TMC246_06 Datasheet, PDF (16/26 Pages) List of Unclassifed Manufacturers – High Current Microstep Stepper Motor Driver with sensorless stall detection, protection / diagnosis and SPI Interface

TMC246 / TMC246A DATA SHEET (V2.03 / Nov. 6th, 2006)
16
Calculation of the external components
Sense Resistor
Choose an appropriate sense resistor (R_S) to set the desired motor current. The maximum motor
current is reached, when the coil current setting is programmed to “1111”. This results in a current
sense trip voltage of 0.34V when the internal reference or a reference voltage of 2V is used.
When operating your motor in fullstep mode, the maximum motor current is as specified by the
manufacturer. When operating in sinestep mode, multiply this value by 1.41 for the maximum current
(I_max).
R_S = V_TRIP / I_max
In a typical application:
R_S = 0.34V / I_max
R_S:
V_TRIP:
I_max:
Current sense resistor of bridge A, B
Programmed trip voltage of the current sense comparators
Desired maximum coil current
Examples for sense resistor settings
R_S
0.47Ω
0.43Ω
0.39Ω
0.33Ω
0.27Ω
0.22Ω
I_max
723mA
790mA
870mA
1030mA
1259mA
1545mA
High side overcurrent detection resistor R_SH
The TMC246 detects an overcurrent to ground, when the voltage between VS and VT exceeds
150mV. The high side overcurrent detection resistor should be chosen in a way that 100mV voltage
drop are not exceeded between VS and VT, when both coils draw the maximum current. In a sinestep
application, this is when sine and cosine wave have their highest sum, i.e. at 45 degrees,
corresponding to 1.41 times the maximum current setting for one coil. In a fullstep application this is
the double coil current.
In a microstep application:
R_SH = 0.1V / (1.41 × I_max)
In a fullstep application:
R_SH = 0.1V / (2 × I_max)
R_SH: High side overcurrent detection resistor
I_max:
Maximum coil current
However, if the user desires to use higher resistance values, a voltage divider in the range of 10Ω to
100Ω can be used for VT. This might also be desired to limit the peak short to GND current, as
described in the following chapter.
Attention: A careful PCB layout is required for the sense resistor traces and for the R_SH traces.
Copyright © 2005, TRINAMIC Motion Control GmbH & Co KG