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Original question:

LP87561-Q1: Impact of Cout_Local value on stability performance

LP87561-Q1: Doubts on the EVM design and layout/component recommendations

Anoop Joshi
Expert 1125 points
Part Number: LP87561-Q1

Hi,

I had a few doubts regarding the EVM design and layout/component recommendations for this part, if those could be cleared it would be of great help to finalize my own design with this part:

  1. In the EVM BOM, the snubber capacitor part is mentioned as "GCM155R71H391KA37D" and its value is noted as 0.39uF. Is it a typo? This part number actually corresponds to 0.39nF ie 390pF (as the datasheet recommends). Also, this part number is different from "GCM1555C1H391JA16" that's recommended in the datasheet but I'm assuming that mostly they are interchangeable.
  2. Similarly, in the EVM BOM, the snubber resistor part is mentioned as "CRCW08053R90JNEA" as opposed to "CRCW04023R90JNED" that's recommended in the datasheet. Most importantly, the one used on the EVM is a bigger 0805 1/8W one and the datasheet recommended one is a smaller 0402 1/16W one. Which one out of the two is actually the most recommended one to go with?
  3. Is the recommendation to add ground vias even between the 2 pads of the inductors as shown below encircled in green? This is to reduce the EMI radiated?
  4. Does the copper area shape for the SW_Bx nets shown highlighted in yellow below look acceptable from a layout recommendation point of view? Unfortunately, both the recommended inductor parts "DFE252012PD-R47M" (which is a much smaller package, and is the actual one used on the EVM) and "IHLP1616AB-1A" both don't seem to be in stock with the vendors, and I have decided to go ahead with "IHLP1616ABERR47M01" which is the nearest matching one that I could find relative to "IHLP1616AB-1A". Hence the inductor footprint is a bit bigger than the one used in the EVM. From our side we have tried to make the SW_Bx node metal shape area as small as possible but it has to be at least big enough to accommodate the bigger inductor pad as shown highlighted in yellow below. Do you see any concern with this? Also, hope the placement of the snubber RC components as shown encircled in red looks OK as well?
5. As of now I'm placing two 22uF capacitors and an extra DNI cap option as the local bulk VOUT caps per Buck phase as shown encircled in red in the snapshot below. The dark blue shaded caps belong to one particular buck output ie buck phase and the yellow shaded caps to another phase. Since I'm intending to use the buck in 4 phase mode, I'm shorting the outputs together right after the inductors (inductors are shaded in green) using a metal shape as shown below. Is it OK if I keep the capacitors from both phases as near as shown below?
Thanks,
Anoop

  • 0
    Expert 1125 points

    Just wanted to note that the blue and yellow shaded 22uF capacitors in the image at the very bottom in the previous post that seem to overlap with the green shaded inductors are actually placed on the bottom side of the PCB, and the inductors on the top side.

  • 0
    TI__Mastermind 30280 points

    Anoop,

    This question has been assigned to the experts on the LP87561-Q1 device but they are based in Europe and I do not expect you will receive a response until Monday.

  • 0
    TI__Genius 11020 points

    Hello Anoop,

    1. Yes there is actually an error in the EVM user guide BOM. The capacitor should be 390pF as stated in the datasheet.

    2. 0402 size snubber resistor is usually fine, but you can also calculate the thermal dissipation for the resistor to be sure. For the EVM we wanted to have an option to vary the snubber components.

    3. The more vias you have the less inductance there will be and that will help with the EMI performance. 

    4. I see no concern with the SW node in that picture

    5. Current will anyway find the path with least impedance, so I don't see problem with this. You could draw the current loops including the input capacitors to make sure the loop formed is as small as possible. Please make sure there is sufficient amount of total capacitance regarding stability. See app note for reference: https://www.ti.com/lit/an/snva881/snva881.pdf 

    Thanks.

    Regards,

    Tomi Koskela

  • 0 in reply to Tomi Koskela
    Expert 1125 points

    Thanks Tomi for the quick response!

    Thanks,

    Anoop