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No 3.3v output capacitor on Stellaris/Tiva Launchpads?

Cruz Monrreal
Prodigy 110 points

I've been working on a BoosterPack with the Stellaris Launchpad, where the power domain is the following:

Battery (BoosterPack) -> 5v Regulator (BoosterPack) -> VBUS (Launchpad Pin) -> 3.3v Regulator (Launchpad) -> MCU (Launchpad)

At the moment, the BoosterPack drivers a large inductive load on startup from the 5v Regulator (BoosterPack), which is controlled by the MCU. Due to the relation of the MCU and inductive load, I'm constantly getting a boot loop.

I have sufficient capacitance on the 5v (BoosterPack) rail, but I was surprised to find out that the 3.3v (Launchpad) rail doesn't have a single capacitor coming out of the regulator. I'll be adding more capacitance to the 3.3v line, but I just wanted to point this out.

  • 0
    TI__Guru 58650 points

    Hi Chris,

    I spoke to the designer of the Launchpad, and he had the following comments:

    1) using the MCU to turn on/off its own power supply is not a good idea. Somehow, you should keep the 5 V regulator on until the MCU is fully operational.
    2) seems like you need more current drive to your 5v regulator to not droop it which in turns causes issues with the 3v rail.
    3) there aren't really specs/rules on the power domain for launchpad plus boosterpack combinations.  If you need something more than what is on the launchpad, you have to provide it.  

    Regards,

    Sue

  • 0 in reply to Sue Cozart
    Guru 117855 points

    Sue's advice always first cabin. 

    As regards 3V3 - suspect poster's review of 3V3 regulator spec to be "best source" for that regulator's, "care/handling."

  • 0 in reply to Sue Cozart
    Prodigy 110 points

    1) Sorry, I think I misspoke about the 5v Regulated power supply. This is on as soon as the battery is plugged in. The MCU is not involved in turning this on/off. I'm looking into adding more capacitance on the line, or somehow delaying the 3.3v line to enable the inductive loads to stabilize.

    2) Yes. The regulator that I'm using should be able to push out a peak of 8A, but I think a combination of adding more capacitance and delaying the 3.3v line should do the trick.

    3) Indeed. I had wanted to use the 3.3v regulator on the Launchpad by powering the VBUS line, but I think I'll be powering the 3.3v line instead.

    Thanks your your input.

    Cruz M.

    PS: The BoosterPack in question.