DRV8834 Dual-Bridge Stepper or DC Motor Driver

8.3.1 Current Control

The current through the motor windings may be regulated by a fixed-frequency PWM current regulation (current chopping).

With stepping motors, current control is normally used at all times. Often it is used to vary the current in the two windings in a sinusoidal fashion to provide smooth motion. This is referred to as microstepping. The DRV8834 can provide up to 1/32 step microstepping, using internal 5-bit DACs. Finer microstepping can be implemented using the xPHASE/xENBL signals to control the stepper motor, and varying the xVREF voltages. The current flowing through the corresponding H-bridge varies according to the equation given below. A very high degree of microstepping can be achieved through this technique.

With DC motors, current control can be used to limit the start-up current of the motor to less than the stall current of the motor.

Current regulation works as follows:

When an H-bridge is enabled, current rises through the winding at a rate dependent on the supply voltage and inductance of the winding. If the current reaches the current chopping threshold, the bridge disables the current until the beginning of the next PWM cycle. Immediately after the current is enabled, the voltage on the xISEN pin is ignored for a period of time before enabling the current sense circuitry. This blanking time also sets the minimum on time of the PWM when operating in current chopping mode.

The blanking time also sets the minimum PWM duty cycle. This can cause current control errors near the zero current level when microstepping. To help eliminate this error, the DRV8834 has a dynamic t_BLANK time. When the commanded current is low, the blanking period is reduced, which in turn lowers the minimum duty cycle. This provides a smoother current transition across the zero crossing region of the current waveform. The end result is smoother and quieter motor operation.

The PWM chopping current is set by a comparator which compares the voltage across a current sense resistor connected to the xISEN pins, with a reference voltage supplied to the AVREF and BVREF pins. In indexer mode, the reference voltages are scaled by internal DACs to provide scaled currents used to perform microstepping.

The chopping current is calculated as follows:

Equation 1.

Example: If xVREF is 2 V (as it would be if xVREF is connected directly to VREFO) and a 400-mΩ sense resistor is used, the chopping current will be 2 V / 5 × 400 mΩ = 1 A.

In indexer mode, this current value is scaled by between 5% and 100% by the internal DACs, as shown in the step table in the "Microstepping Indexer" section of the data sheet.

If current control is not needed, the xISEN pins may be connected directly to ground. In this case, TI also recommends connecting AVREF and BVREF directly to VREFO.

8.3.2 Current Recirculation and Decay Modes

During PWM current chopping, the H-bridge is enabled to drive current through the motor winding until the PWM current chopping threshold is reached. This is shown in Figure 7 as case 1. The current flow direction shown indicates positive current flow in the step table below for indexer mode, or the current flow with xPHASE = 1 in phase/enable mode.

Once the chopping current threshold is reached, the drive current is interrupted, but due to the inductive nature of the motor, the current must continue to flow. This is called recirculation current. To handle this recirculation current, 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 through the opposing FETs. As the winding current approaches zero, the bridge is disabled to prevent any reverse current flow. Fast decay mode is shown in Figure 7 as case 2.

In slow decay mode, winding current is recirculated by enabling both of the low-side FETs in the bridge. Slow decay is shown as case 3 in Figure 7.

Figure 7. Decay Modes

The DRV8834 supports fast, slow, and also mixed decay modes. With DC motors, slow decay is nearly always used to minimize current ripple and optimize speed control; with stepper motors, the decay mode is chosen for a given stepper motor and operating conditions to minimize mechanical noise and vibration.

In mixed decay mode, the current recirculation begins as fast decay, but at a fixed period of time (determined by the state of the xDECAY pins shown in Table 1) switches to slow decay mode for the remainder of the fixed PWM period.

Figure 8 shows the current waveforms in slow, 25% mixed, and fast decay modes.

Figure 8. Current Decay Modes

Decay mode is selected by the voltage present on the xDECAY pins. Internal current sources of 10 µA (typical) are connected to the pins, which allows setting of the decay mode by a resistor connected to ground if desired.

It is possible to drive the xDECAY pin with a tristate GPIO pin and also place the resistor to ground. This allows a microcontroller to select fast, slow, or mixed decay modes by driving the pin high, low, or high-impedance. The logic-low voltage must be less than 0.1 V with 10-µA of current sourced from the DRV8834 to attain slow decay.

In indexer mode, only the ADECAY pin is used, and slow decay mode is always used when at any point in the step table where the current is increasing. When current is decreasing or remaining constant, the decay mode used will be fast, slow, or mixed, as commanded by the ADECAY pin.

8.3.3 Protection Circuits

The DRV8834 is fully protected against undervoltage, overcurrent and overtemperature events.

8.4.1 Phase/Enable Mode

In phase/enable mode, the xPHASE input pins control the direction of current flow through each H-bridge. This sets the direction of rotation of a DC motor, or the direction of the current flow in a stepper motor winding. Driving the xENBL input pins active high enables the H-bridge outputs. This can be used as PWM speed control of a DC motor, or to enable/disable the current in a stepper motor.

In phase/enable mode, the M1 input pin controls the state of the H-bridges when xENBL = 0. If M1 is high, the outputs are disabled (high impedance) when xENBL = 0; this corresponds to asynchronous fast decay mode, and is usually used in stepper motor applications to command a "zero current" state. If M1 is low, then the outputs are both driven low; this corresponds to slow decay or brake mode, and is usually used when controlling the speed of a DC motor by PWMing the xENBL pin.

8.4.2 Indexer Mode

To allow a simple step and direction interface to control stepper motors, the DRV8834 contains a microstepping indexer. The indexer controls the state of the H-bridges automatically. Whenever there is a rising edge at the STEP input, the indexer moves to the next step, according to the direction set by the DIR pin.

The nENBL pin is used to disable the output stage in indexer mode. When nENBL = 1, the indexer inputs are still active and will respond to the STEP and DIR input pins; only the output stage is disabled.

The indexer logic in the DRV8834 allows a number of different stepping configurations. The M0 and M1 pins are used to configure the stepping format as shown in Table 3.

The M0 pin is a tri-level input. It can be driven logic low, logic high, or high-impedance (Z).

The M0 and M1 pins can be statically configured by connecting to VINT, GND, or left open, or can be driven with standard tristate microcontroller I/O port pins. Their state is latched at each rising edge of the STEP input.

The step mode may be changed on-the-fly while the motor is moving. The indexer will advance to the next valid state for the new M0/M1 setting at the next rising edge of STEP.

The home state is 45°. This state is entered after power up, after exiting undervoltage lockout, or after exiting sleep mode. This is shown in Table 4 by cells shaded yellow.

Table 4 shows the relative current and step directions for different step mode settings. 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.

8.4.3 nSLEEP Operation

Driving nSLEEP low will put the device into a low-power sleep state. In this state, the H-bridges are disabled, the gate drive charge pump is stopped, all internal logic is reset (this returns the indexer to the home state), the VINT supply is disabled, and all internal clocks are stopped. All inputs are ignored until nSLEEP returns inactive high.

Because the VINT supply is disabled during sleep mode, it cannot be used to provide a logic high signal to the nSLEEP pin. To simplify board design, the nSLEEP can be pulled up directly to the supply (VM) if it is not actively driven. Unless VM is less than 5.75 V, a pullup resistor is required.

The nSLEEP pin is protected by a Zener diode that will clamp the pin voltage to approximately 6.5 V. The pullup resistor limits the current to the input in case VM is higher than 6.5 V. The recommended pullup resistor is 20 kΩ to 50 kΩ.

When exiting sleep mode, the nFAULT pin will be briefly driven active low as the internal power supplies turn on. nFAULT will return to inactive high once the internal power supplies (including charge pump) have stabilized. This process takes some time (up to 1 ms), before the motor driver becomes fully operational.

9.1.1 Sense Resistor

For optimal performance, it is important for the sense resistor to be:

• Surface-mount
• Low inductance
• Rated for high enough power
• Placed closely to the motor driver

The power dissipated by the sense resistor equals I_RMS² × R. For example, if peak motor current is 3 A, RMS motor current is 2 A, and a 0.05-Ω sense resistor is used, the resistor will dissipate 2_A² × 0.05 Ω = 0.2 W. The power quickly increases with higher current levels.

Resistors typically have a rated power within some ambient temperature range, along with a derated power curve for high ambient temperatures. When a PCB is shared with other components generating heat, margin should be added. It is always best to measure the actual sense resistor temperature in a final system, along with the power MOSFETs, as those are often the hottest components.

Because power resistors are larger and more expensive than standard resistors, it is common practice to use multiple standard resistors in parallel, between the sense node and ground. This distributes the current and heat dissipation.

9.2.1 Phase/Enable Mode Driving Two DC Motors

In this configuration, the DRV8834 is used to drive two independent DC motors. Current up to 1 A per motor is possible. The M1 pin is pulled low to allow slow decay PWM from the controller (if desired) to control the motor speed by PWMing the xENBL inputs, and ADECAY and BDECAY are connected to ground to set slow decay mode during current limiting. The value of the RSENSE resistors shown is for a 1-A current limit; if current limiting is not needed, the AISEN and BISEN pins may be connected directly to ground. If the sleep function is not needed, nSLEEP can be connected to VM with an approximate 47-kΩ resistor.

Figure 9. Phase/Enable Mode Driving Two DC Motors

9.2.1.3 Application Curves

Figure 10. Brushed Motor – VM = 8 V,
DRV8834 is Regulating Current

Figure 12. External Microstepping – VM = 8 V, 4000 Steps Per Second,
1/128 microstep mode, 1.2-A Current Regulation

Figure 11. Internal Indexer – VM = 8 V, 1200 Steps per
second, 1/8 microstep mode, 1-A Current regulation

9.2.2 Phase/Enable Mode Driving a Stepper Motor

Figure 13. Phase/Enable Mode Driving a Stepper Motor

9.2.3 Indexer Mode Driving a Stepper Motor

In indexer mode, only a rising edge on the STEP pin is needed to move the motor to the next step. The DIR pin sets which direction the motor rotates, by reversing the step sequence. The internal indexer can operate in full-step, half-step, and smaller microsteps up to 1/32-step, depending on the state of the M0 and M1 pins. The M0 and M1 pins can also be connected directly to ground or to VINT to program the step modes, if desired. If the sleep function is not needed, nSLEEP can be connected to VM with an approximate 47-kΩ resistor. Step sequences for full and half step are shown below.

Figure 16. Indexer Mode Driving a Stepper Motor

9.2.3.3 Application Curves

Figure 17. Full Step Sequence (2-Phase)

Figure 18. Half Step Sequence (1-2 Phase)

9.2.4 High-Resolution Microstepping Using a Microcontroller to Modulate VREF Signals

Figure 19. High-Resolution Microstepping