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INA169: Output fails low impedance

Willem van der Kooij
Prodigy 135 points
Part Number: INA169
Other Parts Discussed in Thread: TLV431,

Hi,

I'm currently debugging a power supply failure where the output voltage exceeds the specified limit of 12.5 volts due to the INA169 shorting its output pin to ground (25 Ohms measured in and out of circuit), at the moment of writing I am not sure if this short is the cause of the power supply failure or an effect. I've attached a schematic overview of the circuit in question to this post, we've concluded that the TLV431 has failed as well (failed short circuit) causing the powersupply voltage to not exceed the forward voltage of the optocoupler and outputting only 2 volts. It seems likely that the short in the TLV431 is an effect of something else though.
. 
The device failure was instant and it doesn't seem temperature or overheating of components contributes to the power supply failure, therefore i'm expecting an excessive voltage to occur at the output, there was a load present on the power supply which has TVS diodes that limited the output voltage of the powersupply to about 22V, which should be within spec for the INA169.

The load only draws about 3A of current, so an overcurrent (and therefore too large a differential voltage) seems unlikely.

My question now is, what sort of event could cause the INA169 to fail with 25 Ohms between its output pin and ground?

Any help here is greatly appreciated as this would help to pinpoint the root cause of the issue.

With kind regards,
Willem

  • 0
    TI__Mastermind 29600 points

    Hello Willem,

    I would not assume a GND short at the output will cause damage as it is current output and will normally just force that current to GND.  If a transient cause the output to go below GND then that could be an issue. This device is a current output and you have loads at the output connected back to the supply creating additional currents.  The inputs also don't look like its connected as normally recommended to get the best gain.  Is this done on purpose.

    It looks like maybe during transients the TLV431 could reach a voltage above the 7V max? 

  • 0 in reply to Javier Contreras4
    Prodigy 135 points

    Hi Javier,

    Thanks you for your reply. I figured that the current output of the INA169 being shorted to ground shouldn't be an issue for the device itself, though something in the IC broke in such a way that it is shorted internally (i've verified this with a measurement out of circuit), I'll checkout if I can see any negative voltages, though these should be caught by a rectifier that is not shown in the attached schematic snippet.

    The INA169 is used to make the output voltage of the powersupply drop slightly with an increase of load current, this is done to achieve load balancing with another power supply, this is why the gain of the INA169 is set so low.

    The TLV431 shown in the schematic is not the TI part, it is the Diodes Inc. TLV431BFTA which supports a cathode voltage of up to 18V.

    With kind regards,
    Willem

  • 0 in reply to Willem van der Kooij
    TI__Mastermind 29600 points

    Willem,

    Could you please give some detail on how the device is damaged and what test you performed with it out of the circuit.  This could give some hints on what could have caused the damage.  The big items for me is the IQ of the device, the IB of the input pins.  When you say shorted internally is that the input pins to each other? Input pins to output?  Input to Vs?  Output to VS?  Also sometimes if not setup correctly external of circuit you may not get what is expected.  If you have undamaged device to compare that would help.