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Original question:

LAUNCHXL-F280049C: Powering LaunchPad with External Supply

LAUNCHXL-F280049C: Power supply on launchpad

Kai Ruetterswoerden
Prodigy 100 points

Part Number: LAUNCHXL-F280049C

Hi,

we tried to use the launchpad as a seperate controller board without any connection via USB. So we power the board via the 3V3 connection.

As we need a 5V supply, we set the jumper on JP9 to use the 5V from the Step-Up-regulator U5.(JP1 ..3 are all disconnected)

There we found a very strange behaviour: With the jumper connected, the controller hangs immediately. If the jumper is left open, program works as expected. The 5V are loaded with only a few mA, the 3V3 supply can deliver about 2 A. So I don't think, an overload would be the reason. Any suggestions, what the reason can be?

Regards

Kai

  • 0
    TI__Guru* 93800 points

    Hello - due to some of our staff being out of office this week, there may be a delay in getting back to you until Friday/Monday.  I will check to see if there is someone who can help sooner.  I appreciate your patience. 

  • 0
    TI__Guru* 85850 points

    Kai,

    The 5V step up supply is for the QEP channels/level shifter in case that needs to be powered up when there is no 5V from the USB(JP3 open).  However, the 5V signal/domain also runs to the booster pack headers at J3-21 and J7-61(if JP8 is populated).

    Is there anything connected to the booster pack headers that may be contending with the 5V?

    If you don't have need of the QEP in your design you can safely leave JP9 open.

    Best,

    Matthew

  • 0 in reply to MatthewPate
    Prodigy 100 points

    Matthew,

    so I'll try to repeat if I understand your post correctly:

    JP9 is only for the QEP level shifter?

    JP8 is for providing the voltage to only the booster pack headers J3 and J7?

    Our intention was to use the supply for some low power sensors on the booster pack headers. No other supply is connected there. We referred to the schematics in SPRUII7B on page 19. There JP9 seems to connect/disconnect the compelete 5V0 supply. It is not clear, if JP9 connects only the QEP level shifter. So JP9 was shunted in our case as well as JP8.

    But in my opinion there is still no reason for the controller to crash, if JP9 is shunted, and not, if JP 9 is left open.

    Regards

    Kai

  • 0 in reply to Kai Ruetterswoerden
    TI__Guru* 85850 points

    Kai,

    Yes, you are correct that JP9 will enable/disable the on-board Booster 5V to the whole board. 

    I was unclear in my explanation; I was trying to say that the reason that we placed the 5V boost was due to the QEP encoder(U7) device on the LaunchPad needed 5V and this was our soln if only 3.3V was supplied externally. 

    J3 and J7 just bring the 5V bus to the bottom booster headers if present(from the USB or otherwise).

    The boost supply is LMR62421MFX/NOPB, and according to that data sheet is capable of 2.1A.  According to the DS for the QEP level shifter it should only consume 4uA, so I agree you should be OK powered some other typical components.

    The only time I've seen issues is when customers leave JP3 populated and then populate JP9, which potentially causes contention if the USB 5V is very different from the Boosted 5V.  Most of the time this just means the boost supply gets hot, etc.

    Could you verify that if JP3 is open and JP9 is open measuring the 5V bus you don't see any voltage?  Perhaps there is a rogue charged cap, but I think that would resolve on its own.

    Best,
    Matthew