 VcapH and VcapL Pins

VcapH and VcapL are high and low voltage thresholds respectively that the LM74610-Q1 uses to detect when to

turn the charge pump circuitry ON and OFF. The capacitor charging and discharging time can be correlated to

the duty cycle of the MOSFET gate. Figure 9 shows the voltage behavior across the Vcap. During the time

period T0, the capacitor is storing energy from the charge pump. The MOSFET is turned off and current flow is

only through the body diode during this time period. The conduction though body diode of the MOSFET is for a

8

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