Datasheet

TMC2660 DATASHEET (Rev. 1.05 / 2016-JUL-14) 31
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8.1 Sense Resistors
Sense resistors should be carefully selected. The full motor current flows through the sense resistors.
They also see the switching spikes from the MOSFET bridges. A low-inductance type such as film or
composition resistors is required to prevent spikes causing ringing on the sense voltage inputs
leading to unstable measurement results. A low-inductance, low-resistance PCB layout is essential.
Any common GND path for the two sense resistors must be avoided, because this would lead to
coupling between the two current sense signals. A massive ground plane is best. When using high
currents or long motor cables, spike damping with parallel capacitors to ground may be needed, as
shown in Figure 8.1. Because the sense resistor inputs are susceptible to damage from negative
overvoltages, an additional input protection resistor helps protect against a motor cable break or
ringing on long motor cables.
SRA
SRB
10R to 47R
10R to 47R
optional input
protection resistors
MOSFET
bridge
MOSFET
bridge
GND
TMC2660
Power
supply GND
no common GND path
not visible to TMC2660
470nF
470nF
optional filter
capacitors
R
SENSE
R
SENSE
Figure 8.1 Sense resistor grounding and protection components
The sense resistor needs to be able to conduct the peak motor coil current in motor standstill
conditions, unless standby power is reduced. Under normal conditions, the sense resistor sees a bit
less than the coil RMS current, because no current flows through the sense resistor during the slow
decay phases.
The peak sense resistor power dissipation is:
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For high-current applications, power dissipation is halved by using the lower sense resistor voltage
setting and the corresponding lower resistance value. In this case, any voltage drop in the PCB traces
has a larger influence on the result. A compact power stage layout with massive ground plane is best
to avoid parasitic resistance effects.