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DP83848I Power Feedback cap

paddu1
Expert 1195 points
Other Parts Discussed in Thread: DP83848I

Hi,

The DP83848I datasheet(SNLS207E) "1.9  Special Connections" mentions
Parallel cap 10µ F (Tantalum preferred) for the Power Feedback Output(PFBOUT).
Could you please let us know what is the reason for preferring the "Tantalum"
capacitor.But I have heard that Tantalum caps are usually costly and they tend to have failures.
Will there be a problem not using Tantalum cap for PFBOUT circuit?

Best Regards.
Paddu

  • 0
    TI__Mastermind 31600 points

    Paddu,

    The Tantalum capacitor is the load for an internal regulator which generates a 1.8V supply for the device.  The regulator design takes advantage of the parasitics associated with the Tantalum caps to improve the stability of the regulator.  Lowering the parasitics by using a ceramic cap will increase the bandwidth at the regulator output, but will also decrease the stability slightly.  The design should still have enough phase margin to provide the necessary stability though.  Thus, the use of the Tantalum cap would constitute best practice and would provide more stability for the regulator, but many designs have used ceramic caps instead and have not seen any issues.

    Patrick

  • 0 in reply to Patrick OFarrell
    TI__Mastermind 31600 points

    I moved this post to the Ethernet forum so that it will be grouped with posts of similar content.  Going forward, the Ethernet forum will be the best place for posts related to the TLK1xx and DP83xxx devices.

    Patrick

  • 0 in reply to Patrick OFarrell
    Intellectual 640 points
    In case one wants more stability margin, a small resistor can be inserted in series with the ceramic cap. The exact value of this resistor is equivalent to the amount of ESR wanted to improve stability.