L9935

Functional description

Doc ID 5198 Rev 10

17/29

5.6

Short from an output to ground

The current through the short will be detected by the protection of the source transistor. The

source transistor will turn off exceeding a current of typically 1.8 A. Minimum overload

detection current is 1.2 A. To obtain proper current regulation (by the sink transistors and not

by source transistor shut down) the maximum current of the PWM regulator should be set to

a maximum value of 1.1 A.

5.7

Diagnosis of a short to ground

Detecting an overload will set an overcurrent error (Error2 = LOW) (bit6). To reset the error

flag a new byte must be written into the interface. (Reset of the error flag takes place at the

rising slope of CSN).

5.8

Shorted load

With a shorted load both, the sink- and the source protection or the PWM alone will

respond. In either case there will be no flyback pulse.

5.9

Diagnosis of a shorted load

Shorting the load two events may take place:

–

overload (of the high side transistor) while low side transistor overcurrent is

detected will set the following combinations:

bit6 = LOW

bit7 = HIGH

–

overload is marginal. So the low side driver may turn off before overload is

detected. This leads to the combination bit6 = HIGH and bit7 = LOW.

5.10

Open load

An open load will not lead to any flyback pulses. Error detection will take advantage of the

flyback pulse. Missing the flyback pulse after reversing the polarity of a motor winding bit7

will become LOW.

Open load will not be tested in the low current mode (current bits HL) to avoid the risk of

instable diagnosis at low flyback currents. Open load immediately after reset or power down

may on random be detected in the low current mode too. This diagnosis however will not

persist longer than 8 changes of polarity.

We strongly suggest to test open load at a high current mode (combination LL).

While circuit clock speed passes the stepper motor resonant points during

acceleration/deceleration phase, it can happen that flyback energy is temporarily insufficient

for a proper open load detection. Under specific circumstances, pending on motor and load

characteristics, this could lead to sporadic faulty open load error messages despite proper

system operation. The recommended solution is an appropriate software filter approach.

A detailed description is available in the application note AN2378 on www.st.com.