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

6.3.3 Decay Mode

During PWM current chopping, the H-bridge is enabled to drive through the motor winding until the PWM current chopping threshold is reached. This is shown in Figure 6-2, Item 1. The current flow direction shown indicates positive current flow in the step table below.

Once the chopping current threshold is reached, the H-bridge can operate in Fast Decay, Slow Decay, or Smart Tune Dynamic Decay mode. The decay mode setting is latched at power-up or when exiting sleep mode.

Table 6-1 Decay Mode Settings
DECAYRCADecay Mode
012kΩ to 100kΩFast Decay
112kΩ to 100kΩSlow Decay
0.21 × VCC < VDECAY < 0.6 × VCC12kΩ to 100kΩMixed Decay
XGNDSmart Tune Dynamic Decay

Fast Decay Mode

In fast decay mode, once the PWM chopping current level has been reached, the H-bridge reverses state to allow the winding current to flow in a reverse direction. If synchronous rectification is enabled (SRn pin logic low), the opposite FETs are turned on; as the winding current approaches zero, the bridge is disabled to prevent any reverse current flow. If SRn is high, current is recirculated through the body diodes or external Schottky diodes. Fast-decay mode is shown in Figure 6-2, Item 3.

Slow Decay Mode

In slow-decay mode, the winding current is re-circulated by enabling both of the low-side FETs in the bridge. This is shown in Figure 6-2, Item 2.

If SRn is high, current is recirculated only through the body diodes, or through external Schottky diodes. In this case, fast decay is always used.

DRV8818A Decay ModesFigure 6-2 Decay Modes

Mixed Decay Mode

The DRV8818A also supports a mixed decay mode. Mixed decay mode begins as fast decay, but after a while switches to slow decay mode for the remainder of the fixed off time.

Fast and mixed decay modes are only active if the current through the winding is decreasing; if the current is increasing, then slow decay is always used.

Which decay mode is used is selected by the voltage on the DECAY pin. If the voltage is greater than 0.6 × VCC, slow decay mode is always used. If DECAY is less than 0.21 × VCC, the device operates in fast decay mode when the current through the winding is decreasing. If the voltage is between these levels, mixed decay mode is enabled.

In mixed decay mode, the voltage on the DECAY pin sets the point in the cycle that the change to slow decay mode occurs. This time can be approximated by:

Equation 4. DRV8818A

Mixed decay mode is only used while the current through the winding is decreasing; slow decay is used while the current is increasing.

Operation of the blanking, fixed off time, and mixed decay mode is illustrated in Figure 6-3.

DRV8818A PWM Figure 6-3 PWM

Smart Tune Dynamic Decay

The smart tune dynamic decay current regulation scheme is an advanced current-regulation control method compared to traditional fixed off-time current regulation schemes. Smart tune current regulation helps the stepper motor driver adjust the decay scheme based on factors such as:

  • Motor winding resistance and inductance
  • Motor aging effects
  • Motor dynamic speed and load
  • Motor supply voltage variation
  • Motor back-EMF difference on rising and falling steps
  • Step transitions
  • Low-current versus high-current dI/dt

DRV8818A Smart tune Dynamic Decay ModeFigure 6-4 Smart tune Dynamic Decay Mode

Smart tune Dynamic Decay greatly simplifies the decay mode selection by automatically configuring the decay mode between slow, mixed, and fast decay. This eliminates the need for motor decay tuning by automatically determining the best mixed decay setting that results in the lowest ripple and best performance for the motor. Smart Tune Dynamic Decay is best for applications that require minimal current ripple, at the same time maintain a fixed frequency with the current regulation scheme.

Select Smart Tune Dynamic Decay mode by connecting the RCA pin to GND.

After the current is enabled (start of drive phase) in an H-bridge, the current sense comparator is ignored for some time (tBLANK) before enabling the current-sense circuitry. The blanking time also sets the minimum drive time of the PWM. The blanking time is approximately 1.25µs.

The decay mode setting is optimized iteratively each PWM cycle. If the motor current overshoots the target trip level, then the decay mode becomes more aggressive (increases fast decay percentage) on the next cycle to prevent regulation loss. If a long drive time must occur to reach the target trip level, the decay mode becomes less aggressive (decreases fast decay percentage) on the next cycle to operate with less ripple more efficiently. With decreasing current steps, smart tune Dynamic Decay automatically switches to fast decay to reach the next current step quickly.

The off time tOFF during Smart Tune Dynamic Decay mode can be selected by setting the RCB pin to Hi-Z or ground as shown in Table 6-2.

Table 6-2 OFF Time Settings in Smart Tune Dynamic Decay Mode
RCARCBOFF Time
GNDHi-Z (Floating)16µs
GNDGND32µs