SLVAFT0 July   2024 DRV8706-Q1 , DRV8714-Q1 , DRV8718-Q1

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Background and Theory
    1. 2.1 Factors Affecting Bulk Capacitor Sizing
    2. 2.2 Pulse Width Modulation
    3. 2.3 Estimating Motor Current Variation
    4. 2.4 General Guideline Calculations Assuming Ideal Capacitors
  6. 3Real-World Results
    1. 3.1 Example Measurements
    2. 3.2 Revised Practical General Guidelines
    3. 3.3 Other Considerations
  7. 4Summary
  8. 5References

2.2 Pulse Width Modulation

If pulse-width modulation (PWM) is used to control the effective motor voltage, the bulk capacitance on the motor supply becomes even more important. Otherwise the current variation during PWM operation can cause voltage ripple which can propagate throughout the system, causing electromagnetic interference with other components. Figure 2-2 shows typical waveforms in a motor drive system with PWM at 20kHz. The red trace marked MOTOR CURRENT shows the effect of rising current as the motor is activated, with ripple current corresponding to the PWM frequency.

Typical Waveforms in a PWM Motor Drive, From DRV8718-Q1 Data Sheet Figure 2-2 Typical Waveforms in a PWM Motor Drive, From DRV8718-Q1 Data Sheet

In this example, the scale is 1A per major division of the current trace, so the variation in motor current is on the order of 200mA due to the PWM switching. This is approximately 10% of the peak motor current of about 2 amps.