Tokmas MAX16171ATA/VY(TOKMAS)

When power is first applied, load current flows from source to drain through the MOSFET's body diode. Once the voltage across the body diode exceeds VSD(REG), the MAX16171ATA begins charging the MOSFET gate through a 30μA (typical) charge pump current source. During forward operation, the MOSFET gate is charged until it reaches the internal 14V gate-to-IN pin Zener diode clamp voltage of the MAX16171ATA.

The MAX16171ATA is designed to regulate the MOSFET gate-to-source voltage. If the MOSFET current decreases such that the voltage across the MOSFET falls below the VSD(REG) voltage regulation point of 30mV (typical), the GATE pin voltage will decrease until the voltage across the MOSFET is regulated to 20mV. If the source-to-drain voltage is greater than VSD(REG), the gate-to-source voltage will increase and ultimately reach the 14V gate-to-IN pin Zener clamp level.

If the MOSFET current reverses — potentially due to an input power failure — causing the voltage across the IN and OUT pins of the MAX16171AT to become more negative than the VSD(REV) voltage of -15mV (typical), the MAX16171AT will rapidly discharge the MOSFET gate through a robust gate-to-IN pin discharge transistor. If the input power suddenly fails, such as when the supply is shorted directly to ground, reverse current will temporarily flow through the MOSFET until the gate is fully discharged. This reverse current originates from the load capacitance and the paralleled power supplies. The MAX16171AT typically responds to a voltage reversal condition within 50ns. The actual time required to turn off the MOSFET depends on the charge held by the gate capacitance of the MOSFET used. A MOSFET with an effective gate capacitance of 47nF can typically be turned off within 260ns. This fast turn-off time minimizes voltage disturbances at the output and current transients from redundant power supplies.

The VS pin of the MAX16171AT is the primary power supply pin for all internal bias circuitry and serves as the auxiliary supply for the internal gate drive charge pump. For typical MAX16171AT applications, the VS pin can be connected directly to the OUT pin. The capacitance value should be the minimum needed to provide acceptable filtering of voltage noise. If the VS pin is powered while the IN pin is floating or grounded, approximately 0.5mA of current will leak from the VS pin into the IC, and approximately 2mA of current will leak from the OUT pin into the IC.

The OFF pin is a logic-level input that controls the gate drive to the external MOSFET. The maximum operating voltage on this pin is 5.5V. When the OFF pin is high, the MOSFET is turned off regardless of the detected IN and OUT voltages. In this mode, load current flows through the MOSFET's body diode. If the MOSFET is conducting through the body diode, the voltage difference between the IN and OUT pins is approximately 700mV. The OFF pin has an internal pull-down current of 5μA (typical). If the OFF function is not needed, this pin can be left floating or connected to ground.

Systems requiring high availability typically use multiple redundant power supplies connected in parallel to improve reliability. Schottky OR-ing diodes are commonly used to connect these redundant supplies to a common point at the load. The drawback of using OR-ing diodes is the forward voltage drop, which reduces the available voltage and introduces associated power loss as load current increases. Replacing OR-ing diodes with N-channel MOSFETs adds modest complexity but can reduce or eliminate the need for diode heat sinks or large thermal relief copper areas in the PCB layout for high-power applications.

The MAX16171AT is a positive-voltage (i.e., high-side) OR-ing controller that drives an external N-channel MOSFET to replace an OR-ing diode. The MAX16171AT monitors the voltage across the MOSFET source and drain pins at the IN and OUT pins, while the GATE pin drives the MOSFET to control its operation based on the monitored source-to-drain voltage.

• Wide operating input voltage range VIN: 5V ~ 100V
• 110V transient voltage
• Charge pump gate driver for external N-channel MOSFET
• 50ns fast response to current reversal
• 2A peak gate turn-off current
• Ultra-low VDS turn-off voltage for reduced turn-off time
• 8-pin DFN2×3 package

• Active OR-ing for redundant (N+1) power supplies