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OPAMP Leakage

Jay Carlo De Claro
Prodigy 160 points

Other Parts Discussed in Thread: LMV324, LMR62421

Hi All,

I'm designing a circuit using Opamp LMV324IDR. Voltage follower configuration and I need to pump 5A pulse for few micro seconds. The circuit is working however when there's no signal on the (+) input I can still get an output voltage which can turn on the mosfet causing the MOSFET to leak.

  • 0
    TI__Guru**** 169816 points

    Hello Jay,

    What is the LMV324 output voltage with no input?
    When there is no input signal is the non-inverting input 0V?
    If the output did go to zero, how much current must the op-amp sink in the application?

    Please see figures 15 and 16 in the data sheet.

    Regards,
    Ron Michallick

  • 0 in reply to Ron Michallick
    Prodigy 160 points

    Hi Ron,

    Thanks for your reply.

    I can get 960mV voltage on the opamp output on 1 channel. other channel don't have. FYI i used 3 channel (Some boards 2 channel has 960mV-1V on the output)

    I can get 80mV on the non inverting input when there is no input signal.

    I need atleast 300nA  forward leakage and reverse leakage current for the mosfet.

    Regards,

    Jay

  • 0 in reply to Jay Carlo De Claro
    TI__Guru**** 169816 points

    Jay,

    For unity gain setup and non-inverting pin = 80mV, the output should follow unless output current is too high, >5mA.

    What is connected to the LMV324 output pin(s).

    Regards,
    Ron Michallick

  • 0 in reply to Ron Michallick
    Prodigy 160 points

    Hi Ron,

    Thanks for your reply.

    I know I should expect 80mV also on the output.

    The output pins is connected to a mosfet which will turn on the 3 LED in series which will turn on the mosfet every 1 sec with 200uS duration for 5A.

    I used 3 out of 4 output of the LMV324 to connect to this mosfet to switch the LED. One scenario 2 output of the LMV324 is 1V and the other output is 0V which is kind of weird which cause the the 2 output LED to leak.

    Regards,

    Jay

  • 0 in reply to Jay Carlo De Claro
    TI__Guru**** 169816 points

    Jay,

    Is it possible that the parallel outputs are fighting each other (one high and the others low).
    Also check the LMV324 outputs for any oscillation.
    Would you like me to review a partial schematic?

     

     

  • 0 in reply to Ron Michallick
    Prodigy 160 points

    Hi,

    I attached the schematic of one string of my LED total is 3 strings. The supply on my OPAMP is 4V. I have a boost converter(LMR62421) from 4V to 17V.

    Even without input on the non inverting I can get a 1V on the output of the OPAMP (gate of the mosfet).  The input of the non inverting pin comes from the CPU.

    Please advise what are the possibilities why this are happening.

    Thanks and regards,

    Jay

  • 0 in reply to Jay Carlo De Claro
    TI__Guru**** 169816 points

    Jay,

    The LMV324 output should not have any problem going low. It won't be instantly because of the LMV324 slew rate and MOSFET capacitance.

    It's time for some troubleshooting...
    Measure every pin at the DUT, Try replacing the LMV324, disconnect output, ect.