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LMZ31710: Problem starting up - switch node stops switching

Christopher Perkins
Prodigy 10 points
Part Number: LMZ31710

Hi -

   Anyone else ever have a problem where the switch node (PH) stops switching and the regulator won't fully come up to voltage?

   I'm making VOUT = 1.0V from VIN = 5.0V and after about 2.5ms, VOUT is only about half-way up to 1.0V and the PH node stops switching and VOUT decays back to zero.  After this, there is no hiccup-mode restart or any other attempt at coming up to voltage.  VIN is solidly at 5.0V with no sag or undervoltage.  SS looks normal.  INH is enabled (high) the whole time and doesn't sag or anything.  I also checked VSENSE and it looks fine.  This voltage powers VCCINT on a Xilinx FPGA.

   Here are some scope traces that show VOUT only coming up to 0.5V, PH stops switching, while VIN is solid and SS is normal :

    Here is a zoom-in at the point that PH stops switching :

   Here is the schematic :

   In the scope traces above I tried removing C59 to see if a smaller output load capacitance would help but it didn't.  With C59 included, VOUT actually shot up to > 1.0V very quickly (in about 0.2ms it went to 1.5V) and PH stopped switching at the same point as seen above (~2.5ms).  I have these scope traces if it would help debug.

   We made a batch of these boards and some of them have this startup problem while some of them work fine - VOUT comes up to 1.0V in the expected ~1.2ms and PH switches as expected.

   Any idea why the PH node would stop switching and never try to startup again?  Or why some boards would work and some don't?  Or anything else to try?

thanks,

Chris

  • 0
    TI__Mastermind 32350 points

    Hello Chris, 

    Thanks for the detailed information. 

    • Since this is happening on only few boards. The first thing I would try to isolate the issue is to do swap test (ABA test). Basically take a good known board and populate the IC that has this issue, and do the same with the bad known board and populate it with a good known board. This way you will have a better idea is the issue is related to the BOM or the IC itself.   
    • Soldering issues can be the root cause, but by doing the above test can tell. Because if the device is not soldered properly you may see all king of issues.   
    • The other thing that you need to double check is your external SYNC clock, make sure that is within the acceptable level. 
    • I do not know where this VIN is coming from, just for sanity check it right at the pin of the device not at the source, because you may see drop at the pin and not seeing at the source.  

    Thanks! 

    Tahar