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LM2700MT-ADJEV: Board Modifications

Jim Balcerovich
Prodigy 40 points
Part Number: LM2700MT-ADJEV
Other Parts Discussed in Thread: TPS61175, LM5000

I attempted to modify this Eval board for a 12V input and 17V output based on the Webbench Design Report.  I did the following:

Added Cc2, 220pF Cap, using PN GRM31A5C2J221JW01D

Replaced Cc with 2.2nF Cap, using PN GRM3195C1H222JA01D

Replace Rc with 162K Res, using PN CRCW1206162KFKEA

Replace R1 with 1.05K Res, using CRCW12061K05FKEA

Replace R2 with 13K Res, using CRCW120613K0FKEA

When I connected to a power supply, set to 12VDC, 1A limit, the supply output dropped to ~1.6VDC.  

I'm running at 600Khz.  Its not clear, absent actually in the users manual, but I've had the SD jumpers in both On and Off, although I assume from the schematic that floating is fine?

Why would my input be practically shorted when I apply power?  I can't see why there would be a problem w/ changing the above values on the eval board based on the Webbench report.  I didn't change was Cout, I left it as 22uF, and Cin as a single 4.7uF, but this shouldn't cause what I'm seeing should it?  

Please point me at what I may be doing wrong here. 

  • 0
    Prodigy 40 points

    A change to my original post, R1 and R2 are swapped in terms of values above.  The Eval board and the Webbench did not retain the same Ref Des.  So on the Eval board, I now have R1 at 13K, and R2 at 1.05K.  Doesn't make any difference however, my input source is still current limiting.  

  • 0 in reply to Jim Balcerovich
    TI__Mastermind 46905 points
    Hi Jim,
    it seems the input is short to ground caused the assembling issue or the damaged.
    please test the impedance between VIN and GND. if it is very small, remove the components one by one to check which components is damaged.
    the 12V is equal to the limit of this device. if there is variation in the input voltage, the IC could be damage.

    what is the output current in the application?
  • 0 in reply to Jasper Li
    Prodigy 40 points

    Good Morning, and thank you for the reply.

     The application I have will step up a 12vdc switching supply to a 15V signal, which I will then linearly regulate back down to 12 for a clean source.  My output current is <1A peak, ~500mA nominal.

     I am concerned about the 12V max on this input, as my source supply is 12V, and there is always a +/- associated with that, not to mention switching or noise spikes.  Do you have a device which has a higher tolerance or input voltage threshold but operates in the same manner?

    My current theory is I may have damaged the chip by having the feedback resistors swapped.  The Webench design tool output and the Eval board have Rfb1 and Rfb2 swapped between them.  I have ordered more chips, but won't have them before next week.

  • 0 in reply to Jim Balcerovich
    TI__Mastermind 46905 points
    Hi Jim,
    I would suggest TPS61175 for this application. The input voltage can be up to 18V. you can apply an EVM for evaluation.
  • 0 in reply to Jasper Li
    Prodigy 40 points
    Thank You. Would this be better than the LM5000? Appreciate your help.
  • 0 in reply to Jim Balcerovich
    TI__Mastermind 46905 points
    Hi Jim,

    yes. higher current capability, smaller IC size, thermal pad helping to improve the thermal performance.