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Original question:

LM317L: Regarding waveform at startup and startup period

LM317L: LM317L

Tracy Brewer
Prodigy 10 points
Part Number: LM317L

Tool/software:

Hello Support Team,

I have the following circuit:

My Problem is that I get a large transient on the output when I switch on the Vin (24V). I'm using a toggle switch.

Blue = Vin

Yellow = Vout

The output is approx. 3.3V and the input is 24V. There is no load on the output.

There is a transient on the output when the 24V is switched on. It's -2.2V to +2.4V (4.5V total).

The amplitude varies and sometimes reaches over 7-10V which causes problems when used in my overall design.

What would be the best way to eliminate this transient?

Thanks, Tracy

  • 0
    TI__Guru 50025 points

    Hi Tracy,

    Do you have a way (i.e. a pad on board) to slow the input with a surge resistor? 

    Have you measured a similar transient on other nodes when you switch on the input or is it just the LM317L output? I'm asking because I'm wondering if there's some coupling through ground with such a fast rise time. 

    Regards,

    Nick

  • 0 in reply to Nick Butts
    Prodigy 10 points

    Hi Nick,

    These are really good suggestions:

    I did put a 240 Ohm resistor in series with Vin to limit the inrush current but did not see an improvement. It was not with the stand alone 3.3V supply though (the full ckt. was in connected). I should repeat this with the stand alone 3.3V supply to keep it in it's simplest form.

    Checking for the transient in other locations is a great idea. If there's a transient on the ground then that may be a good clue. I did check for any overshoot or transient on the 24Vin and didn't see anything unusual.

    On Friday I'll try the 240 Ohm resistor once again and check the feedback pin and ground on the regulator.

    Thanks for your input Nick and have good 4th,

    Tracy

  • +1
    Prodigy 10 points

    I found that the transient is from the switch. I used a common mode choke between the switch and the input of the supply and it nearly eliminated the transient (switch bounce).

    Nick, thanks for your suggestions which lead me to the solution (I repeated the load test and realized the switch was the culprit).

    Tracy

  • 0 in reply to Tracy Brewer
    TI__Guru 50025 points

    Hi Tracy,

    Glad I could (indirectly) help!

    Regards,

    Nick