Does the short to ground protection detect whether two motor coils are short-circuit to each other?
The TMC26x monitors the voltage drop across the high-side MOSFET by comparing VS to BM. If the coil voltage at BM does not reach the detection level within the time defined by the internal detection delay setting (TS2G parameter) an overload is detected and the bridge is switched off. To detect an overload the current needs a sufficiently high value. If two motor coils are short-circuited to each other the low-side MOSFET and the sense resistor will be limiting the current. For this reason, an overload may not be necessarily detected.
By adding an additional high-side shunt resistor the short to ground detection can more be fine-tuned and can be adjusted to the nominal motor current. The TS2G-detection time has to be shorter than the blank time in order that the normal chopper does not prevent an overload detection.
The size of RSHUNT can be calculated by the short to ground detector threshold VBMS2G provided in the data sheets, the RDS(ON) resistance of the high-side MOSFET and the desired upper limit current at which an overload detection shall occur:
Example: For the above circuit consisting of a TMC262 as driver and 4 TMC1620 as 2 H bridges RDS(ON) equals 75mΩ and VBMS2G equals 1.5V. The value of RSHUNT for an overload detection at 5A is: