High Noise Levels in DRV8870DDAR : Diagnosing PWM Frequency Problems

High Noise Levels in DRV8870DDAR: Diagnosing PWM Frequency Problems

When working with motor drivers like the DRV8870DDAR, high noise levels can be a common issue, especially when using Pulse Width Modulation (PWM) for controlling the motor speed. This noise can manifest as electrical interference, causing instability or malfunctions in other parts of the system. Here’s an in-depth analysis of the problem and a step-by-step guide to help diagnose and solve it.

Understanding the Issue

The DRV8870DDAR is a popular motor driver used for controlling DC motors and stepper motors. It works by modulating the voltage applied to the motor via PWM signals, which can lead to high-frequency noise if not managed properly. This noise is typically a result of issues related to the PWM frequency and how it interacts with the motor, Power supply, and the surrounding circuit.

Causes of High Noise Levels

Several factors can contribute to high noise levels in a DRV8870DDAR circuit:

Incorrect PWM Frequency: The DRV8870 allows users to set the PWM frequency, and an improperly chosen frequency can lead to excessive noise. If the frequency is too high, the switching transients become more pronounced, creating unwanted noise.

Improper Filtering: Insufficient decoupling capacitor s or improper placement of filters on the power supply lines can fail to reduce the noise generated by the switching process of the PWM.

PCB Layout Issues: Poor PCB design can lead to noise coupling between the power and signal lines, which increases noise levels. Long traces or inadequate grounding can also contribute to noise.

Motor Characteristics: Some motors, particularly those with lower inductance, are more susceptible to noise caused by rapid switching in PWM drivers.

Ground Loops: Grounding problems can create additional paths for noise currents to flow, which amplifies the overall system noise.

How to Diagnose the Noise Problem

Check the PWM Frequency: The first thing to verify is whether the PWM frequency is appropriate. For the DRV8870, a frequency of around 20 kHz to 100 kHz is usually ideal. Frequencies above or below this range can cause more noise. You can use an oscilloscope to measure the frequency of the PWM signal and check if it falls within the recommended range.

Examine the Power Supply: Measure the voltage levels at the Vcc pin of the DRV8870. If you notice spikes or irregularities, the power supply might not be adequately filtering out high-frequency noise. Inspect the power lines for noise using an oscilloscope.

Look for PCB Layout Issues: Inspect your PCB layout to ensure that power and ground traces are as short and thick as possible. Try to separate the motor power circuitry from the control and signal lines to avoid coupling noise.

Test Motor Performance: If possible, test the system with different motors. If the noise is reduced with a higher inductance motor, this could point to motor characteristics being a contributing factor.

Check for Ground Loops: Verify the grounding system of your circuit to ensure that there are no ground loops or high impedance paths that could cause noise to circulate.

Step-by-Step Solution to Reduce High Noise

Adjust PWM Frequency: Reduce the PWM frequency to fall within the ideal range. You can try frequencies between 20 kHz and 100 kHz. Ensure the frequency is not too close to the resonant frequency of the motor or the system to avoid amplifying noise. Improve Filtering: Add appropriate capacitors (e.g., 0.1 µF ceramic and 10 µF electrolytic capacitors) at the Vcc pin and across the motor terminals to reduce high-frequency noise. Use inductive filters or LC filters to further clean up the signal. Optimize PCB Layout: Keep high-current traces as short and wide as possible. Implement star grounding, ensuring that all ground connections come to a single point to avoid creating ground loops. Use proper decoupling capacitors near the power supply pins to reduce switching noise. Use Snubber Circuits: Consider adding snubber circuits across the motor to absorb voltage spikes caused by the inductive nature of the motor windings. Replace the Motor if Necessary: If you are using a motor with low inductance or other characteristics that make it more susceptible to noise, consider switching to a motor with higher inductance or lower switching transients. Shielding and Enclosure: If noise continues to be a problem, consider placing the motor driver circuit in a metal enclosure that can act as a shield, reducing EMI (Electromagnetic Interference). Ensure Proper Grounding: Verify that the ground connections are solid and there are no loops that could introduce noise into the system. Use a single-point ground if possible.

Conclusion

By following the above steps, you should be able to diagnose and solve the issue of high noise levels in your DRV8870DDAR motor driver circuit. Start with adjusting the PWM frequency, improving filtering, and ensuring proper PCB layout. These solutions should help you reduce or eliminate the noise and improve the overall performance of your system.