Datasheet

TMC5130A DATASHEET (Rev. 1.11 / 2015-OCT-08) 69
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11 RDSon Based Measurement Eliminates Sense
Resistors
The TMC5130A provides the option to eliminate external sense resistors. In this mode the external
sense resistors become omitted (shorted) and the internal on-resistance of the power MOSFETs is
used for current measurement (see Figure 3.3). As MOSFETs are both, temperature dependent and
subject to production stray, a tiny external resistor connected from +5VOUT to AIN/IREF is used to
provide a precise absolute current reference. This resistor converts the 5V voltage into a reference
current. Be sure to directly attach BRA and BRB pins to GND in this mode near the IC package. The
mode is enabled by setting internal_Rsense in GCONF.
11.1 Limitations of RDSon Sensing
While the RDSon based measurements bring benefits concerning cost and size of the driver, it gives
slightly less precise current setting when compared to external sense resistors. External sense
resistors provide the possibility to adapt the driver to a wide range of motor operation currents
without trading in more noise due to scaling down chopper comparator reference voltage. Therefore
the use of RDSon based measurement should be considered for motor currents between 0.4A RMS
and 1.2A RMS. For lower and higher current motors, the performance should be evaluated first. Low
current motors (below 0.4A RMS) also perform well when using RDSon measurement in combination
with the stealthChop mode, because digital scaling does not directly increase chopper noise and
reduce effective microstep performance. RDSon sensing may suffer from increased chopper noise and
reduced microstep precision in combination with spreadCycle, because the current measurement
required for spreadCycle does not guarantee the same precision and symmetry as a sense resistor can
deliver. Therefore, consider using classic constant off time chopper instead of spreadCycle in case
audible high pitch chopper noise appears.
11.2 Dimensioning of Reference Resistor
For RDSon measurement, up to 1.5A (2A max.) peak current can be driven into the motor (with
vsense=0). An external reference current into the AIN/IREF pin is used as a reference current. AIN/IREF
input resistance is about 1kOhm. In order to realize a certain current a single resistor (R
REF
) can be
connected between 5VOUT and AIN/IREF (pls. refer the table for the choice of the resistor). The
resulting current into AIN/IREF is amplified 3000 times. Thus, a current of 0.5mA yields a motor
current of 1.5A peak. When using reference currents above 0.5mA resulting in higher theoretical
current settings of up to 2A, the resulting current decreases linearly when chip temperature exceeds a
certain maximum temperature. For a 2A setting it decreases from 2A at up to 100°C down to about
1.5A at 150°C. The resulting curve limits the maximum current setting in this mode. For calculation of
the reference resistor, the internal resistance of AIN/RREF needs to be considered additionally.
vsense=1 allows a lower peak current setting of about 55% of the value yielded with vsense=0 (as
specified by V
SRTH
/ V
SRTL
). For fine tuning use the current scale CS.