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WEBENCH® Tools/LM25118: Function of Cboot: incorrect Value causes High FET to smoke. IRDA, Micrograbber

Jeff cohen
Prodigy 80 points
Part Number: LM25118

Tool/software: WEBENCH® Design Tools

Dear TI Support,

I'm really an FPGA/CPLD  VHDL and Verilog designer. I took over a board that was mostly completed. I inherited the LM25118 part of the design which was already laid out and routed.

I just got in my first boards which used a Webbench generated reference design for the LM25118. 

The design is for 12V output at 3A. The input is 12-36V.

The power supply feeding the design was set for 25V at 2A.  The output is about 0.2V, and eventally the HB FET smoked. I checked the schematic and the value of Cboot was not correct. It is supposed to be 100 nF, but it is 180pF. Would this cause the FET to fail? Why?

Also: The reference design specifies a 100nF capacitor for Cinx X7R Kemet C0805C104K5RACTU  ESR = 64,0 mOhm, VDC = 50.0 IRMS=1.64A. The capacitor specified for Cboot is a Yageo X7R CC0805KRX7R8BB104, 100nF VDC=25V IRMS =0.0A, can I use Cinx capacitor for Cboot?  I have some extra parts. What is IRMS?

While I have your attention: Is there a grabber test lead that is small enough to hook onto one of the pins?  Could you recommend anything?

Thank you very much for all of your help and support.

Jeff Cohen Hytorc Inc. Mawah, New Jersey USA

  • 0
    TI__Mastermind 21720 points
    Hi Jeff,

    After the MOSFET is damaged and the CBOOT value is changed and the MOSFET be replaced is the circuit still functioning? It could be possible for a small CBOOT value to damage the MOSFET. The CBOOT provides energy for the gate driver when the high-side FET is on. For example if the HB to SW voltage drops drastically when the HO signal is high and drives the FET below the threshold voltage but above the HB to SW UVLO threshold this would cause the FET to have a large RDSON leading to a possible over heating condition. The Yageo capacitor should be fine.

    Normally instead of using a grabber directly on the pin it is easier to solder a twisted pair close to the pin. Once the twisted pair is on the board this can be probed with a normal oscilloscope probe.

    Thanks,

    Garrett
  • 0 in reply to Garrett Roecker
    Prodigy 80 points
    Hello Garrett,

    Thank you very much for the speedy reply. I was able to replace the capacitor. I need help replacing the transistor.
    It should be done on Monday. Thanks once again

    Jeff
  • 0 in reply to Jeff cohen
    Prodigy 80 points

    Hello Garrett,

    The regulator is now working. There was a short in the copper pour that goes between the inductor output, the two 

    diodes and ground. I was able to cut the copper pour. Thank you very much for all of your help. 

    Jeff