Purpose of L1 and C5?

Discussion in 'General Electronics Chat' started by spinnaker, Dec 10, 2011.

  1. spinnaker

    spinnaker Thread Starter Well-Known Member

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    Can someone please tell me the purpose of L1 and C5 in this schematic?

    http://www.olimex.com/dev/images/PIC/PIC-USB-STK-A-sch.gif

    The connector is for SD memory.


    The cap seems to be an over sized decoupling cap. But if I had to guess I would say the combo has do do with hot swapping of the SD memory chip.

    Does it reduce voltage spikes on the chip when it is connected?
  2. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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    It appears to be a simple LC power line filter. It is going to the Vdd power connection from the 3.3VDC power input. For anyone having trouble finding it it is on the lower right hand side of the schematic.
  3. spinnaker

    spinnaker Thread Starter Well-Known Member

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    Why would the SD need a filter if the pic does not? I have seen this configuration with other SD projects too.


    Maybe this explains it?

    http://elm-chan.org/docs/mmc/mmc_e.html

    Near the bottom of the page. It prevents a surge current. I was kind of close if this article is right.
  4. JMac3108

    JMac3108 Member

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    C5 is a bulk storage capacitor for the SD card. Its standard practice to put a bulk cap, usually tantalum or electrolytic, on external devices or interfaces. The capacitor stores energy locally so that it can provide it to the load, in this case an SD card, during high currrent draw conditions without having to draw currnent through the PCB traces which could possible cause a voltage dip at the SD card. Good design practice says you should also have a smaller ceramic bypass cap in parallel with the bulk cap.

    The inductor is just there to filter the power. Perhaps the 3.3V power supply is noisy and needs extra filtering for the SD card. Perhaps the designer was just being cautious.
  5. thatoneguy

    thatoneguy AAC Fanatic!

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    Reading/writing SD Cards takes a lot of power, relatively, I suspect those components are there to keep the supply voltage from dropping out at a critical moment.

    I'll have to dig out a datasheet, but I know a standard fullsize 2GB / "old style" SD Card gets quite warm to the touch when writing 1GB of files to it. I don't think the new SDHC micro cards are as power hungry, but the early SD cards were.

    --ETA: 65mA read and 75mA write, that has come down a bit since 2003, well, a lot.
    Last edited: Dec 11, 2011
  6. GetDeviceInfo

    GetDeviceInfo Senior Member

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    I'm thinking that the inductor prevents dips on the main during 'hot swap', while the cap filters dips during card activity.
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