SLVSDS7B August 2019 – November 2019 DRV8876
The DRV887x family of devices integrate current regulation using either a fixed off-time or cycle-by-cycle PWM current chopping scheme. The current chopping scheme is selectable through the IMODE quad-level input. This allows the devices to limit the output current in case of motor stall, high torque, or other high current load events.
The IMODE level can be set by leaving the pin floating (Hi-Z), connecting the pin to GND, or connecting a resistor between IMODE and GND. The IMODE pin state is latched when the device is enabled through the nSLEEP pin. The IMODE state can be changed by taking the nSLEEP pin logic low, waiting the tSLEEP time, changing the IMODE pin input, and then enabling the device by taking the nSLEEP pin back logic high. The IMODE input is also used to select the device response to an overcurrent event. See more details in the Protection Circuits section.
The internal current regulation can be disabled by tying IPROPI to GND and setting the VREF pin voltage greater than GND (if current feedback is not required) or if current feedback is required, setting VVREF and RIPROPI such that VIPROPI never reaches the VVREF threshold. In independent half-bridge control mode (PMODE = Hi-Z), the internal current regulation is automatically disabled since the outputs are operating independently and the current sense and regulation is shared between half-bridges.
|IMODE STATE||IMODE FUNCTION||nFAULT
|Current Chopping Mode||Overcurrent
|Quad-Level 1||RIMODE = GND||Fixed Off-Time||Automatic Retry||Overcurrent Only|
|Quad-Level 2||RIMODE = 20kΩ to GND||Cycle-By-Cycle||Automatic Retry||Current Chopping and Overcurrent|
|Quad-Level 3||RIMODE = 62kΩ to GND||Cycle-By-Cycle||Outputs Latched Off||Current Chopping and Overcurrent|
|Quad-Level 4||RIMODE = Hi-Z||Fixed Off-Time||Outputs Latched Off||Overcurrent Only|
The current chopping threshold (ITRIP) is set through a combination of the VREF voltage (VVREF) and IPROPI output resistor (RIPROPI). This is done by comparing the voltage drop across the external RIPROPI resistor to VVREF with an internal comparator.
For example, if VVREF = 2.5 V, RIPROPI = 1500 Ω, and AIPROPI = 1000 μA/A, then ITRIP will be approximately 1.67 A.
When the ITRIP threshold is exceeded, the outputs will enter a current chopping mode according to the IMODE setting. The ITRIP comparator has both a blanking time (tBLK) and a deglitch time (tDEG). The internal blanking time helps to prevent voltage and current transients during output switching from effecting the current regulation. These transients may be caused by a capacitor inside the motor or on the connections to the motor terminals. The internal deglitch time ensures that transient conditions do not prematurely trigger the current regulation. In certain cases where the transient conditions are longer than the deglitch time, placing a 10-nF capacitor on the IPROPI pin, close to the DRV887x, will help filter the transients on IPROPI output so current regulation does not prematurely trigger. The capacitor value can be adjusted as needed, however large capacitor values may slow down the response time of the current regulation circuitry.
The AERR parameter in the Electrical Characteristics table is the error associated with the AIPROPI gain. It indicates the combined effect of offset error added to the IOUT current and gain error.