TMC249 / TMC249A DATA SHEET (V2.09 / 2009-Jul-30)
21
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 allow a high current
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 three things to be done, if short circuits are expected:
1. Protect SRA/SRB inputs using a series resistance
2. Increase RSH to limit maximum transistor current: Use same value as for sense resistors
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 6V forms a constant current source together with its internal resistance and
RSH. A positive side effect is that only one type of low ohmic resistor is required. The drawback is that
power dissipation increases. 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.
VS
100nF
RDIV
VT
100R
+VM
GND
RSH=RSA=RSB
internal
INA/INB
RDIV values for
Microstep:
Fullstep:
reference up to3V
27R
18R
18R
12R
CVM
SRA
100R
SRB
RSA
RSB
100R
GND
Copyright © 2005, TRINAMIC Motion Control GmbH & Co KG
TRINAMIC [ TRINAMIC MOTION CONTROL GMBH & CO. KG. ]
