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LM1117-N / Behavior of current limit and thermal

S.Satoshi
Guru 21685 points
Other Parts Discussed in Thread: LM1117, TLV1117

Hi All,

Please give your comment for attached behavior.  I'd like to know the reason why Iout is regulated under Vout shorted condition. It looks like the Iout is regulated due to thermal regulation but I understand that the output is disabled when thermal protection is operated. Here is the flow of this behavior.

  • LM1117 is powered-on from 3.3V input with Vout shorted to GND
  • Iout is limited due to current limit
  • Iout is regulated due to thermal protection...?

Best Regards,

Sonoki / Japan Disty

  • 0
    TI__Guru**** 165086 points
    Sonoki-san,

    The device starts in current limit (as you have stated) then it reduces output current to limit temperature. The TLV1117 may linearly lower current to maintain temperature or alternate between on and off to maintain temperature. The loop stability of the thermal limit loop sets which action is taken.
  • 0 in reply to Ron Michallick
    Guru 21685 points
    Hi Ronald-san,

    Regarding thermal protection, it does not shut off output immediately but reduce output current to limit temperature. Is my understanding correct? Then is the output current reduced to zero if the rising of temperature is too much?

    Best Regards,
    Sonoki
  • 0 in reply to S.Satoshi
    TI__Guru**** 165086 points
    Sonoki-san,

    If temperature goes above the limit then the output will turn completely off, however about 1.7mA of current will still flow (minimum current for regulation).

    With the output off, the device will start cooling at it will turn back on.
  • 0 in reply to Ron Michallick
    Guru 21685 points

    Hi Ronald-san,

    The waveform I attached  is still flow about 1A so it is not turned completely off, right? The thermal protection of LM1117 is reduce current when the die is heated and will turn completely off if temperature goes above the limit. Is my understanding correct?

    Best Regards,

    Sonoki

  • 0 in reply to S.Satoshi
    TI__Guru**** 165086 points

    Sonoki-san,

    Your understanding is correct.

    At no time (in the waveform) did the temperature exceed the internal thermal limit. This is why the output did not turn off.
    For the last half of the waveform, the temperature was exactly at the thermal limit.