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TPS65988EVM: Issue with RCP when configured to allow both sink paths to close switch

Will Oliver
Prodigy 10 points

Part Number: TPS65988EVM

Hi,

I am evaluating the internal and external power paths of the TPS65988EVM for use in a dual sink application (no sourcing required). For this application a constant 20V must be supplied to the target system with the ability for the two ports to be hot plugged. To achieve this I have had to configure the multiport sink policy to allow both paths to close, otherwise a significant voltage drop during port switchover can be seen (image 1). 

image 1:

However, as a result of this change there is an issue with the reverse current protection between SYS_PWR and VBUS when:

  • One port is connected, limited to 5V
  • The second port is connected, negotiating 20V
  • During the second ports ramp to 20V the RCP comparator (image 3) will not disconnect the first port from system power, causing it to rise to 20V also (image 2)

image 2:                                                                                   

Image 3:

Currently, I have got round this issue by configuring the external paths to only be enabled once the 20V is negotiated, however this is not desirable as we also wish for 5V to be present on SYS_PWR if that is the highest VBUS voltage present. I have also attempted to tweak the comparator circuit in simulation but have been unable to remove this issue. I have noticed from testing that this issue does not occur when using the internal power paths with the same configuration, but we would prefer to use the external paths for additional flexibility and thermal management.

Could you please advise me on how to design an external power path without this issue or perhaps suggest an alternate solution to changing the multiport sink policy for solving the voltage drop seen in image 1? Or whether in fact the internal paths are the best solution possible?

Regards,

Will

  • 0
    TI__Mastermind 34476 points

    Hi Will, 

    I'm looking into this and will provide feedback early next week. 

    Thanks and Regards,

    Raymond Lin

  • 0
    TI__Mastermind 36756 points

    Will, 

    It looks like you're running into an issue with the discrete power path implementation on the TPS65988EVM. It will likely be easier to use two of the TPS66120 power path ICs instead of this discrete circuit. The TPS66120 integrates reverse current protection. 

    You're not seeing any issues when using the internal power paths on the TPS65988 because the internal power paths also integrate RCP.

    Thank you,
    Eric