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TPS7B84-Q1: Failure mode analysis

Shinji Tsuji
Genius 12358 points
Part Number: TPS7B84-Q1

Hi team,

Could you advise how does the device behaves in the conditions below?

This is the automotive application. 

BATT=9V to 16V

　 IN GND 出力
1 BATT GND BATT
2 BATT GND GND
3 Open GND BATT
4 Open GND GND
5 BATT Open BATT
6 BATT Open GND

Please correct if my idea below is wrong.

1. Protected. Device keeps operation. 

2. Protected. Trigger current limit and eventually trigger thermal shutdown and repeat it. (not good for reliability)

3.Current flow through body diode and charge input capacitor to power up the device. 

VIN<Vout - 1x diode which violate the absolute maximum ratings, but the current is limited(VIN=open). So risk is low?

4. Protected. No problem.

5. Protected, No problem. 

6. Protected. Trigger current limit and eventually trigger thermal shutdown and repeat it. (not good for reliability)

regards,

  • +1
    TI__Genius 12511 points

    Hi Tsuji-san,

    1) The device will be protected but will not be operating, because the BATT potential is higher than the set output level, the internal FET will be closed off as it tries to regulate the output lower. The device will not be harmed, but the part will not be in a standard mode of operation, the passfet will be completely closed.

    2) Correct, this mode of operation will cause the reliability to decrease drastically

    3) If Input is floating, then current will flow through the body diode and the current will be on the order of  Iq ~18uA. If there is an input capacitance, there is slightly more concern as the current draw will be much more significant on first connection. When this occurs there is a concern of the parasitics turning on and drawing current as a short which could destroy the part. We believe the TPS7B84-Q1 should be safe, but there is some concern depending upon the size of Cin

    4) Correct, no potential on any pins, so no risk for harm

    5 and 6) Whenever ground is left floating on a cmos process, you run the risk of an esd diode conducting current as there is no easy pathway to ground. Once an ESD diode turns on there is a short to other pathways to ground which will cause problems. We strongly recommend not leaving the Ground pin floating on these devices.

    Regards,

    John