Some power architectures require that the power-supply sequencer (or system manager) control downstream power MOSFETs to allow power to flow into branch circuits. If the incoming power-supply voltage is at least 5V higher than the power-supply output voltage, a power MOSFET can be placed on the output of the power supply with the addition of some level-shifting circuitry. In this case, a low-voltage system manager like the MAX34440
series can be used to enable and disable the power MOSFET.
In the application shown, the system manager is configured with active-low, open-drain outputs. Even if the system manager is not powered up when power is applied to the power supply, the power MOSFET will not be enabled (since the 2N7002 will be enabled). This in turn disables the power MOSFET.
Most power supplies contain an undervoltage lockout (UVLO), which prevents them from providing power until the input voltage is above a minimum level. The minimum UVLO level needs to be high enough to ensure that the 2N7002 is enabled before the power supply starts delivering power.
When the system manager needs to enable the power MOSFET, the open-drain output is pulled low. This disables the 2N7002, allowing the power MOSFET to turn on. The resistor and capacitor tied to the gate of the power MOSFET can be adjusted to slow the turn-on time of the power MOSFET.
As soon as the system manager releases the open-drain output and allows it to go high impedance, the resistor-divider tied from the power-supply input will immediately enable the 2N7002. This immediately shuts off the power MOSFET.