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TLV9351: opamp missing rail protection

Part Number: TLV9351

Hello, I am actively using an opamp in the circuit I designed and I want the opamp to turn off its output when the gnd pin is disconnected. However, when the gnd pin is disconnected, the gnd leg of the opamp takes a positive value and burns up . How can I find a solution to this problem.I read documents  about gnd loss protection.  https://www.ti.com/lit/an/slvaes9a/slvaes9a.pdf?ts=1669729893383&ref_url=https%253A%252F%252Fwww.google.com%252F.but I don't quite understand what to do.Any idea?

  • Have you checked the BAT54S Schottky protection diode from the ground pin of OPAmp to the output pin of OPAmp?

    https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1170931/tlv9351-op-amp-missing-gnd-rail-protection/4407797#4407797

    Kai

  • Hello Electronx,

    A good way to protect against an open ground is to make system ground connection unnecessary.

    I assume you want a programmable output current. If this current doesn't have to down to zero then send application supply current to the load. 

    Something like this. The op amp connects to BATT the OUTPUT [D1 cathode] (just two wires). I put in an amp local ground (to make the circuit easier to understand). The minimum current is op amp IQ plus any other circuit in the local application).  If application's BATT or OUTPUT opens then load sees zero current (so it can know)    

    The other approach would be far more complex and the wrong output current will flow with ground open. 

  • Electronx,

    If you want to keep your original design idea. Add D2 that turns on if ground opens (switch simulates this). Also add R8 to limit op amp output current in a fault condition. C1 could be added to help tune stability of the circuit.  

  • Hey Ron! Thanks for your feedback. D2 diode is greatly reduced the warming on the IC.However, I want the output to be completely zero when gnd pin is disconnected . So I need to remove  the +rail of the supply as you said. Do you have a solution for this? or second idea  is output must be  in out of range value  (more than what the normal current range at the output provides) so that the ADC at the output can measure it and report an emergency to the MCU.

  • Electronx,

    With the remove V+ idea, pull D2 out and add T2, R10, R11 (see right side). Both V+ and GND must be present, for T2 to pass V+ to rest of the application. 

    The PNP could be a PMOS instead. Test out values for R10, R11 to get saturated PNP on when GND is connected and completely off when GND is open.

     

  • Hey Ron! Thanks for your feedback.

    I tried this solution before, and the resistor values are as shown in the picture. You can see the voltage values on the pins of the transistor in marked with red. (These are the values measured with a multimeter).But the transistor keeps on conducting continuously, so I couldn't find a solution.

  • Electronx,

    I converted the power remover this to a generic solution to make it easy to study and find what may go wrong.

      

    The protected three wire application is represented by resistors IQ and PASS

    Under fault, the IOUT must be 'zero'. To make that happen I had to add D1. With out it, IQ can flow backwards which can keep T1 on.

    D1 should not be Schottky because they can leak badly at high temperatures. 

    Paste in the original application in place of IQ and PASS; it will be powered by T1 collector with "APP GND" as ground.

    Original app has an output diode so the elevated application ground should not be an issue. You could make output diode two in series to make more margin for zero output current during normal operation.     

    Power_remover.TSC