Datasheet

TMC2100 DATASHEET (Rev. 1.07 / 2017-MAY-15) 26
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9 Internal Sense Resistors
The TMC2100 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 4.2). As MOSFETs are both, temperature dependent and
subject to production stray, a tiny external resistor connected from +5VOUT to AIN/IREF provides a
precise absolute current reference. This resistor converts the 5 V 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 tying CFG3 to VCCIO.
COMPARING INTERNAL SENSE RESISTORS VS. SENSE RESISTORS
Item
Internal Sense Resistors
External Sense Resistors
Ease of use
Need to set OTP parameter first
(+) Default
Cost
(+) Save cost for sense resistors
Current precision
Slightly reduced
(+) Good
Current Range
Recommended
200mA RMS to 1.2A RMS
50mA to 1.4A RMS
Recommended
chopper
stealthChop,
spreadCycle shows slightly
reduced performance at >1A
stealthChop or spreadCycle
While the RDSon based measurements bring benefits concerning cost and size of the driver, it gives
slightly less precise coil current regulation when compared to external sense resistors. The internal
sense resistors have a certain temperature dependence, which is automatically compensated by the
driver IC. However, for high current motors, a temperature gradient between the ICs internal sense
resistors and the compensation circuit will lead to an initial current overshoot of some 10% during
driver IC heat up. While this phenomenon shows for roughly a second, it might even be beneficial to
enable increased torque during initial motor acceleration.
PRINCIPLE OF OPERATION
A reference current into the VREF pin is used as reference for the motor current. In order to realize a
certain current, a single resistor (R
REF
) can be connected between 5VOUT and AIN (pls. refer the table
for the choice of the resistor). AIN input resistance is about 1kOhm. The resulting current into AIN is
amplified 3000 times. Thus, a current of 0.5mA yields a motor current of 1.5A peak. For calculation of
the reference resistor, the internal resistance of AIN needs to be considered additionally. When using
reference currents above 0.5 mA resulting in higher theoretical current settings of up to 2 A, the
resulting current decreases linearly when chip temperature exceeds a certain maximum temperature.
For a 2 A setting it decreases from 2 A at up to 100 °C down to about 1.5 A at 150 °C.
CHOICE OF R
REF
FOR OPERATION WITHOUT SENSE RESISTORS
R
REF
[Ω]
Peak current [A]
RMS current [A]
6k8
1.92
1.35
7k5
1.76
1.24
8k2
1.63
1.15
9k1
1.49
1.05
10k
1.36
0.96
12k
1.15
0.81
15k
0.94
0.66
18k
0.79
0.55
22k
0.65
0.45
27k
0.60
0.42
33k
0.54
0.38