# What is the purpose of the op-amp feedback circuit?

This is an existing circuit of buck regulator circuit I have been reviewing with.(Here I have not mentioned the part number of the due to the confidentiality issue)

Following is the screenshot of the feedback resistor design.

Reference voltage of the buck regulator is 1 V and output is designed for 3.3 V . Feedback regulator resistors (51 kΩ , 22.1 kΩ) combination design I have reviewed and verified the values.

Opamp feedback path and its functions are confusing sections for me. Following is my understanding, current feedback across the 100 Ω resistor is taken across to the amplifier input and voltage output comes in parallel with the feedback path ( 22.1 kΩ) through a buffer.

Can anyone suggest the purpose of the opamp-feedback circuit? And what could be possible output voltages at the buffer (op-amp) output.

• My first guess is that it's a current limiter. The right opamp measures the voltage across a $100\Omega$ resistor, amplifies it, and then pulls up the FB pin if it is too large. A large FB will make the regulator think that the output voltage is too high, causing it to source less current. Nov 7, 2018 at 8:31
• @SvenB, can you please just give a reference I can read to understand how buck will source less current? Nov 7, 2018 at 8:44
• I'd refer you to any introduction about buck regulators if you want to know how they can source less current. YouTube has video's explaining the principles. Nov 7, 2018 at 9:07
• 100Ω current sense resistor? You sure that's not a 1Ω or 0.1Ω? It should be. It looks like an attempt to regulate both voltage and current. Nov 17, 2018 at 17:54
• @Misunderstood, good catch. Initially there was a schematic error, Actually it is 0.1 Ω Nov 19, 2018 at 6:54