Voltage Drop and buzzing noise

Hello,

I would suggest changing the resistor in series with the SENSE- pin from 20ohms to 0ohms, changing the sense filter cap from 100nF to 1nF, compensation from 1nF+1.62k//620pF to 4.7nF+10K//100pF and add a 20pF feed-forward capacitor (across the top feedback divider resistor). These are all values taken from the LTC7801 DC2641A schematic. LTpowerCAD indicates your feedback loop has little to no phase margin (depending heavily on the ESR of the output capacitors). I will try to attach the modified DC2641A LTpowerCAD file for you to check my entries and add your ESR, etc. values.

Kurk

7801 DC2641A mod 2025 Apr 3.zip

Hello, first of all, thank you for your response, I would like to add a few things for you to understand the situation better. In my own design, when I test before installing the output capacitors, there is a fluctuating 12V, but as soon as I add the output capacitors, the regulator starts to make a buzzing sound and 12V starts to fluctuate excessively (I put a tantalum capacitor on the output). is there anything you want to add on the advice you give?

Hello, Output capacitors are required. The buzzing sound could be caused by a large signal, low frequency oscillations associated with an unstable feedback loop (motorboating). Have you tried the suggested values changes?

If you are unfamiliar with switching power supply design and compensation, I would suggest using the LTC7801 Evaluation Board DC2641A without change. Otherwise, I would suggest entering your components (including output capacitor ESR) into LTpowerCAD software and adjusting the compensation values. The LTC7801 DC2641A compensation values are a good start. Depending on your output capacitors, you might then want to try increasing the series compensation cap (4.7nF) while decreasing the resistor (10K) by similar ratio.

Kurk

hello, first of all thank you for your help, I applied the changes you said and solved the buzzing.
The drop in the output voltage has improved very little, for example, the 12V voltage level drops to around 10V under load, what do you think is the reason for this?