7.3.1 IN, GATE, and OUT Pins

When power is initially applied, the load current will flow from source to drain through the body diode of the MOSFET. Once the voltage across the body diode exceeds V_SD(REG) then the LM5050-1 begins charging the MOSFET gate through a 32 µA (typical) charge pump current source . In forward operation, the gate of the MOSFET is charged until it reaches the clamping voltage of the 12-V GATE to IN pin Zener diode internal to the LM5050-1.

The LM5050-1 is designed to regulate the MOSFET gate-to-source voltage. If the MOSFET current decreases to the point that the voltage across the MOSFET falls below the V_SD(REG) voltage regulation point of 22 mV (typical), the GATE pin voltage will be decreased until the voltage across the MOSFET is regulated at 22 mV. If the source-to-drain voltage is greater than the V_SD(REG) voltage, the gate-to-source voltage will increase and eventually reach the 12-V GATE to IN pin Zener clamp level.

If the MOSFET current reverses, possibly due to failure of the input supply, such that the voltage across the LM5050-1 IN and OUT pins is more negative than the V_SD(REV) voltage of -28 mV (typical), the LM5050-1 will quickly discharge the MOSFET gate through a strong GATE to IN pin discharge transistor.

If the input supply fails abruptly, as would occur if the supply was shorted directly to ground, a reverse current will temporarily flow through the MOSFET until the gate can be fully discharged. This reverse current is sourced from the load capacitance and from the parallel connected supplies. The LM5050-1 responds to a voltage reversal condition typically within 25 ns. The actual time required to turn off the MOSFET will depend on the charge held by the gate capacitance of the MOSFET being used. A MOSFET with 47 nF of effective gate capacitance can be turned off in typically 180 ns. This fast turnoff time minimizes voltage disturbances at the output, as well as the current transients from the redundant supplies.