TMC246/A ETC2 [List of Unclassifed Manufacturers], TMC246/A Datasheet - Page 15

• Manufacturer Part Number: TMC246/A
• Description: High Current Microstep Stepper Motor Driver with sensorless stall detection, protection / diagnosis and SPI Interface
• Manufacturer: ETC2 [List of Unclassifed Manufacturers]

TMC246 DATA SHEET (V2.01 / Sep. 14th, 2005)
Making the circuit short circuit proof
In practical applications, a short circuit does not describe a static condition, but can be of very different
nature. It typically involves inductive, resistive and capacitive components. Worst events are
unclamped switching events, because huge voltages can build up in inductive components and result
in a high energy spark going into the driver, which can destroy the power transistors. The same is true
when disconnecting a motor during operation: Never disconnect the motor during operation!
There is no absolute protection against random short circuit conditions, but pre-cautions can be taken
to improve robustness of the circuit:
In a short condition, the current can become very high before it is interrupted by the short detection,
due to the blanking during switching and internal delays. The high-side transistors allows up to 10A
flowing for the selected blank time. The lower the external inductivity, the faster the current climbs. If
inductive components are involved in the short, the same current will shoot through the low-side
resistor and cause a high negative voltage spike at the sense resistor. Both, the high current and the
voltage spikes are a danger for the driver.
Thus there are a two things to be done, if short circuits are expected:
1. Protect SRA/SRB inputs using a series resistance
2. Increase R
3. Use as short as possible blank time
The second measure effectively limits short circuit current, because the upper driver transistor with its
fixed ON gate voltage of 7V forms a constant current source together with its internal resistance and
R
power dissipation increases slightly. A high side short detection resistor of 0.33 Ohms limits maximum
high side transistor current to typically 4A. The schematic shows the modifications to be done.
However, the effectiveness of these measures should be tested in the given application.
Copyright © 2005, TRINAMIC Motion Control GmbH & Co KG
SH
VS
VT
GND
SRA
SRB
. A positive side effect is, that only one type of low ohmic resistor is required. The drawback is, that
R
Microstep:
Fullstep:
DIV
values for
100nF
R
SH
SH
100R
100R
R
internal
reference
27R
18R
=R
DIV
to limit maximum transistor current: Use same value as for sense resistors
100R
SA
=R
SB
INA/INB
up to3V
18R
12R
R
SA
R
SB
C
VM
+VM
GND
15