TMC2225 DATASHEET (Rev. 1.15 / 2023-FEB-16)
52
10 Internal Sense Resistors
UART OTP
The TMC2225 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 0). As MOSFETs are both, temperature dependent and
subject to production stray, a tiny external resistor connected from +5VOUT to VREF provide s 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
Ease of use
Internal Sense Resistors
External Sense Resistors
Need to set OTP parameter first (+) Default
Cost
(+) Save cost for sense resistors
Current precision Slightly reduced
(+) Good
Current Range
200mA RMS to 1.2A RMS
50mA to 1.4A RMS
Recommended
Recommended StealthChop,
StealthChop or SpreadCycle
chopper
SpreadCycle shows slightly
reduced performance at >1A
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 (RREF) can be connected between 5VOUT and VREF (pls. refer the table
for the choice of the resistor). VREF input resistance is about 0.7kOhm. An additional current of 40µA
flows internally making up for an adder of roughly 120mA. The resulting current into VREF 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 VREF needs to be considered additionally.
CHOICE OF RREF FOR OPERATION WITHOUT SENSE RESISTORS
RREF [Ω]
Peak current [A]
(CS=31, vsense=0)
RMS current [A]
(CS=31, vsense=0)
6k8
2.11
1.49
7k5
1.94
1.37
8k2
1.80
1.27
9k1
1.64
1.16
10k
1.51
1.07
12k
1.29
0.91
15k
1.07
0.75
18k
0.91
0.65
22k
0.77
0.55
27k
0.65
0.46
33k
0.56
0.39
vsense=1 allows a lower peak current setting of about 55% of the value yielded with vsense=0 (as
specified by VSRTH / VSRTL).
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