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WEBENCH® Tools/LM5121-Q1: Parts Selection

Part Number: LM5121-Q1
Other Parts Discussed in Thread: CSD16340Q3, BQ24640, CSD17577Q3A, , LM5121

Tool/software: WEBENCH® Design Tools


Im trying to understand how the tool selects parts when it does the automatic selection. I tried 4.5Vin 9Vout 6Aout and the tool recommended different part numbers for the switch and forward diode/fet respectively CSD16340Q3 and CSD17577Q3A. I am planning to use CSD17577Q3A for both as well as to my BQ24640 circuit with 7.5V 6A output to simplify BOM. Please advise if this is ok or if I missed something important.



  • Emmanuel,

    Related to FET selection - Initially, the tool performs various voltage and current rating checks to ensure the FETs can work for the given input conditions. We create combinations of M1/M2 pairs for all FETs that could work and then estimate losses, footprint (for M1/M2) and cost for these pairs. If there is a FET combination where the losses are too high and FETs may overheat, then we eliminate that solution and carry on till all pairs are exhausted. Then, we run a ranking algorithm based on the "Design consideration" setting (for example balanced, low cost, etc.) to rank the FET pairs that will work best. The switching frequency is also varied for adjustable frequency parts based on the design consideration and taken into account while FET loss estimation.

    Coming to your question regarding using same FET CSD17577Q3A for LM5121-Q1's M1/M2 position - I do not see this combination in the recommended alternate FET list. It is possible some of the checks may be failing or the pair with same FETs (CSD17577Q3A) is ranking quite low. We can check this out and let you know what may be happening.

    For BQ24640, I would recommend opening another post with that part so we can ask relevant product expert to take a look.



  • Hi Emmanuel,

    Webench has dual FET solution implemented for LM5121/-Q1. This means, you cannot change M1 and M2 independently. The solutions in Webench are such that both fets should work for a particular solution. The problem with using CSD17577Q3A is that it's temperature reaches upto 258 degree (theoretically) if used in the place of M1. Hence, Webench rejects it for the position of M1. 

    Please create a separate post for BQ24640 as Amod suggested.