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TL974: Reference voltage from TLV70025D 2.5V is not steady in some units

user4935639
Prodigy 190 points
Part Number: TL974
Other Parts Discussed in Thread: TLV700, TLV713

Hi Ti team,

We use TL974 as signal voltage supply buffer for Aux in, and use TLV70025 as 2.5V reference voltage for OPAMP positive input port, pls. refer. to the schematic.

But we found a issue: some unit's reference voltage is not 2.5V but 3.8~4.5V.

Tested TLV700025 alone, it outputs 2.5V well.

Does such design have problem? or you have any advises?

Thanks!

Harley.

  • 0
    TI__Guru* 84915 points

    Hi, Harley,

    I'm moving this thread to the Power Management Forum in order to get more information about the TLV70025 device.

    Hi, Ryan and Power Management Forum team,

    Could you help us to get some information about the TLV70025 device? Which conditions would cause a change on the output voltage from 2.5V to 3.8~4.5V?

    Thank you.

    Best regards,
    Luis Fernando Rodríguez S.

  • 0
    TI__Mastermind 43630 points

    Hi Harley,

    Unfortunately, Ryan is no longer with our team.

    I think the problem with using the TLV700 as a reference is that LDO's in general are designed to source current to regulate a voltage. They don't normally sink current.

    The block diagram of the TL974 shows that the input stage using PNP transistors. Connected as above, this is pulling up our output and all the LDO can do is to try to turn its pass FET completely off. We do have a few LDO's with a push-pull output stage (TLV713 and TLV733) which may work a bit better, but accuracy may be the next issue. The reason is the sink capability is there to minimize overshoot.

    I am not as familiar with the TL974 so this is a guess as to what might be happening.

  • 0 in reply to JCHK
    Prodigy 190 points

    Hi John Cummings,

    Thanks for your comments! that makes sense. We will call some TLV713 and TLV733 for trail.

    We had some PCBA builds with a 10K pull down resistor at TLV700025 output, it helps but it seems not suitable, we found at least one of TLV700025 had broken and output always ~4.8V.

    How many current comes from TL974 PNP transistor?

    Is it possible how I can fix this problem without layout change?

    Thanks!

    Harley

  • 0 in reply to user4935639
    TI__Mastermind 43630 points

    HI Harley,

    Unfortunately I am not so familiar with the TL974, but the datasheet claims that the input bias current can be up to 1000nA:

    You might try adding a 2.5KOhm pull-down to keep the node from rising, but this is likely impact the rest of the circuit. Short of a layout change, the only option I see with an existing circuit maybe working is to change C70 to a 1.25KOhm resistor. and maybe short out or significantly lower R414 and R415 might work. Again this suggestion is without understanding any of the nuances of the TL974.

    Someone from our Op-Amp team will have to comment  on this.

    I hope this gets you going in the right direction.