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UA78M: Start up at low temp

masahiko shimizu
Guru 11235 points
Part Number: UA78M

Hi,

We have a issue at start-up with only low temp(under 5 degrees)
UA78M33 is slow start-up at this situation.

Below is waveform and circuit.
At 25 degrees, we don't have this isuue.
This issue is at only low temp.

Could you give me comment about this event, please?


Best regards,
Shimizu

  • 0
    TI__Guru 55940 points
    Hi Shinizu,

    I have not heard of this issue before and unfortunately due to the age of this device (UA78M was originally released in 1976) there is very little information about this device outside of what is published in the datasheet. I will attempt to replicate your waveform in order to help to debug. Please allow me some time to receive the devices and look at this on the bench.

    In the meantime, could you provide me the same scope shot at room and hot?

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    Guru 11235 points

    Hi Ryan,

    Thank you for your support.
    I attach the waveforms at 20, 10 and 5 degrees.
    Unfortunately, my customer didn't measured at hot temp.

    UA78M33C.pptx

    Thank you for your continious support.

    Best regards,
    Shimizu

  • 0 in reply to masahiko shimizu
    TI__Guru 55940 points

    Hi Shimizu,

    I apologize for the delay in response.  We were able to take the same startup temperatures as you; however, we were not able to reproduce your startup waveform.  We did our testing without a load so that we are simply showing the startup of the linear regulator.  On the below scopeshots yellow is Vin and magenta is Vout.

    20 C

    10 C

    5 C

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    Guru 11235 points
    Hi Ryan,

    Thank you for your support.

    Did you have a same value capacitor of input and output like above circuit?
    If it is different, How much capacitor value did you use?

    Best regards,
    Shimizu
  • 0 in reply to masahiko shimizu
    TI__Guru 55940 points
    Hi Shimizu,

    The waveforms provided were without any additional external capacitors (UA78M does not require any external components). This is the true startup waveform of the linear regulator. As long as the input supply has a high enough current limit to supply both the input and output capacitors charging as well as the load, the output waveform will take on the shape of a charging capacitor.

    Very Respectfully,
    Ryan