Output capacitor of TPS51124 (or TPS51125, TPS51218) are designed to satisfy the stability and the ripple voltage as in the data sheet.
In layout, if we add some small bypass capacitors in parallel with the output capacitor to filter the high frequency components, would you please recommend the
1. will bypass capacitors affect the stability and the ripple voltage of the circuit?
2. If yes, should we calculate again the new value of capacitance and ESR of the output capacitor? And how can we calculate those values?
Thank you very much for your help.
Yes, paralleling small ceramic (or other types of capacitors) in parallel with larger capacitance capacitors changes the impedance of the output capacitor and will affect the response of the loop and will affect the ripple. This is true for any regulator but is especially important for DCAP and other hysteretic type topologies that require ripple to regulate.
The math to determine the new impedance is complicated and there is no exact equation that I can give to guarantee stability, etc. At a minimum, I would recommend keeping the capacitance of the ceramics less than 5% (the lower the better) of the sum of the other, higher ESR caps.
I recommend simulating the new capacitor bank in spice and seeing if its impedance is significantly different than without the smaller ceramics. If not, then it is likely ok to add the extra caps. I would highly recommend testing the proposed output cap bank on the EVM before going to production.
Thank you very much for your reply. I appreciate that.
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