Datasheet

TMC2209 DATASHEET (Rev. 1.03 / 2019-JUN-26) 53
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10 Internal Sense Resistors
The TMC2209 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 chapter 3.2). As MOSFETs are both, temperature dependent and
subject to production stray, a tiny external resistor connected from +5VOUT to VREF provides 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 (OTP option).
COMPARING INTERNAL SENSE RESISTORS VS. SENSE RESISTORS
Item
Internal Sense Resistors
External Sense Resistors
Ease of use
Need to set OTP parameter
before motor enable
(+) Default
Cost
(+) Save cost for sense resistors
Current precision
Slightly reduced
(+) Good
Current Range
Recommended
200mA RMS to 1.4A RMS
50mA to 2A RMS
Recommended
chopper
StealthChop or SpreadCycle
SpreadCycle shows slightly more
distortion at >1.4A RMS
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 VREF (pls. refer the table
for the choice of the resistor). VREF input resistance is about 0.45kOhm. The resulting current into
VREF is amplified 3000 times. Thus, a current of 0.33mA yields a motor current of 1.0A peak, or 0.7A
RMS. For calculation of the reference resistor, the internal resistance of VREF needs to be considered
additionally.
CHOICE OF R
REF
FOR OPERATION WITHOUT SENSE RESISTORS
R
REF
[Ω]
Peak current [A]
(CS=31, vsense=0)
RMS current [A]
(CS=31, vsense=0)
6k2
2.26
1.59
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
UART OTP