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Original question:

LP2951: Spike in the output during turn ON

LP2950: Vout overshoot

Go
Genius 4155 points
Part Number: LP2950

Hi,

I'd like to ask about the thread below.

https://e2e.ti.com/support/power-management/f/196/t/819132

Will LP2950 have the same phenomenon?

(If VIN rises quickly, VOUT tracks VIN and overshoots above the nominal output voltage.)

and what value should we reduce a VIN slew rate to prevent this phenomenon?

Is there some target value?

Thanks,

Go

  • 0
    TI__Mastermind 38115 points

    Hello,

    I am looking into your question and I will provide an answer tomorrow (7/28).

    Best regards,

    Nick

  • 0
    TI__Mastermind 38115 points

    Hello,

    The issue of overshoot is common to all LDOs, so the LP2950 will also exhibit the same properties. 

    The good news is that there are well-documented ways to avoid overshoot, which are summarized in this app note. The app note also explains the cause of overshoot on startup - in particular section 2.4 - so I recommend reading it in its entirety to get a good understanding of how to avoid it. 

    The exact cause and consequently the appropriate solution to overshoot depends on the application. In one case, the VIN slew rate should be sufficiently fast so that the input is stable before the output reaches its target voltage. I point this out because you suggested reducing the slew rate, which I understand is counter-intuitive that the opposite should actually be done. As for the actual nominal value for the input slew rate, it is hard to say for this particular device because the datasheet does not specify the slew rate of the reference voltage on startup. The example part used in the app note used an 8V/ms slew rate to achieve a startup with no overshoot, so that may be a good place to start.

    If the app note I linked is not enough and you have more questions, please don't hesitate to come back here with them!

    Best regards,

    Nick

  • 0 in reply to Nick Butts
    Genius 4155 points

    Hi, Nick

    Thank you for your advice. 

    This app note will be a great help.

    I have additional question about Figure 21 in LP295x D/S below,

    Vout overshoot can be seen in this waveform.

    I'm not sure the exact value of Vin in this waveform, but I assume that Vin has already been at the nominal value before EN went low.

    In this case, I think this LDO isn't going  into drop-out. What is the cause of this overshoot?  

    Thanks, 

    Go

  • 0 in reply to Go
    TI__Mastermind 38115 points

    Hi Go,

    It is difficult to know for sure the reason without knowing the entire setup. My best guess is that the output capacitor value used for that scope shot is 10uF, which is larger than the minimum required. Section 8.2.2.1 from the datasheet explains the minimum Cout requirement:

    In Fig. 21, IL = 1mA, and according the the description above, for this load current only 0.1uF is required for stable operation. So back to the app note, having a larger than necessary output cap increases the amount of inrush current, which can still cause the output to overshoot on startup. So depending on the load you expect, you will need to adjust the size of the output cap such that it is a good balance of large enough for stable operation and small enough as to avoid overshoot on startup. 

    Hope this helps,

    Nick