Output Connection for Feedback Resistor 11pin FB

Hi kuramoto ,

Thanks for contacting ADI.

Here are the answers to each question.

1. Is it mandatory to connect a feedback resistor to the FB pin in this type of configuration?

- It is not mandatory to connect a feedback resistor to the FB if we follow the schematic that you provided. The FB pin is designed so that the distance between the Inverting or non-inverting pin and the output node becomes short and that will help reduce the parasitic capacitance of the line. It is also described here regarding signal trace lengths.

2. If the FB pin is left floating, what issues may occur (for example, loop instability, oscillation, internal bias disturbance, or output stage malfunction)?

- The FB pin is also an output pin. The output pin has a low impedance and the effect of floating it will not have instability issues, unless it was soldered down to a signal trace that is long but not terminated. 

3. Is the FB pin internally connected to the compensation network or output stage?

- It is connected to the output stage, you can also see it as an output pin.

4. If the FB pin is not used, what is the recommended way to terminate it?

- When the FB pin is not used, you can treat it as an output pin. You can short all the OUT pins with the FB if you are not using it. However, you can also float the pin and solder it down to a single copper pad that will not have a long trace.

Hope that helps.

Thanks and regards,

JE

Hi JE,

Thank you very much for your detailed explanation.
Your clarification that the FB pin is internally connected to the output stage and can be treated as an output pin is very helpful.

I also appreciate the advice regarding the PCB layout and trace length.
This information will help with our design.

Thanks again for your support.

Best regards,
Kuramoto