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UCD7242: Placement ordering for output capacitors

Larry Shaffer
Prodigy 240 points
Part Number: UCD7242
Other Parts Discussed in Thread: UCD9222,

Hello,

I am working on a new design using the TMS320C6678ACYP DSP. For powering the CVDD and CVDD1 power rails I am using the UCD9222 and the UCD7242 following the TI application notes and the EVM schematic for the output capacitors on the UCD7242, both for capacitor values and the number of capacitors of each value. For these output capacitors, on the other side of the inductor from the UCD7242 SW-A and SW-B outputs, there is also a 1K ohm resistor. My question is: In what order should these components be physically placed on the PCB since some will be very close to the inductor and some will be somewhat further away? In the attached file “IC2002” is the UCD7242 and “IC2001” is the UCD9222. The figure in the attached document shows which components are closest to the SW-node inductor and which are further away. Is this the correct arrangement, or how should the physical placement be done?

 Document3.docx

Regards,

Larry

  • 0
    TI__Genius 10000 points

    Larry, 

    I have assigned this thread to the responsible applications engineer, you should hear back by Monday.

    Thanks,

    Nastasha

  • 0 in reply to Nastasha
    TI__Expert 3525 points

    Hi Larry,

    You can keep one or two 47uf and one 330uf close to inductor side, then the rest of capacitors close to the load. The 1kohm in parallel is acting as a small load, so it doesn't matter too much where you place it as long as it can provide bias current. The voltage divider may already provide small load current so you can decide if you need extra load resistor based on your validation.

    The key is to sense output voltage across capacitor as close as possible to the load, then route the positive and negative differential sense signals as in differential pair back to the controller.  Keep the voltage sense traces away from noisy sources on board.

  • 0 in reply to Qingquan Tang
    Prodigy 240 points

    Great,

    thanks for your help,

    Larry