Datasheet

TMC262 / TMC262C DATASHEET (Rev. 2.22 / 2019-FEB-22) 34
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8.1 Sense Resistors
Sense resistors should be carefully selected. The full motor current flows through the sense resistors.
As 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. A compact power stage layout
with massive GND plane for low-inductance and low-resistance is essential to avoid disturbance by
parasitic effects. Any common GND path for the two sense resistors must be avoided, because this
would lead to coupling between the two current sense signals. Use the massive ground plane for all
GND connections. 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
R
SENSE
SRB
R
SENSE
10R to 47R
10R to 47R
optional input
protection resistors
MOSFET
bridge
MOSFET
bridge
GND
TMC262
Power
supply GND
no common GND path
not visible to TMC262
470nF
470nF
optional filter
capacitors
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 less
than 50% of 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, halve power dissipation by using the lower sense resistor voltage
setting and the corresponding lower resistance value.
Set the desired maximum motor current by selecting an appropriate value for the sense resistor. The
following table shows the RMS current values which are reached using standard resistors. The
resulting application current may vary due to slow decay phase effects and trace resistance.
CHOICE OF R
SENSE
AND RESULTING MAX. MOTOR CURRENT FOR CS=31
R
SENSE
[Ω]
RMS current [A] VSENSE=0
RMS current [A] VSENSE=1
0.33
0.7
0.35
0.27
0.8
0.43
0.22
1.0
0.53
0.15
1.5
0.8
0.12
1.8
1.0
0.10
2.2
1.2
0.075
3
1.6
0.066
3.3
1.8
0.050
4.4
2.3