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 drop (V_f). The minimum value of V_f required to enable the charge pump circuitry is 0.48V. Once the MOSFET is 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 across Anode and Cathode pins depends on the R_DSON and load current. If voltage difference across source and 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 10 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 very small period of time (2% typical) which rules out the chances of overheating the MOSFET, regardless of the output current. Once the capacitor voltage reaches its high threshold, the MOSFET is turned ON and charge pump circuity is deactivated until the Vcap reaches its lower voltage threshold again (T1). The voltage difference between Vcap high and low threshold is typically 1.15V. The LM74610-Q1 charge pump has 46µA charging capability with 5-8MHz frequency.

Figure 10. Vcap Charging and Discarding by the Charge Pump

The Vcap current consumption is 1 µA (typical) to drive the gate. The MOSFET OFF time (T0) and ON time (T1) can be calculated using the following expression

Equation 1.

Where:

• C = Vcap Capacitance
• dV = 1.15V
• dI = 46 µA for charging
• dI = 0.95 µA for discharging

Note: Temperature dependence of these parameters – The duty cycle is dependent on temperature since the capacitance variation over temperature has a direct correlation to the MOSFET OFF and ON periods and the frequency. If the capacitor varies 20% the periods and the frequency will also vary by 20% so it is recommended to use a quality X7R/COG cap and not to place the cap in close proximity to high temperature devices. The variation of the capacitor does not have a thermal impact in the application as the duty cycle does not change.

7.3.3.3 Gate Drive Pin

When the charge pump capacitor is charged to the high voltage level of 6.3 V (typical), the Gate Drive pin provides a 6.8 µA (typical) of drive current. When the charge pump capacitor reaches its lower voltage threshold of 5.15 V (typical), Gate is pulled down to the Anode voltage (Vin). During normal operation, the gate turns ON and OFF with a slow 2msec slew rate in order to avoid switching noise and EMI issues. To protect the gate of the MOSFET, a built-in internal 11.5V Zener clamp the maximum gate to source voltage (V_GS(MAX)).

7.3.3.4 Gate Pull Down Pin

The Gate Pull Down pin is connected to the Gate Drive pin in a typical application circuit. When the controller detects negative polarity, possibly due to failure of the input supply or voltage ripple, the Pull-Down quickly discharges the MOSFET gate through a discharge transistor. The Gate Pull Down pin can discharge the MOSFET gate capacitance with 160-mA pull down current to speed up the MOSFET turn OFF time. This fast pull down reacts regardless of the Vcap charge level. If the input supply abruptly fails, as would happen if the supply gets shorted to ground, a reverse current will temporarily flow through the MOSFET. This reverse current can be due to parallel connected supplies and load capacitance and is dependent upon the R_DSON of the MOSFET. When the negative voltage across the Anode and Cathode pins reaches -20mV (typical), the LM74610-Q1 immediately reacts and discharges the MOSFET gate capacitance as shown in Figure 11 . A MOSFET with 5nF of effective gate capacitance can be turned off by the LM74610-Q1 within 2µs (typical). The fast turnoff time minimizes the reverse current flow from MOSFET drain by opening the circuit. The reverse leakage current does not exceed 110µA for a constant 13.5V reverse voltage across Anode and Cathode pins. The reverse leakage current for a Schottky diode is 15mA under the same voltage and temperature conditions.

Figure 11. Gate Pull Down in the Event of Reverse Polarity