Kontron COMe-cBTi6R E3845 8GB/16S User Manual page 32

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MOSFET. In some cases, the peak pulse power dissipation in the resistor(s) is low enough that specialty "pulse

withstanding" resistors are not required.

4) Medium Voltage Rails: These 3.3V and 5V rails typically have relatively small output capacitances and peak discharge

currents <1A. The discharge circuits for these rails are typically implemented with conventional resistor(s) and a low-

RDSon MOSFET.

Shutdown input circuit details

The shutdown input pin to the RXT module is coupled through a series Schottky diode and a small series resistor to the

gates of all crowbar MOSFETs, connected in parallel. All crowbar MOSFETs are N-channel "logic level" parts that have are

specified for operation at Vgs = 4.5V. Three additional components are connected in parallel between the MOSFET gates

and ground:

» A capacitor that provides energy storage to keep the MOSFETs conducting for several mS after the shutdown signal

is de-asserted.

» A high-value resistor that provides a discharge path for the capacitor as well as a pulldown resistance (to insure

that the shutdown circuits remain inactive if the shutdown pin is left floating).

» A 6.2V zener diode that protects the MOSFET gates from damage due to input ESD or input overdrive.

In order to insure that the crowbars do not "fight" active switching regulators while the input capacitors are being

discharged, the shutdown circuit rapidly crowbars the 5V rail, with a time constant <10uS. The 5V rail powers most of the

remaining switching regulators, and as its voltage falls below about 4V those regulators enter under-voltage lockout

mode and cease to operate. Additionally, by using the UVLO mechanism in the design of the RXT module, Kontron

minimizes the risk of inadvertently affecting the standard power sequencing logic for such RXT modules. Two of the

switching regulators do not require the 5V supply for operation, and in those two cases it will be necessary to clamp the

enable inputs to ground when shutdown begins.

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