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LMP8602: LMP8602 always in saturation

Gilad Barak
Prodigy 70 points
Part Number: LMP8602
Other Parts Discussed in Thread: DRV8870

I designed a current sensing circuit based on the LMP8602.

The setup is quite simple:

current sensing resistor is connected to inputs (pins 1, 8)

offset pin is connected to supply as I would like bidirectional measurement

preamplifier goes directly to output amplifier (pins 3-4 shorted).

Supply voltage is +5V at pin 6 (and 7 as already mentioned). ground to pin 2.

As soon as power is provided the output goes to 5V (should be 2.5V).

I suspected some stray voltages at the differential input so as a test shorted the input pins together - output still going to 5V.

What am I missing?

  • 0
    TI__Mastermind 21720 points
    Hello Gilad,

    Thanks for considering to use Texas Instruments in your design. From those details, it sounds like you are doing everything right. Next step I would propose is to remove the LMP8602 and see if the output trace is shorted to the supply by any stray wire, solder, or downstream broken part. If there is not a short, replace the part and be sure to probe the inputs at power-on. You do have a bypass capacitor on your supply pin, right?
  • 0 in reply to Patrick Simmons
    Prodigy 70 points

    Thanks for the reply.

    I do not think it is an arbitrary problem with the circuit  or defective component as this repeats itself on 3 boards which were professionally manufactured (4 layers PCB with separate power and ground planes, automatic soldering of components, no loose wires etc.).

    I did not understand "be sure to probe the inputs at power-on" - what do you mean by that?

  • 0
    Guru 140200 points
    Hi Gilad,

    can the input bias currents of LMP6802 flow back to GND?

    Kai
  • 0 in reply to kai klaas69
    Prodigy 70 points

    Here is the circuit.

    It might be possible that the LMP8602 is not suitable for my actual task, but anyhow I am trying  to understand what is wrong.

    U5 is an H bridge, and imagine an external load at the right end of the lines coming out of OUTx , so basically current flows between the OUTx pins in either direction. R16 is the sensing resistor - I know it looks very high but the load resistance is even much higher and the current is very small. The LMP8602 (marked by mistake as 6802) is wired as I described initially.

    Now, even if I remove U5, the external load removed and R16 shorted the output remains high at 5V

  • 0 in reply to Gilad Barak
    TI__Mastermind 21720 points

    Hello Gilad,

    From your schematic, your device should not have any issues as it looks like you are operating under acceptable conditions.  However, it seems likely that device is broken.  This could be from a transient from when you power on your system or from something else.  I would replace the part, isolate the sense inputs from the DRV8870 and short them together.  Be sure to give them a reasonable common mode voltage and then test whether the output still rails to supply.  If the output is at midrail, then there must be an event on the DRV8870 output line that is violating the part specifications.  If the device still rails, you will need to replace the part again and isolate the other pins from your system to pin-point which pin is experience the over stress condition.  You may even need to use an external supply with a common ground to your system to power the device.