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SD/MMC ESD protection necessary ?

Younès Gahnem
Intellectual 300 points

Hello,

I saw several evalboard schematics for the AM335x and observed there is a ESD-protection between the SD-Host and SD card.

Knowing that most of the SD-Card contains some ESD-protection, do you think it is really necessary to add a ESD-protection circuit ?

As far as I'm concerned, I never used any ESD-protection for SD-memory. Nevertheless, I always care that the SD itself if industrial type.

Thanks in advance,

Regards

Younes

  • 0
    TI__Guru**** 393215 points

    Hi Younes,

    This ESD protection ensures there is no damage to the processor pins if there is an ESD discharge on the SD Card cage pins while there is no card present.

  • 0 in reply to Biser Gatchev-XID
    Intellectual 300 points
    Hi Biser,

    This is clear for me. But I thought maybe the host has an ESD protection.
    And like I said, I designed board without that.

    I'm talking about that because we're trying to reduce cost as much as possible and like you know ESD protection are not that cheap.
    But ok , I think you've pointed out something interesting. There is a risc when when there is no card.

    Thank you very much ,
    I think this is ok

    Regards

    younes
  • 0 in reply to Younès Gahnem
    TI__Guru**** 393215 points

    Hi Younes,

    younes GHANEM said:
    But I thought maybe the host has an ESD protection.

    Please note that the processor's level of ESD protection only ensures there will be no damage to the device during handling/assembly. This is specified in section 5.2 of the AM335X Datasheet, Rev. H.

  • 0 in reply to Younès Gahnem
    Genius 3685 points

    If your SD card connector is inside a housing, then:

    - nobody can put fingers at the CPU SD card I/Os.

    - if a SD card is inserted, you can not have your fingers on the SD card contacts while inserting.

      So there can be no contact between fingers and CPU SD card I/Os.

    - There only can be a contact between CPU SD card I/Os and the SD card contacts.

      But as the Capacitance of SD card against earth is low, the ESD pulse will be weak.

    We manufacture devices with SD card connectors without ESD protection. So far

    there a no complains.

    The 3 biggest problems for SD card insertion in practice are:

    1) at SD card insertion, you have to wait ca. 500ms from "card detect" to the first access to the SD card.

      (Customer may insert the SD card slowly).

    2) Inserting the SD card may produce a short current spike at 3,3 Volt because the capacitor on the SD card is loaded.

      You have to provide some sort of capacitor to block this current at the SD card connector.

    3) The write protect switch at the SD card tend to be unreliable (moving at insertion).

    It's the best to disable the write protect detection. For most embedded devices, write protect detection is useless.

    regards

    Wolfgang

  • 0 in reply to Wolfgang Muees1
    Intellectual 300 points
    Thank you very much Wolfgang.
    Just one point concerning the
    "- There only can be a contact between CPU SD card I/Os and the SD card contacts.
    But as the Capacitance of SD card against earth is low, the ESD pulse will be weak"

    I would say : There is always a contact between CPU SD-Card I/Os and the SD-card contacts. But I think that is what you wanted to say.
    Regarding the capacitance of SD card, you say it is low. What do you mean by low and is there any document where I can find that ? Is this specified somewhere on a datasheet ?
    When you're saying the ESD will be weak, what do you mean by weak ?

    Thank you,

    Regards,

    younes