Pulse Density Modulation for LRA Motor Drive

Pulse Density Modulation (PDM) is commonly used in the application of LRA (Linear Resonant Actuator) motors to control their vibration intensity and frequency. LRA motors are used for generating haptic feedback in various devices such as smartphones, gaming controllers, and haptic feedback devices.

To apply PDM to an LRA motor, a fixed-frequency high-frequency pulse train is typically used. The density of the pulses determines the vibration intensity, where a higher density of pulses results in stronger vibrations. By dynamically adjusting the number and width of the pulses, different vibration frequencies and intensities can be achieved.

Here’s a general outline of how PDM is applied to an LRA motor:

  1. Pulse Generation: A pulse train with a fixed frequency is generated, typically at a high frequency. The frequency is selected based on the desired vibration characteristics.
  2. Pulse Density Control: The density of the pulses in the pulse train is controlled to adjust the vibration intensity. This is achieved by modulating the number of pulses within a given time frame. Increasing the number of pulses increases the vibration intensity, while decreasing the number reduces it.
  3. Pulse Width Adjustment: The width of the individual pulses in the pulse train may also be adjusted to further fine-tune the vibration characteristics. Modulating the pulse width can help achieve specific vibration patterns or effects.
  4. Driving the LRA Motor: The PDM signal, consisting of the pulse train with controlled density and width, is used to drive the LRA motor. The motor responds to the pulses by generating vibrations corresponding to the PDM signal.

By utilizing PDM, precise control over the vibration intensity and frequency of the LRA motor can be achieved. This allows for customized haptic feedback experiences, enhancing the user’s interaction with the device.

It’s worth noting that the exact implementation details may vary depending on the specific LRA motor and its associated control circuitry. Manufacturers often provide guidelines and specifications on how to apply PDM for their LRA motors, which can help optimize the performance and achieve the desired haptic effects.

PDM vs PWM for LRA Motor Drive

In the context of LRA (Linear Resonant Actuator) motors, both Pulse Density Modulation (PDM) and Pulse Width Modulation (PWM) can be used for controlling the motor’s vibration. Here are the differences between PDM and PWM in LRA motor applications:

  1. Control Principle:
    • PDM: PDM controls the motor’s vibration intensity by varying the density or number of pulses within a specific time frame. A higher pulse density leads to stronger vibrations, while a lower density results in weaker vibrations.
    • PWM: PWM controls the motor’s vibration intensity by adjusting the width or duration of the pulses within a fixed-frequency pulse train. Wider pulses produce stronger vibrations, whereas narrower pulses generate weaker vibrations.
  2. Control Precision:
    • PDM: PDM offers high precision and fine-grained control over the motor’s vibration intensity. By adjusting the pulse density with greater granularity, it allows for more precise control of the vibration level.
    • PWM: PWM provides good control precision, but it may not offer the same level of granularity as PDM. The control resolution is limited by the fixed frequency of the pulse train and the number of discrete pulse width steps available.
  3. Energy Efficiency:
    • PDM: PDM can be more energy-efficient compared to PWM in certain scenarios. Since PDM controls the vibration intensity by adjusting the pulse density, it can deliver the required vibration energy more efficiently by utilizing higher pulse densities during active vibration and reducing or eliminating pulses during idle periods.
    • PWM: PWM, on the other hand, maintains a constant frequency pulse train, regardless of the required vibration intensity. This can result in relatively higher energy consumption since the pulses are always present, even if the motor’s vibration level needs to be reduced.
  4. Implementation Complexity:
    • PDM: Implementing PDM control for LRA motors typically requires more complex circuitry and algorithms to generate and control the pulse density accurately. It may involve additional components or signal processing techniques to achieve the desired control precision.
    • PWM: PWM control is relatively simpler to implement as it involves generating a fixed-frequency pulse train and adjusting the pulse widths based on the desired vibration intensity. PWM-based control circuits are commonly available and can be easily integrated into motor control systems.

In summary, PDM and PWM offer different approaches to control the vibration intensity of LRA motors. PDM provides higher control precision and potentially better energy efficiency by adjusting pulse density, while PWM offers a simpler implementation but with slightly lower control precision and potentially higher energy consumption. The choice between PDM and PWM depends on the specific requirements of the application and the desired trade-offs between control precision, energy efficiency, and implementation complexity.

VitalSigns PDM Verilog IP


  • Pulse Density Modulation for LRA Motor Drive
  • Full H-bridge or Half bridge output drive
  • Adjustable modulate clock rate
  • Full Digital PDM modulation
  • Audio to haptic effect


Port Description



Port Name






Asynchronous Reset (low active)








Pulse Density Modulate clock rate, If keep pmd_ck = 1, modulate clock = CLK_Audio




Synchronous reset (high active)




Output mute, set spk_p/spm_m  to low




Input data




H-bridge MOS drive input signal




H-bridge MOS drive input signal

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