If the ENN input is set high in normal operation of the TMC262(without use of external gate drivers) - then the motor will freewheel since all NMOS FET's will be switched off i.e. both top and bottom FET drive outputs.
However if an external bridge driver is used as shown in figure 23 of the TMC262 datasheet, then if the ENN pin is pulled high on the TMC262 it will set the LA1, LA2 outputs low - thus the external driver will switch the top FET's on since the external driver will simply complement the bottom gate drive signals in order to drive the upper FET's.
Therefore in order to avoid this the external high voltage FET drivers will need to be controlled via typically the shutdown input (SDN) on the external gate drivers. The ENN input on the TMC262 should be connected to 0V.
This will now support the motor being freewheeled or indeed will provide protection from any external short-to-ground circuit that can disable all FET's in the event of a s/c to ground being detected.
However I am concerned that if this situation arises for a period longer than a few seconds, the bootstrap capacitor on each gate drive chip will discharge due to there being no path to ground to keep the bootstrap capacitors charged.
In order to avoid this we could place say 100K suitably rated resistor from at least one half-bridge centre point (assuming that the motor winding will be present).
To be sure even in the event of an open circuit motor winding we would need a resistor to ground from each half bridge centre point.
With a high voltage motor supply (say up to 150V) these resistors will dissipate relatively low power in normal operation, however the resistors will prevent the boost capacitors from totally discharging during periods when the bridge FET's are all disabled.
Are there any other solutions possible for this potential problem?