This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM5175: Which FETs to use?

Part Number: LM5175
Other Parts Discussed in Thread: CSD17571Q2, CSD17308Q3, CSD17311Q5

Hi guys,

We generated a webench report, and the chosen FETs are a little fishy so just wanted to check with you guys. 


It calls out CSD17571Q2 and lists it as 30V 44amps.

The datasheet shows it as a 30V 22A.

The CSD17308Q3 is a 50A part, but states the gate is optimized for 5V so not sure if that is a better option. 

Any recommendations from you guys for this spec?



  • Hi Brain,

    Thanks for reaching out with your questions.

    Is the design being used for an automotive application? If so you will need to select an automotive qualified MOSFET.

    To help with thremal management I suggest using a MOSFET that is 5mmX6mm. This is a common package size and will allow you to modify the change the MOSFETs as needed once the board is built. The size also helps keep the MOSFETs from heating up. 30V should be enough margin on the MOSFET voltage rating. The CSD17311Q5 would be a good alternative.


  • I was concerned that the program was telling me to use a 44A part, whereas the part suggested is listed as being 22A. Aside from the Webench software part library needing to be updated to properly reflect datasheet parameters, I want to be sure that 44A isn’t really needed. I assume that the 22A is the closest value made since the actual current through the part should never be higher than 18A in my design (at 4V in and a 12V, 6A load [72W]).



  • Hi guys,

    Any comment here? Your suggestion is a 32A part, can we really not use a 22A part?

  • Hi Brian,

    I recommend going with the 32A rated device. This will allow for more margin than the 22A rated device. In this case it is safer to add a little more margin to ensure the MOSFET is not damaged.


  • Hi Garrett,

    Thanks again, but I'm not sure this helps me. It's hard to point my customer to the more expensive FET without anything to back up why.

    So, separately from the error in webench needing to be fixed (let me know if i should just open a new thread for that) where it recommends a FET for my customer's condition that you guys don't recommend, can you help me understand why a 22A part is not safe?

    The webench report shows this:

    So, max current is close, but below 22A. The webench report also mentions that the junction temp of the FET will be 66C, which doesnt seem like it's close to burning itself up. 

    I started to look at the FET SOA, but then calculated that the M3 FET is only turned on for less than 2uS at a time, given 363kHz switching and ~70% duty cycle at the 4Vin, 12Vout 6A condition. SOA is shown in DC to a millisecond, so I would think 2uS gives plenty of margin.

    In addition to needing to aid my customer, I am genuinely trying to learn here, in case there are other things I should be taking in consideration when looking at SMPS power stages. 



  • Hi
    The responsible engineer is out of office now. He will reply by 4/23.