SLVSAX9E September   2011  – January 2016 DRV8818

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Pin Configuration and Functions
  6. 6 Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 ESD Ratings
    3. 6.3 Recommended Operating Conditions
    4. 6.4 Thermal Information
    5. 6.5 Electrical Characteristics
    6. 6.6 Timing Requirements
    7. 6.7 Motor Driver Timing Switching Characteristics
    8. 6.8 Typical Characteristics
  7. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1 PWM H-Bridge Drivers
      2. 7.3.2 Current Regulation
      3. 7.3.3 Decay Mode
      4. 7.3.4 Microstepping Indexer
      5. 7.3.5 Protection Circuits
        1. 7.3.5.1 Overcurrent Protection (OCP)
        2. 7.3.5.2 Thermal Shutdown (TSD)
        3. 7.3.5.3 Undervoltage Lockout (UVLO)
    4. 7.4 Device Functional Modes
      1. 7.4.1 RESETn, ENABLEn, and SLEEPn Operation
      2. 7.4.2 Decay Modes
  8. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Application
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
        1. 8.2.2.1 Stepper Motor Speed
        2. 8.2.2.2 Current Regulation
      3. 8.2.3 Application Curves
  9. 9 Power Supply Recommendations
    1. 9.1 Bulk Capacitance
  10. 10Layout
    1. 10.1 Layout Guidelines
      1. 10.1.1 Heatsinking
    2. 10.2 Layout Example
    3. 10.3 Thermal Considerations
      1. 10.3.1 Power Dissipation
  11. 11Device and Documentation Support
    1. 11.1 Documentation Support
      1. 11.1.1 Related Documentation
    2. 11.2 Community Resources
    3. 11.3 Trademarks
    4. 11.4 Electrostatic Discharge Caution
    5. 11.5 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information

7 Detailed Description

7.1 Overview

The DRV8818 is a highly configurable, integrated motor driver solution for bipolar stepper motors. The device integrates two H-bridges, current sense and regulation circuitry, and a microstepping indexer. The DRV8818 can be powered with a supply voltage between 8 V and 35 V and is capable of providing an output current up to 2.5 A full-scale.

A simple STEP/DIR interface allows for easy interfacing to the controller. The internal indexer is able to execute high-accuracy microstepping without requiring the controller to manage the current regulation loop.

The current regulation is highly configurable, with three decay modes of operation. They are fast, slow, and mixed decay, which can be selected depending on the application requirements. The DRV8818 also provides configurable mixed decay, blanking, and off time in order to adjust to a wide range of motors.

A low-power sleep mode is incorporated which allows for minimal power consumption when the system is idle.

7.2 Functional Block Diagram

DRV8818 fbd_slvsax9.gif

7.3 Feature Description

7.3.1 PWM H-Bridge Drivers

DRV8818 contains two H-bridge motor drivers with current-control PWM circuitry, and a microstepping indexer. A block diagram of the motor control circuitry is shown below.

DRV8818 motor_lvsax9.gif Figure 6. Motor Control Circuitry

7.3.2 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. DRV8818 equat1_lvs865.gif

Example:

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

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 Microstepping Indexer section.

When a winding is activated, the current through it rises until it reaches the chopping current threshold described above, 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. DRV8818 equat2_lvs865.gif

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. DRV8818 equat3_lvs865.gif

7.3.3 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 7, 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 two different states, fast decay or slow decay.

In fast decay mode, once the PWM chopping current level has been reached, the H-bridge reverses state to allow 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 through external Schottky diodes. Fast-decay mode is shown in Figure 7, Item 2.

In slow-decay mode, winding current is re-circulated by enabling both of the low-side FETs in the bridge. This is shown in Figure 7, Item 3.

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.

DRV8818 drv8811_decay_lvs865.gif Figure 7. Decay Mode

The DRV8818 also supports a mixed decay mode. Mixed decay mode begins as fast decay, but after a period of time 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. DRV8818 equat4_lvs865.gif

Mixed decay mode is only used while the current though 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 8.

DRV8818 drv8811_pwm2_lvs865.gif Figure 8. PWM

7.3.4 Microstepping Indexer

Built-in indexer logic in the DRV8818 allows a number of different stepping configurations. The USM1 and USM0 pins are used to configure the stepping format as shown in Table 1:

Table 1. Microstepping Selection Bits

USM1 USM0 STEP MODE
0 0 Full step (2-phase excitation)
0 1 1/2 step (1-2 phase excitation)
1 0 1/4 step (W1-2 phase excitation)
1 1 Eight microsteps/step

Table 2 shows the relative current and step directions for different settings of USM1 and USM0. At each rising edge of the STEP input, the indexer travels to the next state in the table. The direction is shown with the DIR pin high; if the DIR pin is low the sequence is reversed. Positive current is defined as xOUT1 = positive with respect to xOUT2.

Note that the home state is 45°. This state is entered at power-up or device reset. The HOMEn output pin is driven low in this state. In all other states it is driven logic high.

Table 2. Microstepping Indexer

FULL STEP
USM = 00		 1/2 STEP
USM = 01		 1/4 STEP
USM = 10		 1/8 STEP
USM = 11		 AOUTx CURRENT
(% FULL-SCALE)		 BOUTx CURRENT
(% FULL-SCALE)		 STEP ANGLE
(°)
1 1 1 100 0 0
2 98 20 11.325
2 3 92 38 22.5
4 83 56 33.75
1 2 3 5 71 71 45 (home state)
6 56 83 56.25
4 7 38 92 67.5
8 20 98 78.75
3 5 9 0 100 90
10 –20 98 101.25
6 11 –38 92 112.5
12 –56 83 123.75
2 4 7 13 –71 71 135
14 –83 56 146.25
8 15 –92 38 157.5
16 –98 20 168.75
5 9 17 –100 0 180
18 –98 –20 191.25
10 19 –92 –38 202.5
20 –83 –56 213.75
3 6 11 21 –71 –71 225
22 –56 –83 236.25
12 23 –38 –92 247.5
24 –20 –98 258.75
7 13 25 0 –100 270
26 20 –98 281.25
14 27 38 –92 292.5
28 56 –83 303.75
4 8 15 29 71 –71 315
30 83 –56 326.25
16 31 92 –38 337.5
32 98 –20 348.75

7.3.5 Protection Circuits

7.3.5.1 Overcurrent Protection (OCP)

If the current through any FET exceeds the preset overcurrent threshold, all FETs in the H-bridge will be disabled for a period of approximately 800 µs, or until the ENABLEn pin has been brought inactive high and then back low, or power is removed and reapplied. Overcurrent conditions are sensed in both directions; that is, a short to ground, supply, or across the motor winding will all result in an overcurrent shutdown.

Note that overcurrent protection does not use the current sense circuitry used for PWM current control and is independent of the Isense resistor value or VREF voltage. Additionally, in the case of an overcurrent event, the microstepping indexer will be reset to the home state.

7.3.5.2 Thermal Shutdown (TSD)

If the die temperature exceeds safe limits, all drivers in the device are shut down and the indexer is reset to the home state. Once the die temperature has fallen to a safe level operation resumes.

7.3.5.3 Undervoltage Lockout (UVLO)

If at any time the voltage on the VM or VCC pins falls below the VM or VCC undervoltage lockout threshold voltage, all circuitry in the device will be disabled, and the indexer will be reset to the home state. Operation will resume when VM and VCC both rise above their UVLO thresholds.

7.4 Device Functional Modes

7.4.1 RESETn, ENABLEn, and SLEEPn Operation

The RESETn pin, when driven active low, resets the step table to the home position. It also disables the H-bridge drivers. The STEP input is ignored while RESETn is active.

The ENABLEn pin is used to control the output drivers. When ENABLEn is low, the output H-bridges are enabled. When ENABLEn is high, the H-bridges are disabled and the outputs are in a high-impedance state.

Note that when ENABLEn is high, the input pins and control logic, including the indexer (STEP and DIR pins) are still functional.

The SLEEPn pin is used to put the device into a low power state. If SLEEPn is low, the H-bridges are disabled, the gate drive charge pump is stopped, and all internal clocks are stopped. In this state all inputs are ignored until the SLEEPn pin returns high.

7.4.2 Decay Modes

The DRV8818 supports three different decay modes: slow decay, fast decay, and mixed decay. The current through the motor windings is regulated using a fixed off time scheme.

This means that the current will increase until it reaches the current chopping threshold (ITRIP), after which it will enter the set decay mode for a fixed period of time. The cycle will then repeat after the decay period expires.

The blanking time tBLANK defines the minimum drive time for the current chopping. ITRIP is ignored during tBLANK, so the winding current may overshoot the trip level.