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# LM25066I: Accuracy for LM25066i application

Part Number: LM25066I

Hi Praveen,

I found there are different tolerances between different widths of trace.

If the trace(vin) is 10 mil (1 oz. copper, Rs=1 mohm), we can get around -20% tolerance with 1A load.

Besides, if the trace(vin) is 10 mil (2 oz. copper, Rs=0.83 mohm), the tolerance will be reduced to around 5%.

I think it's a big parameter for the trace width that will impact the accuracy.

But I can't find any suggestion for the trace width in datasheet.

Could you inform me how much the recommended line width should be? and base on how many oz. copper? how many routing length from Rs to chip?

Thanks.

• Hi Chris,

You are right ! The resistance in the path from Rs to vin pin is critical for the Sense accuracy.

The reason is that there is  Iin=8mA (Max) flowing through  Vin pin and this current along with Trace resistance causes  a voltage drop in the path. Unfortunately it is not mentioned in the datasheet.

You can use PCB resistance calculators to find the resistance of the path with different trace width and Copper thickness. Then Calculate offset voltage (8mA * Trace Resistance). Check the % with respect to 25mV. Say, you have 3m Ohms of trace resistance, it causes 3A error with 1m Ohm of Rs.  So, calculate trace resistance based on acceptable tolerance and then use trace resistance calculator to calculate trace width and Copper Thickness.

Apart from this the Sense lines layout and soldering also plays an important role in accuracy.  Please follow Kelvin sensing method with wide trace from Rs.

Regards,

Praveen

• Hi Praveen,

Is there any calculating book with excel format to get parameters just like copper thickness, trace length & width for LM25066i's application?

Will TI update datasheet for the missing parts into layout guide?

If the answers are yes, please provide us the files and we can implement in projects in the future. Thanks.
• Hi Chris,

As of now we don't have a calculator for measuring the trace resistance. But you can use online calculators like the one in below link. You can find the  simple equation used for calculating the resistance with which you can calculate the resistance yourself as well.

We will consider updating the datasheet or releasing an App note regarding this but it may take time do so. Meanwhile, you can consider the suggestions given in the above reply for your design.