7.3.1 PWM Motor Drivers

The DRV8812 contains two H-bridge motor drivers with current-control PWM circuitry. A block diagram of the motor control circuitry is shown in Figure 5. A bipolar stepper motor is shown, but the drivers can also drive two separate DC motors.

Figure 5. Motor Control Circuitry

Note that there are multiple VM motor power supply pins. All VM pins must be connected together to the motor supply voltage.

7.4.1 Bridge Control

The xPHASE input pins control the direction of current flow through each H-bridge. The xENBL input pins enable the H-bridge outputs when active high. Table 1 shows the logic.

7.4.2 Current Regulation

The current through the motor windings is regulated by a fixed-frequency PWM current regulation, or current chopping. When an H-bridge is enabled, current rises through the winding at a rate dependent on the DC voltage and inductance of the winding. Once the current hits the current chopping threshold, the bridge disables the current until the beginning of the next PWM cycle.

For stepping motors, current regulation is normally used at all times, and can changing the current can be used to microstep the motor. For DC motors, current regulation is used to limit the start-up and stall current of the motor.

The PWM chopping current is set by a comparator which compares the voltage across a current sense resistor connected to the xISEN pins, multiplied by a factor of 5, with a reference voltage. The reference voltage is input from the xVREF pins, and is scaled by a 2-bit DAC that allows current settings of 100%, 71%, 38% of full-scale, plus zero.

The full-scale (100%) chopping current is calculated in Equation 1.

Equation 1.

Example:

If a 0.5-Ω sense resistor is used and the VREFx pin is 3.3 V, the full-scale (100%) chopping current will be 3.3 V / (5 x 0.5 Ω) = 1.32 A.

Two input pins per H-bridge (xI1 and xI0) are used to scale the current in each bridge as a percentage of the full-scale current set by the VREF input pin and sense resistance. The function of the pins is shown in

Table 2.

Note that when both xI bits are 1, the H-bridge is disabled and no current flows.

Example:

If a 0.5-Ω sense resistor is used and the VREF pin is 3.3 V, the chopping current will be 1.32 A at the 100% setting (xI1, xI0 = 00). At the 71% setting (xI1, xI0 = 01) the current will be 1.32 A x 0.71 = 0.937 A, and at the 38% setting (xI1, xI0 = 10) the current will be 1.32 A x 0.38 = 0.502 A. If (xI1, xI0 = 11) the bridge will be disabled and no current will flow.

7.4.3 Decay Mode

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 6 as case 1. The current flow direction shown indicates the state when the xENBL pin is high.

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. As the winding current approaches zero, the bridge is disabled to prevent any reverse current flow. Fast decay mode is shown in Figure 6 as case 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 6 as case 3.

Figure 6. Decay Mode

The DRV8812 supports fast decay, slow decay and a mixed decay mode. Slow, fast, or mixed decay mode is selected by the state of the DECAY pin - logic low selects slow decay, open selects mixed decay operation, and logic high sets fast decay mode. Note that the DECAY pin sets the decay mode for both H-bridges.

Mixed decay mode begins as fast decay, but at a fixed period of time (75% of the PWM cycle) switches to slow decay mode for the remainder of the fixed PWM period.

7.4.4 Blanking Time

After the current is enabled in an H-bridge, the voltage on the xISEN pin is ignored for a fixed period of time before enabling the current sense circuitry. This blanking time is fixed at 3.75 μs. Note that the blanking time also sets the minimum on time of the PWM.

7.4.5 nRESET and nSLEEP Operation

The nRESET pin, when driven active low, resets the internal logic. It also disables the H-bridge drivers. All inputs are ignored while nRESET is active.

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, the V3P3OUT regulator is disabled, and all internal clocks are stopped. In this state all inputs are ignored until nSLEEP returns inactive high. When returning from sleep mode, some time (approximately 1 ms) needs to pass before the motor driver becomes fully operational.

7.4.6 Protection Circuits

The DRV8812 is fully protected against undervoltage, overcurrent and overtemperature events.