Kontron COMe-bHL6 - Manual (Page 36)

COMe-bHL6 / Features and Interfaces
4.2 Rapid Shutdown
Overview
For “R” or the “RXT” version of the COMe-bHL6, Kontron has implemented a rapid shutdown function. It works as follows:
1) An active-high shutdown signal is asserted by the COM Express Eval Type 2 carrier board via pin C67 of the COM Express
connector. The characteristics of the shutdown signal are as follows:
» Amplitude 5.0V +/- 5%
» Source impedance < = 50 ohms
» Rise time 1uS ⇐
» Duration >= 20uS
The assertion of this signal causes all power regulators to be disabled and the internal power supply rails to be discharged
by crowbar circuits. The shutdown circuitry provides internal energy storage that maintains crowbar activation for at least
2mS following the de-assertion of the shutdown signal. The circuit also incorporates a weak input pulldown resistor so
that the RXT module will operate normally in systems where the rapid shutdown functionality is not used and pin C67 of
the COM Express is left unconnected.
2) Simultaneously with the leading edge of shutdown, the 12V (main) input power to the RXT module is removed and
these input power pins are externally clamped to ground though a crowbar circuit located on the COM Express carrier
board. This external clamping circuit must maintain a maximum resistance of approximately 1 ohm and be activated for a
minimum of 2mS.
3) Simultaneously with the leading edge of shutdown, the 5V (standby) input power to the RXT module is removed, if
present. External clamping on these pins is not necessary.
Crowbar implementation details
As a tool for designing the internal crowbars, Kontron developed tallied the total capacitance present on each of the
internal power rails, and calculates the required discharge resistance in order to achieve the desired voltage decay time
constant. The principal design criteria are that each supply rail must decay to 37% of initial value (equivalent to 1RC)
within 250uS, and to below 1.5V within 2mS. Analysis shows that the power rails fall into four general classes. Each class
of power rails has a corresponding discharge strategy.
1) Power Input Rails: The main 12V power input rail incorporates about 300uF of distributed capacitance. This rail must be
discharged by an external crowbar located on the carrier board, which must provide a shunt resistance of approximately 1
ohm. The peak power dissipation in this crowbar resistance will be relatively high (on the order of 150W when the crowbar
is activated), but will diminish very rapidly as the input capacitors discharge.
2) Low Voltage, High Power Rails: Each of these 5 “major” internal supply rails has an output voltage in the 1.0 V to 1.5V
range, and each rail has between 1500uF and 3300uF of output capacitance. The required discharge resistances for these
rails are in the range of 0.1 to 0.2 ohm, and peak discharge currents are in the range of 8 to 16A.
The discharge circuit for each rail is implemented with a “pulse withstanding” thick-film SMT resistor in series with a low-
RDSon MOSFET. The resistor peak powers are in the 8W to 20W range; depending on PCB layout considerations either a
single resistor or multiple smaller resistors may be used to achieve sufficient pulse handling capability.
Because of the relatively high currents in the discharge paths, these crowbar circuits require wide copper traces and
careful component placement adjacent to the output components of the corresponding power supplies.
3) Low Voltage, Low Power Rails: These rails have voltages of 1.8V or less and capacitances under 1000uF, with peak
discharge currents <3A. The discharge circuits for these rails are also implemented with resistor(s) and a low-RDSon
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