Table 5. Design Parameters

9.2.1.2.1 Motor Voltage

The motor voltage to use will depend on the ratings of the motor selected and the desired RPM. A higher voltage spins a brushed DC motor faster with the same PWM duty cycle applied to the power FETs. A higher voltage also increases the rate of current change through the inductive motor windings.

9.2.1.2.2 Power Dissipation

The power dissipation of the DRV8834 is a function of RMS motor current and the FET resistance (_RDS(ON)) of each output.

For this example, the ambient temperature is 35°C, and the junction temperature reaches 65°C. At 65°C, the sum of R_DS(ON) is about 1 Ω. With an example motor current of 0.8 A, the dissipated power in the form of heat will be 0.8 A² × 1 Ω = 0.64 W.

The temperature that the DRV8834 reaches will depend on the thermal resistance to the air and PCB. It is important to solder the device PowerPAD to the PCB ground plane, with vias to the top and bottom board layers, in order dissipate heat into the PCB and reduce the device temperature. In the example used here, the DRV8834 had an effective thermal resistance R_θJA of 47°C/W, and:

9.2.1.2.3 Motor Current Trip Point

When the voltage on pin SENSE exceeds V_TRIP (0.5 V), overcurrent is detected. The R_SENSE resistor should be sized to set the desired I_TRIP level.

To set I_TRIP to 2 A, R_SENSE = 0.5 V / 2 A = 0.25 Ω.

To prevent false trips, I_TRIP must be higher than regular operating current. Motor current during start-up is typically much higher than steady-state spinning, because the initial load torque is higher, and the absence of back-EMF causes a higher voltage and extra current across the motor windings.

It can be beneficial to limit start-up current by using series inductors on the DRV8834 output, as that allows I_TRIP to be lower, and it may decrease the system’s required bulk capacitance. Start-up current can also be limited by ramping the forward drive duty cycle.

Table 6. Design Parameters

9.2.2.2.1 Stepper Motor Speed

The first step in configuring the DRV8834 requires the desired motor speed and microstepping level. If the target application requires a constant speed, then a square wave with frequency ƒ_step must be applied to the STEP pin.

If the target motor start-up speed is too high, the motor will not spin. Make sure that the motor can support the target speed or implement an acceleration profile to bring the motor up to speed.

For a desired motor speed (v), microstepping level (nm), and motor full step angle (θ_step),

θstep can be found in the stepper motor data sheet or written on the motor itself.

For the DRV8834, the microstepping level is set by the MODE pins and can be any of the settings in Table 6. Higher microstepping will mean a smoother motor motion and less audible noise, but will increase switching losses and require a higher f_step to achieve the same motor speed.

9.2.2.2.2 Current Regulation

In a stepper motor, the set full-scale current (I_FS) is the maximum current driven through either winding. This quantity will depend on the xVREF analog voltage and the sense resistor value (R_SENSE). During stepping, I_FS defines the current chopping threshold (I_TRIP) for the maximum current step. The gain of DRV8834 is set for 5 V/V.

To achieve I_FS = 1.25 A with R_SENSE of 0.2 Ω, xVREF should be 1.25 V.

9.2.2.2.3 Decay Modes

The DRV8834 supports three different decay modes: slow decay, fast decay, and mixed decay. The current through the motor windings is regulated using a fixed-frequency PWM scheme. This means that after any drive phase, when a motor winding current has hit the current chopping threshold (I_TRIP), the DRV8834 will place the winding in one of the three decay modes until the PWM cycle has expired. Afterward, a new drive phase starts.

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

Table 7. Design Parameters

9.2.3.2.1 Stepper Motor Speed

The first step in configuring the DRV8834 requires the desired motor speed and microstepping level. If the target application requires a constant speed, then a square wave with frequency ƒ_step must be applied to the STEP pin.

If the target motor start-up speed is too high, the motor will not spin. Make sure that the motor can support the target speed or implement an acceleration profile to bring the motor up to speed.

For a desired motor speed (v), microstepping level (nm), and motor full step angle (θ_step),

θstep can be found in the stepper motor data sheet or written on the motor itself.

For the DRV8834, the microstepping level is set by the MODE pins and can be any of the settings in Table 6. Higher microstepping will mean a smoother motor motion and less audible noise, but will increase switching losses and require a higher f_step to achieve the same motor speed.

9.2.3.2.2 Current Regulation

In a stepper motor, the set full-scale current (I_FS) is the maximum current driven through either winding. This quantity will depend on the xVREF analog voltage and the sense resistor value (R_SENSE). During stepping, I_FS defines the current chopping threshold (I_TRIP) for the maximum current step. The gain of DRV8834 is set for 5 V/V.

To achieve I_FS = 1.25 A with R_SENSE of 0.2 Ω, xVREF should be 1.25 V.

9.2.3.2.3 Decay Modes

The DRV8834 supports three different decay modes: slow decay, fast decay, and mixed decay. The current through the motor windings is regulated using a fixed-frequency PWM scheme. This means that after any drive phase, when a motor winding current has hit the current chopping threshold (I_TRIP), the DRV8834 will place the winding in one of the three decay modes until the PWM cycle has expired. Afterward, a new drive phase starts.

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

Table 8. Design Parameters