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LM2830: LM2830 Overshoot on Startup

Dan Kletti
Prodigy 20 points
Part Number: LM2830

I have an LM2830X 5V to 3.3V design.  If the 5V is supplied from within our product, the 3.3V output has an overshoot to 3.6V before coming back down to regulate at 3.3V.  If I use a different 5V source with faster ramp rate, the LM2830X 3.3V supply does not have this overshoot.  Do you have an explanation for why this is happening, and how I can compensate for it?

  • 0
    TI__Mastermind 33341 points

    Hi Dan,

    Can you do some experiments so I can better understand the non-monotonic rise of the output voltage? Please see the details below.


    1. Disconnect the resistor connected between VIN and EN pins. Connect the EN pin to an external source (> 2.0V). Apply the 5.0V on VIN, then turn on the external source that supplies the EN pin (ensure that the VIN is already 5.0V before turning on the supply for EN pin). Capture the VIN, EN, and Vout waveforms.


    The purpose of this experiment is to find out the cause of the non-monotonic rise of the output. And if it is related to the VIN supply ramp behavior where the device goes to max duty cycle during the ramp-up phase.

    Best regards,

    Excel

  • 0 in reply to Excel Regidor
    Prodigy 20 points

    Hi,

    Thank you for your reply.  Yes, I have run the experiment you described.  I actually added a voltage supervisor to monitor the 5V, such that it releases the enable pin of the LM2830 when the 5V is above a certain voltage.  I used TLV840CADL29DBVR, which is 2.9V, plus a 80us time delay, because I had some on hand.  With this supervisor added, the LM2830 5V to 3.3V supply comes up much better and nearly monotonic.  While this is sort of a workaround to the issue, I'd still like to know what is causing the overshoot when the supervisor is not in place.

    Thanks,

    Dan

  • 0 in reply to Dan Kletti
    TI__Mastermind 33341 points

    Hi Dan,

    Thanks for the update.

    I think the device goes to max duty cycle (86% ~ 94%) during the ramp-up stage causing the output voltage overshoot. Because it takes time for the control loop to reduce the duty cycle to the desired level when the Vout exceeds the target level (see figure below for details). Hence, it is necessary to turn on the device when the supply voltage already to avoid this kind of event. 

    Best regards,

    Excel