Does the short-to-ground protection detect whether two motor coils are short-circuited 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 be tuned finer and can be adjusted to the nominal motor current. The TS2G-detection time has to be shorter than the blank time so the normal chopper doesn't prevent an overload detection.

The size of R_SHUNT can be calculated by the short to ground detector threshold V_BMS2G provided in the datasheets, the R_DS(ON) resistance of the high-side MOSFET and the desired upper limit current at which an overload detection shall occur:

R_{\text{SHUNT}}=\frac{V_{BMS2G}}{I_{OVERLOAD}}-R_{DS(ON)}

Example: For the above circuit consisting of a TMC262 as driver and 4 TMC1620 as 2 H bridges, R_DS(ON) equals 75mΩ and V_BMS2G equals 1.5V. The value of R_SHUNT for an overload detection at 5A is:

R_{\text{SHUNT}}=\frac{1.5V}{5A}-75m\omega = 225m\omega