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I am trying to design a 5 V -> 3.3 V buck converter as a design exercise and am implementing my own analog control circuit. At the moment I am not considering compensation or loop response as I am simply trying to regulate to the correct output before I consider the response and stability. Below is a photo of the control circuit, which to my understanding should be regulating to the target voltage. Note that Vfb is taken on the output from a 10 kΩ and 20 kΩ resistor divider, so should ideally be 2.178 V when 3.3 V is on the output. Additionally, the Vramp is a 600 kHz, 3.3 V peak ramp wave. With the current loop seen below my output voltage is actually 1.27 V. I am stuck at where this is going wrong and feel as though I have tried many different configurations. I have trouble thinking that a lack of compensator is causing this issue as the output is a pretty smooth 1.27 V, only 2 mV peak to peak. total schematic

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    \$\begingroup\$ Have you tried simulating your circuit? If not, why not? If so, what are the results you get? \$\endgroup\$
    – Andy aka
    Commented Jul 10 at 11:49
  • \$\begingroup\$ Are you saying Vref = 2.178V in your design? If not, what is your Vref? If Vfb is ~2/3 of Vout, the 20k divider resistor is in parallel with R4. 1.27V Vout would suggest a Vref of about 1.25V. If that is the case, you will need to adjust the impedance of either the divider string orR4/R9/R6/R14. \$\endgroup\$
    – JkingNH
    Commented Jul 10 at 11:52
  • \$\begingroup\$ @Andyaka I have simulated, the output of 1.27V is about what I am getting. All else looks okay, but I can poke around a bit more \$\endgroup\$
    – fzizzo21
    Commented Jul 10 at 11:58
  • \$\begingroup\$ @JkingNH Yup Vref is 2.178. I did attempt to buffer the inverting terminal before R4 to solve the impedance issue and provide accurate feedback but did not really notice much of a change. \$\endgroup\$
    – fzizzo21
    Commented Jul 10 at 12:00
  • \$\begingroup\$ What opamp and comparator are you using and how are they powered? A full schematic would be more helpful than little pieces and text descriptions. \$\endgroup\$
    – Finbarr
    Commented Jul 10 at 12:13

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