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Omap-L1/C6747 - Conflicting information about power sequence

Mariana
Mastermind 24340 points
Other Parts Discussed in Thread: TMS320C6747

Hi all,

In the TMS320C6747 data sheet section 6.3.1 specifies the power-on sequence as 1.2v -> 1.8v -> 3.3v.


Power supply examples included such as SLUA491a (page 4) show 1.2v -> 3.3v -> 1.8v as the sequence.

http://focus.ti.com/lit/an/slua491a/slua491a.pdf

They just have reverse order of 3.3v and 1.8v.  There are actually 3 power supply examples that are all the same but different from the datasheet.

Which one is correct?

  • 0
    TI__Guru 62975 points

    Mariana,

    It is ok to power up 3.3V before 1.8V.  There was a requirement that the 3.3V supply must stay below 1.65V until the 1.2V supply has reached 0.9V.  To avoid this complicated wording in the datasheet, it was easier to recommend a 1.2V -> 1.8V -> 3.3V power-up sequence.

    -Tommy

  • 0 in reply to tlee
    Expert 2860 points

    I had the same question than Mariana so I've been worked on my EVMOMAPL137 to test the power sequence at power-up. Well, I'd like to post the result that shows 1.2V -> 3.3V -> 1.8V sequence as well.

    ch1: 1.2V

    ch2: Alternate 1.2V

    ch3: 3.3V

    ch4: 1.8V

    Board: EVMOMAPL137 Rev.F

    Regarding to the power-off sequence. what could happen if I power off the 1.2 and 1.8V while I keep the 3.3V active?

    gaston

     

  • 0 in reply to Gaston Schelotto
    TI__Expert 8785 points

    Gaston said:
    Regarding to the power-off sequence. what could happen if I power off the 1.2 and 1.8V while I keep the 3.3V active?

    If you power on the 3.3V while the other rails are unpowered, I believe it will stress the transistors inside the device. 

    How long are you planning to leave the 3.3V active with everything else unpowered?  If it's a very short time (e.g., milliseconds or less), then it would probably not stress the transistors too much.  However, I would not recommend leaving it on for too long. 

    Is there a reason why you would like to leave the 3.3V active?

    --Christina

  • 0 in reply to clam
    Expert 2860 points

    clam said:
    Is there a reason why you would like to leave the 3.3V active?

    Oh no, I wouldn't like to do it. Especially, taking into account your suggestion :)

    Actually I've got some C6747 custom boards that definitely do not work. Simply, devices are not able to connect to the emulator through JTAG interface. This could be an assembly issue (Maybe a X-ray inspection could get me out of doubts) but I wonder If I was wrong when I powered the device with only 3.3V. Since in our power supply design there were also some bugs.

    Gaston

  • 0 in reply to Gaston Schelotto
    Mastermind 7560 points

    Hi Tommy,

    I'd like to ask you a question.

    As you mentioned　in your previous post, there is a requirement that the 3.3V supply must stay below 1.65V until the 1.2V supply has reached 0.9V when the power-up sequence is 1.2V -> 3.3V -> 1.8V.

    So, in case of the sequence as 1.2V  -> 1.8V -> 3.3V, is there still the same requirement?

    Thanks in advance for your cooperation

    Best regards,
    j-breeze