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

6.3.2 Current Regulation

The PWM chopping current is set by a comparator, which compares the voltage across a current sense resistor, multiplied by a factor of 8, with a reference voltage. The reference voltage is input from the VREF pin. The full-scale (100%) chopping current is calculated as follows:

Equation 1. DRV8818A

Example:

If a 0.22Ω sense resistor is used and the VREFx pin is 3.3V, the full-scale (100%) chopping current is
3.3V / (8 × 0.22Ω) = 1.875A.

The reference voltage is also scaled by an internal DAC that allows torque control for fractional stepping of a bipolar stepper motor, as described in the Section 6.3.4 section.

When a winding is activated, the current through the winding rises until the chopping current threshold described above is reached, then the current is switched off for a fixed off time. The off time is determined by the values of a resistor and capacitor connected to the RCA (for bridge A) and RCB (for bridge B) pins. The off time is approximated by:

Equation 2. DRV8818A

To avoid falsely tripping on transient currents when the winding is first activated, a blanking period is used immediately after turning on the FETs, during which the state of the current sense comparator is ignored. The blanking time is determined by the value of the capacitor connected to the RCx pin and is approximated by:

Equation 3. DRV8818A

Note that in Smart Tune Dynamic Decay mode the RCA pin must be tied to GND. Refer to Table 6-2 for how to select the off time in this mode using the RCB pin. RCB pin left Hi-Z selects tOFF of 16μs, and RCB pin logic high selects 32μs.