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LM25066A: External supply for VDD

Christopher Arntzen
Intellectual 630 points
Part Number: LM25066A
Other Parts Discussed in Thread: LM25066EVK, LM25066

Is it appropriate to power the VDD pin with an external supply of 5.5V while the LM25066A is in normal operation with VIN = 12V?

The datasheet shows that the VDD pin can handle up to 6V, but just want to make sure that applying an external voltage higher than the sub-regulation voltage does not result in any negative effects.

  • 0
    TI__Expert 5940 points
    Chris,

    Normally, you would just let the Vin drive the Vdd internal linear regulator and skip the external 5.5v. I assume you have a design that can range from < 3.3v to 12v, hence the question.

    Yes, you can apply 5.5v externally with 12v Vin. The internal linear regular pass FET will simply shut down as it would sense a higher voltage than its desired 4.5v regulation. This will not cause adverse effect on the operation.

    Brian
  • 0 in reply to Brian Daugherty
    Intellectual 630 points
    Hi Brian,

    What if there is zero volts on Vin and I am externally supplying Vdd with 5V. Will the 5V at Vdd back-feed Vin through the LDO?

    Thanks,
    Chris
  • 0 in reply to Christopher Arntzen
    TI__Expert 5940 points
    Chris,

    Its ok to have Vin at 0v and Vdd at 5v. Direct backfeed is prevented by an internal circuit. There may be small leakage, but that is it.

    Brian
  • 0 in reply to Brian Daugherty
    Intellectual 630 points
    Hi Brian,

    I am testing out the LM25066 on the LM25066EVK, and when supplying Vdd with an external 5V LDO it backfeeds to the input (less a diode drop, so Vin = 4.3V). If I put a load on Vin then my 200mA LDO current limits and the voltage collapses, so there is definitely more than a small leakage current being supplied from the Vdd pin to Vin. This is counter to what you stated above; however, the discrepancy is that I am evaluating the LM25066 in the lab, whereas I was inquiring about the LM25066A before. From what I could tell, the LM25066 and LM25066A are functionally identical, and the only difference between them is the telemetry accuracy.

    Can you reconcile what you stated previously with the results I am seeing in the lab?

    Thanks,
    Chris
  • 0 in reply to Christopher Arntzen
    TI__Genius 12621 points
    Hey Chris,

    In rare and extraordinary circumstances, the topology discussed above may be used for debugging purposes but in most system level topologies, it is not suitable for operation.

    Unfortunately yes, the LM25066 will backfeed to the input through the internal LDO. We have an E2E thread discussing this topic:
    e2e.ti.com/.../430240

    Thanks,
    Alex
  • 0 in reply to Alex Triano
    Intellectual 630 points
    Hi Alex,

    How would it be possible not to back-feed Vin through Vdd? I am curious about the topology that allows this even if it may not be suitable for most applications.

    Thanks,
    Chris
  • 0 in reply to Christopher Arntzen
    TI__Genius 12621 points
    Hi Chris,

    Today we don't have a solution to not backfeed VIN unless you put something like a series diode (high power one or several in parallel) or something similar like a high precision ORing controller and external FET. There may be cases where you have a 12V in, add a VDD hold up supply externally, and then remove VIN's main power supply but still want the LM25066 to retain memory. I believe this would work in this case, without causing damage to other system components, but you would need to be careful as the loads could still be powered through the VDD supply, through the LDO to VIN, through the external FET to VOUT.

    Thanks,
    Alex