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LM2904B-Q1: Output not Hi-Z when no power

Frank QIN
Genius 9440 points
Part Number: LM2904B-Q1


Tool/software:

Hi Team,

Could you please confirm if the output of LM2904B-Q1 is low or Hi-Z when it is not powered? According to the measurements, it is output low, while we hope it can output Hi-Z.

After removed the Q10000, D33034, R10002, the output still 5V.

The main problem is when there is no power, device is not operating, the output is LOW.

Could you please confirm the output status and advise if it can output Hi-Z?

Thanks

Best,

Frank

  • 0
    TI__Mastermind 26868 points

    Frank,

    • No the op amp output is not in high impedance if the power supplies are powered down.
    • This is because there are parasitic diodes across the output transistors.  These diodes will turn on when a signal is applied to the output of the op amp.
    • This is really true for the vast majority of op amps.  That is, the inputs and outputs of an op amp are generally not high impedance if the device is not powered.  Even most amplifiers with a shut down feature will not be high impedance when in shut down mode (see so-what-exactly-is-an-op-amp-shutdown-pin-supposed-to-do ).  Note there is at least one that will go to a high-impedance state in shutdown ( tlv4120.pdf ).  Unless it is specified that the output will go into a high-z state when shut down, you should assume that the output is not high-z.
    • Below is the op amp schematic from the data sheet with the parasitic diodes added in red.
    • Here is a good related E2E post:  Op Amps Maintain High Input Impedance When Powered Down.  This post focuses on input impedance but the same can be said for output impedance.

    I hope this helps.  Best regards, Art

  • 0 in reply to Art Kay
    Prodigy 10 points

    "This is because there are parasitic diodes across the output transistors.  These diodes will turn on when a signal is applied to the output of the op amp."

    There is no signal acting on the output in the figure above, how can it cause the diode turn on？


    What is the path of the current if the diode turn on? From the circuit point of view, how can the output be a low voltage level？

  • 0 in reply to chang wang
    TI__Mastermind 26868 points

    Chang Wang,

    If there is no acting signal on the output, the diodes will not turn on.  Normally when an engineer is concerned about the op amp being in a high-impedance state or a non-high impedance state the reason for the concern is that there is a signal fed back to the output through some other path and the concern is that this signal would be loaded down.  If there is no signal to load down by the output not being in a high-impedance state, than I don't see how the impedance of the output is relevant.  

    I would say that in general it is more common that engineers are concerned by the input being in a high impedance state when shut-down or powered down, but I suppose there are cases where the output is also a concern (as in this post).

    I hope this helps.  Best regards, Art