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TPS706: Overshoot at start up

Kazuki Itoh
Genius 9635 points
Part Number: TPS706
Other Parts Discussed in Thread: TPS709, , TLV755P

Hello team,

My customer uses TPS70618 for 3.3V/1.8V conversion.

My customer observes the output voltage goes as high as 2.25V when VIN start up and it takes a couple of ms to settle to 1.8V.

I have a schematic but it looks good.

I have waveform which I can share with you offline.

Could you please help me to address this issue?

Regards,

Itoh

 

  • 0
    TI__Mastermind 33625 points

    Hi Itoh,

    I believe what you are describing is the device turning on while in dropout.  Please see the following app note on this topic.  The app note uses the TPS709 but the TPS706 is very similar but with a different current limit.

    www.ti.com/.../sbva060.pdf

    Thanks,

    - Stephen

  • 0 in reply to Stephen Ziel
    TI__Genius 9635 points

    Hi Stephen-san,

    As you can find from the internal link here, my customer tried EN start up following the appnote, but it didn't solve the issue.

    Could you please check the schematic and waveform from the link?

    Best regards,

    Itoh

  • 0 in reply to Kazuki Itoh
    TI__Mastermind 33625 points

    Hi Itoh,

    I received the waveform.  I believe the TPS706 is still in dropout during turn on due to the current needed to charge up the load capacitance in the 50us ramp time shown in the customer waveforms.  With C = 12.4uF, dV = 1.8, dt = 50us, you can solve for the current needed to charge the load capacitance and I = 446mA.  I would try reducing the 10uF load capacitor to a smaller value such as 2.2uF to see if the problem goes away.  This would yield I = 166 mA which looks to be closer to 1V of dropout needed based on the EC table.  The customer has 1.5V so this is probably okay.

    Thanks,

    - Stephen

  • 0 in reply to Stephen Ziel
    TI__Genius 9635 points

    Hello Stephen-san,

    The customer cannot reduce the output capacitance due to the load-side IC requirement.

    Also, controlling the input voltage slew rate is challenging. As the layout is done, the customer cannot use the LDO with SS function.

    Could you please help us to address this issue?

    For example, the alternative LDO with the same footprint (SOT-23-5) with larger current limit would be helpful.

    Best regards,

    Itoh  

     

  • 0 in reply to Kazuki Itoh
    TI__Mastermind 33625 points

    Hi Itoh,

    If they need the reverse current protection, the TPS706 is usually the best solution for this application.  An option which does not have reverse current protection but should eliminate the overshoot concerns is the TLV755P.

    Thanks,

    - Stephen

  • 0 in reply to Stephen Ziel
    TI__Genius 9635 points

    Hello Stephen-san,

    Thank you for your suggestion!

    I confirmed TLV755P is p2p to TPS706.

    The cause of the dropout should be current limit of TPS706. So you recommend TLV755P because it has higher output current limit 560mA (min.) which is larger than 446mA, correct?

    Regards,

    Itoh

  • 0 in reply to Kazuki Itoh
    TI__Mastermind 33625 points

    Hi Itoh-san,

    You are correct, that's exactly what we did.  Let me know if you need anything else.

    Thanks,

    - Stephen