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Last Post 18 Dec 2015 03:24 PM by  Adam Ford
3.3V_or_1.8V: How much current is available for user circuits
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Scott Battocchi
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03 Dec 2015 12:49 PM

    The OMAP-L138 SOM-M1 Hardware Specification / Hardware Documentation cautions that for the 3.3V_or_1.8V voltage rail:"I/O Voltage Output from SOM. Do not use this as a general purpose power source. Use this pin to power level shifters etc."

    Looking at the schematic it seems like the TPS65070 DCDC2 channel can supply 1.5A of output current.  I would like to use the 3.3V_or_1.8V rail to power a couple of circuits that do not provide a separate IOV/VDDIO pin:

    • A microSD card (<100mA peak, much lower average)
    • A couple of clock oscillators (~15mA total)

    It seems like these should still be well within the safe range of the DCDC converter but I just wanted to confirm that it would be reasonable to connect these directly to 3.3V_or 1.8V saving me the use of level shifters.

    Thanks,
    Scott

     

    Adam Ford
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    03 Dec 2015 01:38 PM
    I will inquire with the hardware developer. I think the limitation also factors the connector, board layout and some other items. Let me see if I can get a better estimate, and I will let you know.

    adam
    Adam Ford
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    16 Dec 2015 10:05 AM
    Nobody here has gone through the work of vetting the max current that can be drawn. I'm going to try do this exercise in between projects, but I can't promise it quickly with the holidays approaching.

    My initial thought was it would probably work, but I cannot guarantee anything.

    adam
    Adam Ford
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    16 Dec 2015 10:07 AM
    If you register our SOM, you can get access to our baseboard schematics. From a support perspective, the SD interface in a tested interface and it you match our schematic, the chances of other issues goes down.

    adam
    Scott Battocchi
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    16 Dec 2015 10:24 AM
    Adam,
    Thanks for getting back to me. I've been happily loading down the 3.3V_or_1.8V rail with an electronic load set to 200mA since December 3 with no noticeable impacts to the system performance.
    We are actually following the baseboard schematic right now (including the TXS level translator), but we are hoping to simplify the system by dropping the translator and running the card straight off the IO voltage rail. We have also noticed a significant amount of EMI emission from the TXS parts on our board which is another reason we'd like to drop it.
    If you get around to verifying the max (or even a reasonable-but-less-than-max) current we can draw off of that rail I'd still be interested to hear it, but I understand it may not happen for a while.
    Thanks again,
    Scott
    Adam Ford
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    17 Dec 2015 01:34 PM
    I am going to double check with one of the Hardware engineers to see if he has any concerns about the max current.

    I was able to locate this web site at TI: http://processors.wiki.ti.com/index...on_Summary

    In it, there is a spreadsheet to download. I would suggest that you fill it out enabling the various peripherals, etc. for your application if you don't have the Wattson hardware for measuring power.

    Because the development kit was obsoleted a while ago by TI (then by us), it took me a while to find one. I'll try to have something for you tomorrow on what the base Linux installation uses for power.

    adam
    Adam Ford
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    18 Dec 2015 03:24 PM
    Running a Demo CD with the video going and the DSP, etc. I took measurements on the following

    Ch mW V Name
    1 1800 4.9 5V Main
    2 530 3.28 3.3 PMIC Switcher
    3 148 3.31 1.8V/3.3 IO Voltage PMIC Switcher
    4 295 1.2 1.2V PMIC Switcher
    5 150 1.2 1.2V PMIC LDO
    6. 248 1.79 1.8V PMIC LDO
    7 9.9 1.79 1.2V RTC LDO

    Looking at the 1.8V / 3.3V rail, it looks like we're well under the peak current. I have put in a request with the EE who worked on this SOM, but it looks promising for what you want to do. I'll let you know when I get confirmation.

    adam
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