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LM25145: Mosfet requirements

Terry Gould
Prodigy 45 points
Part Number: LM25145
Other Parts Discussed in Thread: CSD18513Q5A, LM5145

I cannot understand why there is the need to used different mosfets for the high and low side.

1. All application examples and webench configurations use different but very similar Mosfets on the low and high side, why is this?

2. For BOM consolidation and cost I would like to use the CSD18513Q5A for both high and low side switches, is this ok?

Part will be used for two applications:

9-22V Input, 3.3V 15A Output

9-22V Input, 5V 20A Output

  • +1
    TI__Mastermind 28660 points

    Hello Terry,

    For applications where Duty cycle is low, suggest using dissimilar MOSFETs, this is because HS MOSFETs with low duty can sacrifice RDSon for lower gate charge, lower gate charge in the HS MOSFET will result in lower switching losses.  For the LS MOSFET with low duty cycle will result in the LS MOSFET being on most of the time period in the switching cycle, this means sacrificing gate charge for low RDSon.  Also, the LS MOSFET effectively is a zero-voltage switching device and does not have the same switching losses that occurs in the HS MOSFET, suggest following WEBENCH guidance for highest efficiency.

    You can also make use of the calculator found in the product folder of the LM5145 and add the MOSFET parameters therein to see how efficiency is affected by MOSFET selection.

    LM5145DESIGN-CALC Calculation tool | TI.com

    Hope this helps,

    David.