DocID14109 Rev 3

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AN2650

Shorted coil detection

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Shorted coil detection

During free-wheeling time, the L9942 can use several decay modes that are programmable

by the SPI.

However, only on Auto decay, fast decay without delay time it is possible to detect

shorted coil because, during free-wheeling time, both opposite transistors (HS and LS) are

switched on and high current can rise. During free wheeling the current in the output stage is

monitored, but not regulated, with PWM. Because of this, the short overcomes the over

current filter time and can be detected.

In other decay modes, the decay can periodically change during wheeling of the motor (for

example, every 200 ms) to Auto decay, fast decay without delay time for few microseconds

and than change it back to previous value. This can be done in one clock step.

The decay mode cannot be changed at all values of the phase counter because fast decay

is not active during the whole period but only during decreasing current phase in the coil

(during the rising current phase, the coil is active slow decay mode).

A detailed explanation of Auto decay, fast decay without delay time for all step modes is

given in Figure 3.