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LM5149-Q1EVM-400: 15-72V input range limit

Part Number: LM5149-Q1EVM-400
Other Parts Discussed in Thread: LM5149

What is limiting the input voltage on this eval board to 72V (part is rated to 80V)? Noticed that C26 (on VIN) is only rated to 50V which is a problem even for 72V operation.

  • Hi Rick,

    With switching converters, it is possible that switch node can spike and overshoot the VIN.

    It is heavily dependent on SW slew rate, decoupling capacitance and some layout, it doesnt appear significantly on this but it is present.

    If operating at max operating voltage, and then if SW node spikes exceeds 85V ABSMAX, then it could potentially damage the device or MOSFETs.

    C26 has been updated to 100V on the board, the same P/N as C3.

    Hope this helps,


  • for my application, VIN can be as high as 75V. If needed, could I maybe put a small series resistance in the SW line and zener clamp it to 80V or do you have a better suggestion? I could also increase the FET rating to 100V, but want to make sure that the LM5149 doesn't get damaged.



  • Hi Rick,

    You can protect SW node overshoot with either bootstrap resistor, high-side FET gate resistor, or an switch node RC snubber.

    These all will slow down FET gate rise time and SW node rise time. 

    This might negatively impact efficiency but provide protection.

    The EVM only has a spot for bootstrap resistor R9.

    I'd say start at 72V, and monitor the SW node voltage (with a low inductance probe technique: see figure 3 in appnote).

    Once you increase VIN and see SW touching 80V then I recommend increasing boot resistor incrementally, maybe 0.5-1Ω.

    In your final design you can include spots for a SW node snubber (RC to GND) and a gate resistor if you want. 

    Hope this helps,