TMC249 / TMC249A DATA SHEET (V2.09 / 2009-Jul-30)
20
Calculation of the external components
Sense Resistor
Choose an appropriate sense resistor (RS) 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
(Imax).
RS = VTRIP / Imax
In a typical application:
RS = 0.34V / Imax
RS:
Current sense resistor of bridge A, B
VTRIP
:
Programmed trip voltage of the current sense comparators
Desired maximum coil current
Imax
:
Examples for sense resistor settings
RS
Imax
723mA
1030mA
1545mA
2267mA
3400mA
0.47
0.33
0.22
0.15
0.10
High side overcurrent detection resistor RSH
The TMC249 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:
RSH = 0.1V / (1.41 Imax
In a fullstep application:
RSH = 0.1V / (2 Imax
)
)
RSH:
Imax
High side overcurrent detection resistor
Maximum coil current
:
However, if the user desires to use higher resistance values, a voltage divider in the range of 10 to
100 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 RSH traces.
Copyright © 2005, TRINAMIC Motion Control GmbH & Co KG
TRINAMIC [ TRINAMIC MOTION CONTROL GMBH & CO. KG. ]
