Hi,
I am using the MPM3683-10 to generate a 0.85V rail and when I run simulations with MPSmart, there is a subharmonic that I can’t remove so far, the magnitude is low <5mVpp but it is still present.
The sw frequency is set to 1MHz and the subharmonic is around 50kHz.
The output ripple is 1.65mV and the overall ripple with the subharmonic is 3.2mV, is the output ripple to small for the COT?
What differences are there between the default MPsmart MPM3683-10 schematic and the one you have now? Have you tried running the base schematic and observed these sub-harmonics?
I used the original MPMSmart model found on the MPM3683-10 web page and changed the Feedback resistor as RFB=4k53 and RFT=2k to set output to 0.85V and the sub-harmonics are still there.
This is something I can bring up and check in with the simulation team if this is the performance of the simulation file upon downloading from the website without making any changes to the model.
You’ll hear from me after I speak with them.
OK I will wait on your feedback. Thanks
Hi, do you have any news?
Apologies for the delayed response.
I had spoken to the software engineers regarding this issue and there was an active investigation on the model.
There should have also been a local team that reached out to you.
They concluded that there was an excessively large feedforward capacitance value that caused these subharmonics to show up. No errors with the simulation file were found.
Best,
Krishan
Hi Krishan,
Do you have any formula, instructions on how to select the right Cff value, the datasheet recommends the 4.7nF for the whole vout range;
since our Vout is not listed in this table (0.85V) I run simulations with 47p and 470p in comparison with the original 4.7nF and it seems that the subharmonic is still present;
I see, I also noticed that the recommended feedforward capacitance was 4.7nF.
Thank you for testing these values at lower feedforward capacitances given Vout = 0.8V. Let me discuss this further with the local team that has been interfacing with you.
