
L9942
Appendix
Doc ID 11778 Rev 6
7
Appendix
7.1
Stall detection
The L9942 contains logic blocks designed to detect a motor stall caused by excessive
mechanical load.During a motor stall condition the load current rises much faster than
during normal operation. The L9942 measures this time and compares it to a programmed
value.
This is done by summing the PWM on times for one full quadrant. For a full wave stepping
this is just one value (step 0). For microstepping this includes 8 separate values added
together, one for each step. This measurement is only done on phase A during the
quadrants where the current is increasing naturally (quadrants 1 and 3 of
Figure 15); e.g.
stall detection is active during phase counter values 1 to 8 and 17 to 24 for DIR=0. During
the quadrants where the current is decreasing fast decay recirculation interferes with
accurate measurement of this time. If the sum of the PWM on time is less than a
programmed threshold stored in D0-D7, stall is detected and indicated as a logic “1” in the
stall (ST) bit found in register 6 bit 8 (
Figure 15 bottom). If bit 11 of register 6 is set to logical
“1” then the ST bit is mirrored to the PWM pin providing detection externally.The register
values DT7-DT0 store the threshold value in 16us intervals. These bits can be found
interstitially in register 3 (D0, D1), register4 (D2, D3, D4) and register5 (D5, D6, D7).
Care should be taken when deciding the threshold timing. Motor current slew rates are
dependant on the driving voltage, the actual speed of the motor, the back EMF of the motor
as well as the motor and the inductance. Be sure to set your threshold well away from what
can be seen in normal operation at any temperature.
7.2
Step clock input
The Step clock input allows to run one device in micro-step mode, or several devices
simultaneously with cost effective 8 bit Controller. In case of the L9942, the SPI
communication link provides only the settings for motor operation mode. Motor commutation
as high duty process is outsourced to a parallel driven pin. Without this step clock input, the
SPI command would also have to clock the motor, leading to a high SPI speed. For full
micro-step operation or simultaneous motor drive, an 8 bit Controller could be rapidly
overloaded.
7.3
Load current control and detection of overcurrent (shortages
at outputs)
The L9942 controls load current in the two full bridges by using a pulls with modulation
(PWM) regulator. The mirrored output current of active HS switch is compared with a
programmed reference current (e.g. in figure A2 HSA1 and HSB2). Bridge is switched off if
current has exceeded the programmed limit value. A second comparator of the related LS
switch uses the mirrored load current to detect an overcurrent to ground during ON state of
bridges (e.g. in
Figure 16 LSA2 and LSB1). The event of shortage from output to supply
voltage VS is detectable, but short current between outputs is limited through PWM
controller and so an overcurrent failure will not occur.
Load currents decrease more or less fast during OFF state of bridges depending on
selected decay mode. Slow decay mode is released by activating the HS switches of the