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LM317LC current limit delay.

John Bennett1
Prodigy 10 points
Other Parts Discussed in Thread: LM117, LM317, TLVH431

Using a pulsed 5V supply to a LM317LCPK, SOT-89 package, as a current regulator (83mA), there is a delay of 50us as the current rises to the limit value. Most of the delay is the current rising from ~22mA to ~33mA in ~40us. Has anyone noticed "slow LM317s in current limit and found out why?

John

  • 0
    TI__Mastermind 22610 points

    Yes, I have heard of this phenomenon before. Root cause is unknown.

    Additionally, this start-up delay time reportedly varies noticably between different manufacturers of LM117.

    One unconfirmed solution that a customer allegedly found by trial and error, where LM317 was set to limit at several hundred milli-amps and was being used with a PWMed input supply was to put a resistor from Vin to Vout (k ohms). I never saw the before/after waveforms, or the full application, so I can't offer anything official.

  • 0
    TI__Guru**** 164476 points

    John,

    I have recently used the attached circuit using a TLVH431 (at 2mA) and found that the turn on time was less than 1us.

    It should work also well at higher current.