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LM139: Inverting input sinking current

peter flannigan
Prodigy 30 points
Part Number: LM139
Other Parts Discussed in Thread: TLV1704, TLV3404

Currently have the below circuit, which should take a 5V input on the non-inverting input, 2.5V on the inverting. However in reality, the non-inverting can only ever reach 3.7V and when used over the temp range the Opto isolator is sinking more current so the input voltage on the pin can eventually reach a state where the output of the LM139 switches states.

Doesn't create this scenario on the other devices in the quad package. If we remove the device then the voltages can be measured as predicted. Its only when this device is in circuit

any idea why the input on pin 9 would act like this?

  • 0
    TI__Genius 17085 points

    Hello Peter,

    Can you let us know the exact part number you are using.   TI offers a number of different versions of the LM139 and we would like to be sure we are talking about the same part.

    You mention a circuit but I don't see anything in the post.  Did you mean to attach something?

    In general, the inputs can take up to 36V over temperature and the input bias current will remain within the limits shown in the datasheet, regardless of the supply votlage.  

  • 0 in reply to Kirby Kruckmeyer96848
    Prodigy 30 points

    we're using the LM139D part, it wouldn't let me pick that as an option from the drop down list

    yeah twice I tried to attach a visual . I thought perhaps it just didn't show in the previewcct.pptx

  • 0 in reply to peter flannigan
    TI__Genius 17085 points

    Hi Peter,

    Since this is the commercial version of the product, I am  moving this thread to the commercial amp forum.

    However, I think the answer is pretty much the same.

  • 0
    TI__Guru 53536 points

    Hello Peter,

    5V is above the specified input range (V_ICR). The LM139 is NOT a rail to rail input device.

    The "legal" input range is GND to 1.5V below V+, or +3.5V on a +5V supply (2V below V+ over temp).

    Bringing the input above 3.5V pinches off the internal current source and current is redirected through the base (see the internal schematic in section 8.2) . The result is the input will tend to "float" to equilibrium around the internal bias point.

    The datasheet mentions that damage will not occur and that the output will be correct, but it does not mention that there will be a HUGE change in the bias current as you "drag" the input stage above the Vs-1.2V point.

    You need to attenuate the input slightly to avoid going above 3.5V, or use a Rail to Rail input device. Unfortunately, there does not seem to be any R-R input quad devices in Military temp range, though the closest commercial parts would be the TLV3404 or TLV1704 - but they are micropower and will be slower and may not be able to fully sink the 10k output pullup (change to 100k).

    You could try adding a 22k resistor from the bottom of R1544 to GND to drop the open circuit voltage.

    Regards,

  • 0 in reply to Paul Grohe
    Prodigy 30 points

    a few follow up question if you can answer any of the following please

    1. Is there a figure for the bias current as the input stage rises above Vs-1.2? based on our measurement of 3.7V on this input pin

    2. Would this bias current change over temperature? This would help us relate to the fault we have seen (voltage drop over temperature)

    3. If the circuit was changed to apply 3V (Vcc -2) to the input would the device then operate correctly or have we damaged it by applying 5V?

    regards

    Pete