A
The following are two possible causes of the ringing sound.
- Ringing caused by the MPS (Maintain Power Signature)
- Ringing caused by the light load mode
The former occurs because the DC-DC converter does not satisfy the MPS (Maintain Power Signature) requirements.
When the MPS (Maintain Power Signature) requirements are not satisfied, the PSE on the power supply side repeats a cycle consisting of Detection → Classification → 48V application → OFF.
Therefore, the electrical charge is repeatedly charged and discharged in the multilayer ceramic capacitor that is equipped in the DC-DC converter. This charging and discharging causes distortion due to the piezoelectric effect of the multilayer ceramic capacitor. This distortion may come into contact with the substrate and cause a ringing sound.
For this reason, be sure to satisfy the MPS (Maintain Power Signature) requirements. Use the following method to set the consumption current of the PSE output so that it exceeds 10mA at all times.
- Have it consume power at the PSE output
- Add a dummy resistor, etc. to the DC-DC converter output to have it consume power
The latter type of ringing is caused by the light load mode.
When the 12Vo output current is set to Iout = 0.15A or more, it operates at or above the audio frequencies including variations, and the ringing sound stops.