SLVSAB7D May 2010 – December 2015 DRV8840
The DRV8840 is an integrated motor driver solution for printers, scanners, and other automated equipment applications. The device integrates a single NMOS H-bridge, charge pump, current sense, current regulation, and device protection circuitry. The DRV8828 can be powered from a single voltage supply from 8.2 V to 45 V, and is capable of providing a continuous output current up to 5 A.
A simple PHASE/ENBL interface allows for easy interfacing to an external controller. A 5 bit current control scheme allows for up to 32 discrete current levels. The current regulation method is adjustable between slow and fast decay.
Integrated protection circuits allows the device to monitor and protect against overcurrent, undervoltage, and overtemperature faults which are all reported through a fault indication pin (nFAULT). A low power sleep mode is integrated which allows the system to lower power consumption when not driving the motor.
The DRV8840 device contains one H-bridge motor driver with current-control PWM circuitry. A block diagram of the motor control circuitry is shown in Figure 5.
There are multiple VM, ISEN, OUT, and VREF pins. All like-named pins must be connected together on the PCB.
The PHASE input pin controls the direction of current flow through the H-bridge, and hence the direction of rotation of a DC motor. The ENBL input pin enables the H-bridge outputs when active high, and can also be used for PWM speed control of the motor. Note that the state of the DECAY pin selects the behavior of the bridge when ENBL = 0, allowing the selection of slow decay (brake) or fast decay (coast). Table 1 shows the logic.
The control inputs have internal pulldown resistors of approximately 100 kΩ.
The maximum current through the motor winding is regulated by a fixed-frequency PWM current regulation, or current chopping. When the 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 DC motors, current regulation is used to limit the start-up and stall current of the motor. Speed control is typically performed by providing an external PWM signal to the ENBLx input pins.
If the current regulation feature is not needed, it can be disabled by connecting the ISENSE pins directly to ground and the VREF pins to V3P3.
The PWM chopping current is set by a comparator which compares the voltage across a current sense resistor connected to the ISEN pin, multiplied by a factor of 5, with a reference voltage. The reference voltage is input from the VREF pin, and is scaled by a 5-bit DAC that allows current settings of zero to 100% in an approximately sinusoidal sequence.
The full-scale (100%) chopping current is calculated in Equation 1.
If a 0.25-Ω sense resistor is used and the VREFx pin is 2.5 V, the full-scale (100%) chopping current will be 2.5 V / (5 × 0.25 Ω) = 2 A.
Five input pins (I0 - I4) are used to scale the current in the bridge as a percentage of the full-scale current set by the VREF input pin and sense resistance. The I0 - I4 pins have internal pulldown resistors of approximately
100 kΩ. The function of the pins is shown in Table 2.
(% FULL-SCALE CHOPPING CURRENT)
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.
The DRV8840 device supports fast decay and slow decay mode. Slow or fast decay mode is selected by the state of the DECAY pin - logic low selects slow decay, and logic high sets fast decay mode. The DECAY pin has both an internal pullup resistor of approximately 130 kΩ and an internal pulldown resistor of approximately 80 kΩ. This sets the mixed decay mode if the pin is left open or undriven.
DECAY mode also affects the operation of the bridge when it is disabled (by taking the ENBL pin inactive). This applies if the ENABLE input is being used for PWM speed control of the motor, or if it is simply being used to start and stop motor rotation.
If the DECAY pin is high (fast decay), when the bridge is disabled fast decay mode will be entered until the current through the bridge reaches zero. Once the current is at zero, the bridge is disabled to prevent the motor from reversing direction. This allows the motor to coast to a stop.
If the DECAY pin is low (slow decay), both low-side FETs will be turned on when ENBL is made inactive. This essentially shorts out the back EMF of the motor, causing the motor to brake, and stop quickly. The low-side FETs will stay in the ON state even after the current reaches zero.
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.
The DRV8840 device is fully protected against undervoltage, overcurrent and overtemperature events.
An analog current limit circuit on each FET limits the current through the FET by removing the gate drive. If this analog current limit persists for longer than the OCP time, all FETs in the H-bridge will be disabled and the nFAULT pin will be driven low. The device will remain disabled until either nRESET pin is applied, or VM is removed and re-applied.
Overcurrent conditions on both high and low side devices; i.e., 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.
If the die temperature exceeds safe limits, all FETs in the H-bridge will be disabled and the nFAULT pin will be driven low. Once the die temperature has fallen to a safe level operation will automatically resume.
If at any time the voltage on the VM pins falls below the undervoltage lockout threshold voltage, all circuitry in the device will be disabled and internal logic will be reset. Operation will resume when VM rises above the UVLO threshold.
The nRESET pin, when driven active low, resets the internal logic. It also disables the H-bridge driver. 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. Note that nRESET and nSLEEP have internal pulldown resistors of approximately 100 kΩ. These signals need to be driven to logic high for device operation.