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LM5007 startup after shutdown

Dave White
Prodigy 130 points
Other Parts Discussed in Thread: LM5007, LM25011, TPS54060A

Hi,


I am working on a design where we shut down the LM5007, it seems when the SD pin is released it goes pretty much directly back the the 1.5-2V level in normal run mode, but the convertor does not start up for ~600ms. I Can't see anything in the data about it - indeed it says it has no soft start but this is not softstarting - no switching is happening for 600ms then it just gets going as normal. I am rather wondering if we might have to swap chips whcih would be a shame as this is an existinig design - we are just changing it to allow it to run from battery...


Input is 20-24V output 13V.


Many thanks in advance for any input!


Dave

  • 0
    TI__Genius 11490 points

    Hi Dave,

    Is is possible to post the circuit? You can also reach me at vijay.choudhary@ti.com. In particular I am interested in knowing what is the output capacitor and output voltage waveform when the converter is not switching. Is there any pre-load on the output? Does the output drop voltage drop before the part starts switching?

    Thanks and regards,

    Vijay

  • 0 in reply to Vijay86929
    TI__Intellectual 1435 points

    Is there much load on the output when the converter is off?   The 600ms delay might be the time required for the output capacitor to discharge to a level that is ~5V below the 7V VCC regulator voltage  (Vout ~ 2V).   If Vout lingers above this level in the shutdown mode, the BST capacitor will not be charged above the BST-SW UVLO threshold and the switcher will be disabled until the output falls. 

  • 0 in reply to David Pace
    Prodigy 130 points

    David, Vijay,


    Hmm I can see where you are coming from, - sorry for my delayed response I sort of expected an email from the forum... have just logged in to see if there was anything!
    Ho Hum.


    I will get it out today and try some measurements on it - see if I can sort it - but no probably Vout is not low enough to let it go again, as we were deliberately letting it linger to see if we could keep a bit of cct up and running that was not drawing much power... Hmm. I will play and get back to you!


    Dave

  • 0 in reply to Dave White
    Prodigy 130 points

    As you can see yes there is plenty of C on the output - so again it points at what you are suggesting - I'll go measure :-)

  • 0 in reply to David Pace
    Prodigy 130 points

    David,


    Well it certainly looks like you are right, it (Vout) dischareges actually to about 1.5V and then starts again.


    I can't really deliberately discharge it ... let me explain what I am trying to do and see if a solution is possible (with the LM5007 or if not then without)...


    This provides power from various sources - originally wired mains... but latterly a battery. To provide 13V for some LEDs - now then when it (the light) is off - we need to keep the processor running (mostly sleeping) so need to keep the VCC (+5V but can be allowed to droop off) alive, with minimum battery drain, we were trying to run the switcher for the min time and let it droop in between so we had less loss in the switcher... hence we got here!


    Any thoughts?

  • 0 in reply to Dave White
    TI__Intellectual 1435 points

    Hello Dave,

    Based on the described application, I think there are other TI devices you should consider.   What is your  maximum input voltage requirement?

    If a  42V device is sufficient, then the LM25011 could work for you.  It includes a small low side FET that pre-charges the Bootstrap capacitor after turn on.    The current limit can be adjusted with an external resistor to level similar to the LM5007.  The control method is constant on-time which is the same as the LM5007.

     If higher voltage is needed, the 60V TPS54060A has the advantage of lower quiescent current and a bootstrap capacitor charging circuit powered from Vin instead of VCC.   With Vout near 13V and Vin of 18V or higher, the boost capacitor will be recharged immediate when the converter is enabled.    The TPS54060A  uses current mode control which requires a small compensation network on the COMP pin.   The soft-start (SS) capacitor can be small or deleted if a  little output overshoot is acceptable during turn-on,