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DC/DC Converter Output Voltage with sine wave modulation

I would like to design a power supply for analog Partyline. Usually the required voltage is around 30VDC. The sticking point is that, the partyline participants modulate an audio signal over the DC voltage. So the power supply needs a huge inductance to block the ac signal. This inductance can be realized by a gyrator circuit using an linear regulator acting as an inductor. However this solution dissipates a lot of power, since an VIN-VOUT difference of (around 6V drop) is required for proper regulation and to have sufficiant headroom for the audio signal.  

So I was wondering if it would be possible to solve it using an Switching Regulator. I did some simmulations with ltspice and available models of LTC3605 Stepdown Converter, to see if it meight work in principle. The LTC3605 is operating with a max Frequency of 4MHz, but its limited Input Voltage makes it not suitable for this application. Actually I had one on hand. I feed an sinus signal over an voltage controlled current source to the feedback input of the regulator.

The Simmulation shows that it works in principle. But starting from ca. 3kHz the signal is more and more distorted. Actually it must work upto 24kHz.



I tested it on an available LTC3605 electronic circuit, feeding the sine wave signal from an floating signal generator output. It shows similar results. Starting from 3kHz the sine wave was obvoiusly distorted.

Could you tell about the limiting factors of linears switching regulators. Is there a way to solve it using linears switching regulator. I assume, an FPGA controlled switching regulator could do this job.

Thank you for any hints.


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