How to calculate a voltage/current controller bandwidth based on their gains Kp and Ki?

I am doing some experiments with voltage and current controllers, but sometimes my results are not matching what I expected. At the moment, to calculate the bandwidth of a controller, I am taking the absolute value of its open-loop transfer function, and setting it equal to -3dB (0.86), which gave me the following equation for the bandwidth of a PI controller:

bandwidth = $$\\frac{K_i}{\sqrt{0.739 - K_p^2}}\$$

What am I missing?

Thank you in advance for the assistance in this matter.