LM25069: Power Limit Resistor Valve Calculation Discrepancy

George Branck

Part Number: LM25069

There is a discrepancy in the calculation of the Rpwr resistor value when using the LM25069 Quick Start Calculator or the Rpwr equation located on page 14 of the datasheet . Our design uses a .0012 ohm sense resistor. Using the TI provided spreadsheet( which I will upload)) entering the PowerLimt at 11.1watts (col F, row 25) is returns a recommended Rpwr value of 2.49Kohm. Using the Rpwr equation in the data sheet(page 14), Rpwr = 2.32 x 105 x Rs x Pfet(LIM) = 232000 x 0.0012 x 11.1 = 3.09Kohm . If I go back to the TI spreadsheet I have to increase the Power Limit (col F, row 25) to 13.75 watts to end up with a 3.09K resistor value. So which calculation approach is correct? Does that Rpwr resistor value have to be 2.49K or 3.09K to set a Power Limit of 11.1 watts..

Thanks,

George Branck

over 7 years ago

0 George Branck over 7 years ago

Prodigy 190 points

MBK8257_LM25069_QuickStartCalculator_Rev02.xls

0 Brian Daugherty over 7 years ago in reply to George Branck

TI__Expert 5940 points

George,

You are looking at 'maximum' 11.1W on the spreadsheet vs typical on the DS equation. If you input 3.09k into the 'selected Rpwr' on the spreadsheet you will see the typical 11.1W with min/max around it shown.

Brian

0 George Branck over 7 years ago in reply to Brian Daugherty

Prodigy 190 points

Simple enough ..... Thanks Brian
Regards,
George

0 George Branck over 7 years ago in reply to George Branck

Prodigy 190 points

Brian,

I do have another question .... in our design we are using paralled MOSFETs. When entering the optional SOA data into the spreadsheet should I double the SOA values found in the MOSFET 's datasheet since each MOSFET will see 1/2 of the total? If not, how do I take into account the paralled MOSFET configuration in determining the PowerLimit.

Thanks,
George

0 Brian Daugherty over 7 years ago in reply to George Branck

TI__Expert 5940 points

George,

Since Plim/CL puts the FET into linear region, and FETs have different Vgs th and Plateau levels, a single FET may take the entire SOA energy so you have to assume this. Only use a single FET's SOA and derate linearly with starting FET temp vs max FET temp. i.e. if at 75C, then you multiple the currents by (175-75)/(175-25). And remember the entries in the DS SOA are log-log so what may look like 2.7A for instance may only be 2.3A when you use log on the vertical for interpolation. Good luck.

Brian

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LM25069: Power Limit Resistor Valve Calculation Discrepancy