An Unusual Situation From LM5122

[KAKU]

Hello,
When there is no load, the PWM HO and LO outputs will appear PWM periodically and briefly, accompanied by a whistle of the inductor. At this time, the input voltage will also have a downward peak value, and the output voltage will be unstable.
When loaded, the input voltage is no longer a constant level, but a regular sawtooth wave, and only when the input voltage waveform is lower than a certain value, the HO\LO pin will generate PWM, otherwise, it will not generate PWM, at this time the inductance whistle, the output voltage is a sawtooth wave in the opposite direction to the input voltage.
Here is the datasheet link: https://www.utmel.com/productdetail/tex ... pb-7802861
Hope for helping!

Good listening!

[erintse01]

Thanks for this post, here's the LM5122 series info for you. https://www.lisleapex.com/series-lm5122

Hope it helps.

[Lorise]

Hello,

It sounds like you're encountering a classic issue with the LM5122 under different load conditions. Let’s break down what you’re experiencing:

No Load Condition

• PWM Outputs: The periodic and brief PWM signals on the HO (High Output) and LO (Low Output) pins when there’s no load can indicate that the controller is trying to stabilize the output. This behavior is somewhat expected as the LM5122 attempts to regulate the voltage despite the absence of a load.
  
  Inductor Whistle: The whistle from the inductor is often a sign of oscillation. This can occur due to the inductor’s physical properties and how the PWM frequency interacts with them. Without a load, the energy stored in the inductor may not be utilized effectively, causing it to resonate.
  
  Voltage Instability: The downward peak in input voltage and unstable output voltage could stem from the lack of current draw, which might be leading to fluctuations in the feedback loop. It’s essential to ensure that the feedback loop is properly compensated for no-load conditions.

Loaded Condition

• Input Voltage Waveform: When a load is applied and the input voltage transitions to a sawtooth waveform, it suggests that the converter is operating in continuous conduction mode. This is normal for switching converters as they draw more current under load.
  
  PWM Generation: The fact that the HO and LO pins only generate PWM when the input waveform falls below a certain threshold indicates that the LM5122(you can get more details on>>https://www.xecor.com/product/lm5122mhe-nopb) is trying to protect itself from overvoltage conditions or that the duty cycle is being limited by the feedback loop.
  
  Opposite Sawtooth Wave: The sawtooth output voltage that’s in opposition to the input voltage is also a normal characteristic of step-down converters, where the output voltage reflects the energy being transferred through the inductor. If the output is significantly out of range, it might indicate issues with the feedback network or load regulation.

Recommendations

• Load Testing: To better understand the behavior, test the circuit under various load conditions. Use resistive loads to see how the output voltage and inductor response change.
  
  Feedback Compensation: Check the feedback components (resistors and capacitors) to ensure they’re appropriately set for the load conditions you're working with. Sometimes, adjusting the compensation can stabilize performance across a range of loads.
  
  Inductor Selection: Ensure that the inductor is correctly rated for the application and that it's appropriately sized for the expected load and frequency. If the inductor is too small, it might resonate more and produce that whistle.
  
  Monitoring Input Voltage: Use an oscilloscope to monitor both the input and output voltages during these tests. This will help you visualize the waveforms and troubleshoot any anomalies.