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Original question:

LP2951: Spike in the output during turn ON

LP2951: Is the output voltage of LP2951 overshooted when VIN voltage rising time is slower than the reference thread?

Kazuya Nakai59
Guru 20130 points
Part Number: LP2951

 Hello guys,

 One of my customers is considering using LP2951 for their next products.

 They knew LP2951 output voltage is overshooted when VIN is powered up on the following thread.

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

 But the VIN rising time in the thread is very high speed.

 In the customer case, the VIN rising time will be 500us ~ 5ms range.

 Is the output overshoot happened at the rising time range?

 Your reply would be much appreciated.

 Best regards,

 Kazuya.

 

  • 0
    TI__Genius 13085 points

    Hey Kazuya,

    Because the device is in dropout (as mentioned in the previous post) a slower Vin ramp may help to reduce the overshoot however it will not eliminate the overshoot entirely. Vout will track with Vin and the LDO will take approximately 40us  after Vout exceeds the intended voltage to stop tracking Vin. This is why a slower ramping Vin will result in less overshoot, but  it will depend on how much Vin rises in those 40us.

  • 0 in reply to Kyle Van Renterghem
    Guru 20130 points

     Hi Kyle,

     Thank you very much for the prompt reply. I could understood LDO overshoot can not be eliminated completely.

     Could I ask you an additional question?

     I think the 40us is come from internal error amp response time mainly.

     I found the following TI appli note which discribes about LDO overshoot. 

    https://www.ti.com/lit/an/sbva060/sbva060.pdf

     This document says a overshoot is caused by the relation between VIN rising time

    and the internal reference voltage rising time when VIN rising time range is very fast.

     I think the 40us you told is different mechanism from the thread.

     Is my understanding correct and can the 40us be applied when VIN rising time is slower

    (For example, 0.5ms~10ms) than the rising time (about less than 40us) in the first URL?

     Thank you again and best regards,

     Kazuya.

  • 0 in reply to Kazuya Nakai59
    TI__Genius 13085 points

    Hi Kazuya,

    You are correct the 40us comes from the response time of the internal error amp. 

    As for the App Note on avoiding startup overshoot, it says that if the Vin ramp rate is too slow then the LDO will be in dropout and therefore it will track Vin and overshoot (see figure 6 below). 

    But if the input voltage rises fast enough to keep the LDO out of dropout then the output will not overshoot (see figure 5 below)

    This LDO specifically (LP2951) has an overshoot during startup even if it is not recovering from dropout as shown in the datasheet figure 21. So in your case by slowing down your Vin ramp time, you may put the LDO into dropout and reduce the overshoot since Vin is relatively low as the Vout reaches the intended regulation point. 

    I hope that helps explain it a bit better. 

  • 0 in reply to Kyle Van Renterghem
    Guru 20130 points

     Hello Kyle,

     Thank you very much for your reply.

     I got it.

     I will tell the customer to power Vin up with ramp time as slower as they can. 

     Thank you again and best regards,

     Kazuya.