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TPIC44H01 logic I/O damaged

Other Parts Discussed in Thread: TPIC44H01

I have an application where the FET driver power is supplied with 24VDC and logic Vcc is supplied with 5VDC.  The 24VDC is supplied through a separate connecter from the logic interface.

Power Connector: GND and 24VDC

Logic Interface Connector: SDI, SCLK, /CS, /RST, +5VDC  ---> there is no ground reference on this connector to force a single return path through the power connector.


Also, Vcomp1 - 4, VpkA, VpkB are tied directly to Vcc.  In my application, i have 6 TPIC chips connected on a serial bus.  There are actually 3 identical boards connected with 2 TPIC44H01 chips per board.


So here is my problem.  I turn power off safely at the wall.  This takes down both the 24VDC and 5VDC.  Then i remove the Logic Interface Connector.  If i plug the device into the wall, the TPIC44H01 is damaged.  In some cases, i have seen burn marks in the top of the chip and even the PGND leg blown off the chip completely.

Next, i remove power from the wall and then plug the logic interface cable back into the control board.  When I plug the cable back into the wall with a good 5VDC and 24VDC, the TPIC chip is dead.  Sometimes there is no visible damage. 

If i replace one of the boards with a known working board, it will not work correctly until i replace all 3 boards on the bus.


Does anyone have any ideas how this could happen?  Also, if the 5V fuse blows, there is no damage.  It seems there is something about the case of no cable connected.