Issue 4, May 2009

5

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©Diodes Incorporated 2008

ZXGD3102T8

Operation

The operation of the device is described step-by-step with reference to the timing diagram below.

1. The detector monitors the MOSFET Drain-Source voltage.

2. At system start up, the MOSFET body diode is forced to conduct current from the input power

supply to the load and there is approximately -0.6V on the Drain pin.

3. The detector outputs a positive voltage with respect to ground, this voltage is then fed to the

MOSFET driver stage and current is sourced out of the GATEH pin. The turn on time of the MOSFET

can be programmed through an external resistor RG. Refer to “Speed vs. Gate resistance” graph.

4. The current out of the GATEH pin is sourced into the OR’ing MOSFET Gate to turn the device on.

5. The GATEH output voltage is proportional to the Drain-Source voltage drop across the MOSFET

due to the load current flowing through the MOSFET. The controller increases its output gate voltage

when the Drain current is high to ensure full MOSFET enhancement

6. If a short condition occurs on the input power supply it causes the OR’ing MOSFET Drain current to

fall very quickly.

7. When the Drain-Source differential voltage drops below the turn off threshold, the MOSFET Gate

voltage is pulled low by GATEL, turning the device off. This prevents high reverse current flow from

the load to the input power supply which could pull down the common bus voltage causing

catastrophic system failure

MOSFET

Drain Voltage

MOSFET

Gate Voltage

MOSFET

Gate Current