SLVSI46 July   2025 DRV8818A

ADVANCE INFORMATION  

  1.   1
  2. 1Features
  3. 2Applications
  4. 3Description
  5. 4Pin Configuration and Functions
  6. 5Specifications
    1. 5.1 Absolute Maximum Ratings
    2. 5.2 ESD Ratings
    3. 5.3 Recommended Operating Conditions
    4. 5.4 Thermal Information
    5. 5.5 Electrical Characteristics
    6. 5.6 Timing Requirements
    7.     13
    8. 5.7 Typical Characteristics
  7. 6Detailed Description
    1. 6.1 Overview
    2. 6.2 Functional Block Diagram
    3. 6.3 Feature Description
      1. 6.3.1 PWM H-Bridge Drivers
      2. 6.3.2 Current Regulation
      3. 6.3.3 Decay Mode
      4. 6.3.4 Microstepping Indexer
      5. 6.3.5 Protection Circuits
        1. 6.3.5.1 Overcurrent Protection (OCP)
        2. 6.3.5.2 Thermal Shutdown (TSD)
        3. 6.3.5.3 Undervoltage Lockout (UVLO)
    4. 6.4 Device Functional Modes
      1. 6.4.1 Sleep Mode
      2. 6.4.2 Disable Mode
      3. 6.4.3 Active Mode
  8. 7Application and Implementation
    1. 7.1 Application Information
    2. 7.2 Typical Application
      1. 7.2.1 Design Requirements
      2. 7.2.2 Detailed Design Procedure
        1. 7.2.2.1 Stepper Motor Speed
        2. 7.2.2.2 Current Regulation VREF and RSENSE
      3. 7.2.3 Application Curves
    3. 7.3 Power Supply Recommendations
      1. 7.3.1 Bulk Capacitance
    4. 7.4 Layout
      1. 7.4.1 Layout Guidelines
        1. 7.4.1.1 Heat Sinking
      2. 7.4.2 Layout Example
      3. 7.4.3 Thermal Considerations
        1. 7.4.3.1 Power Dissipation
  9. 8Device and Documentation Support
    1. 8.1 Receiving Notification of Documentation Updates
    2. 8.2 Support Resources
    3. 8.3 Trademarks
    4. 8.4 Electrostatic Discharge Caution
    5. 8.5 Glossary
    6. 8.6 Documentation Support
      1. 8.6.1 Related Documentation
  10.   Mechanical, Packaging, and Orderable Information

7.3.1 Bulk Capacitance

Appropriate local bulk capacitance is an important factor in motor drive system design. Having more bulk capacitance is generally beneficial, although the disadvantages include increased cost and physical size. Bulk capacitors near the motor driver act as a local reservoir of electrical charge to smooth out the motor current variation.

Experienced engineers often use general guidelines about bulk capacitance to select the capacitor values. One such guideline says to use at least 1 to 4μF of capacitance for each Watt of motor power. For example, a motor which draws 10 Amps from a 12V supply has a power of 120 Watts, leading to bulk capacitance of 120 to 480μF, using this general guideline.

The voltage rating for bulk capacitors must be higher than the operating voltage, to provide margin for cases when the motor transfers energy to the supply.

A large value of bulk capacitance is desired to provide a constant motor supply voltage during current transitions, such as motor start-up, changes in load torque, or PWM operation. A working estimate of the required capacitance for consistent supply is essential to reduce complexity, cost, and size of board electronics. We can use a general guideline method to find an appropriate capacitor size based on the expected load current variation and allowable motor supply voltage variation:

Equation 8. CBULK>k × ∆IMOTOR×TPWM/∆VSUPPLY

Where:

CBULK is the bulk capacitance

k is a scale factor to account for the ESR for typical capacitors in this type of application; based on the lab measurements with DRV8718-Q1EVM, k ≈ 3 is practical for these cases.

ΔIMOTOR is the expected variation in motor current, imax – imin

TPWM is the PWM period which is the reciprocal of the PWM frequency

ΔVSUPPLY is the allowable variation in the motor supply voltage

Figure 7-5 plots several data points and applies this general guideline, showing relatively good agreement.

DRV8818A Measured Results and 3x General Guideline, Accounting for Real-World Non-Zero ESR Values of Electrolytic CapacitorsFigure 7-5 Measured Results and 3x General Guideline, Accounting for Real-World Non-Zero ESR Values of Electrolytic Capacitors

For more information please see the Application Note Bulk Capacitor Sizing for DC Motor Drive Applications .