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AM3358: change on VDD_MPU pin

Shine
Genius 13655 points

Part Number: AM3358

Hello Champs,

HW:  own AM3358 board

SW: Qt4.8.4 ,  AM335xSDK 06_00_00_00 （customer doesn't want to change to new SDK version）
https://software-dl.ti.com/sitara_linux/esd/AM335xSDK/06_00_00_00/index_FDS.html

Customer used QT to refresh the 8 items every 200ms, but the VDD_MPU pin also changed every 200ms (if remove 10nf capacitor, the phenomena is more obvious). Please see below pictures for details.

What may cause the change on VDD_MPU pin?





Thanks.
Rgds
Shine

  • 0
    TI__Guru 64905 points

    I'm not able to understand what is being tested from the information provided above.  Please provide more details. 

    For example, what is QT?  Also what is the equipment shown in the short video loop and what is it doing?  

  • 0 in reply to peaves
    TI__Genius 13655 points

    Hi Peaves,

    Thanks for your supporting.

    Customer replied that it doesn't need to know the specific equipment. He ran a Qt app on his own board as shown on above image. The second above image shows the VDD_MPU wave without 10uf capacitor. 

    Part of the hardware is below:


    Thanks.
    Rgds
    Shine


  • 0 in reply to Shine
    TI__Guru 64905 points

    I only provide hardware support and do not know anything about software or QT.

    I will not be able to help if the customer is not willing to explain what he is doing to cause the power supply variation. The schematic you inserted is too small for me to read, so I'm not able to comment on the circuit shown.

    It sounds like the system design has a power supply stability issue, where the 10uF capacitor is acting as a filter that helps stabilize the power supply.

    Regards,
    Paul

  • 0 in reply to peaves
    TI__Genius 13655 points

    Hi Peaves,

    Sorry, I resent the schematic, please help check if there is something wrong with the VDD_MPU circuit.

    It seems that the changing frequency of VDD_MPU is 200ms which is the same to the Qt app refreshing frequency 200ms. So customer wanted to know if the 200ms on VDD_MPU may be caused by the Qt app?

    Thanks.
    Rgds
    Shine

  • +1 in reply to Shine
    TI__Guru 64905 points

    This looks like a power supply resonance created by the natural resonate frequency of the inductor F36 and the decoupling capacitors connected between F36 and the AM335x VDD_CORE supply pins. Ask them to short across F36 and see if the problem goes away.

    Inserting a series inductor in the path of a high current transient load is not good design practice. This can only be done when there is enough de-coupling capacitance placed between the inductor and the load to ensure the voltage does not have any significant drop during the high current transients.

    Regards,
    Paul