TMC246A Datasheet, PDF(15/24 Page) List of Unclassifed Manufacturers – High Current Microstep Stepper Motor Driver with sensorless stall detection, protection / diagnosis and SPI Interface

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TMC246A Datasheet, PDF (15/24 Pages) List of Unclassifed Manufacturers – High Current Microstep Stepper Motor Driver with sensorless stall detection, protection / diagnosis and SPI Interface

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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 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 7V 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 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.

100nF

GND

RDIV

100R

RSH=RSA=RSB

RDIV values for

Microstep:

Fullstep:

internal

reference

27R

18R

INA/INB

up to3V

18R

12R

+VM

CVM

SRA

SRB

100R

RSA

RSB

GND

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