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Using M24LR-DISCOVERY with  5V on VPSTX

Question asked by epstein.len on Aug 7, 2017
Latest reply on Aug 9, 2017 by epstein.len

VPSTX is wired to 3.3Volts.  I want to give it +5V to increase the power.  This changes Rout from 27 ohms to 16 ohms.  I tried using the CR95HF EMI FILTER CALULATION tool to work out component values, without success.

As a baseline, I looked at the 3 volt case, since I already had the answer. I opened the tool and found most of the values default to the values on the board.  VPSTX is set to 3 volts and Rin is set to 27 ohms. I had to change Rrx from 0 to 330 and Crx to 1E+12F (infinite capacitor in series).  The result (section 2) was C11 = C12 = 41pF and C2 = 301pF.  That C11 and C12 value is half of the 82pF used on the board.

When I changed VPSTX to +5 and Rin to 16, the tool gave me C11 = C12 = 29pF and C2 = 301pF.  I tried 66pF (and lifted L8 so I could connect +5V) and got much worse performance than before.

In Summary, what am I misunderstanding about the tool, and what are the correct values for attaching +5V to VPSTX?


Thanks for reading this.