Phase current control
L6472
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DocID022729 Rev 5
At the end of the predictive ON state the power stage is set in the OFF state for a fixed time,
as in a constant t
OFF
current control. During the OFF state both slow and fast decay can be
performed; the better decay combination is automatically selected by the L6472, as
described in
As shown in
, the system is able to center the triangular wave on the desired
reference value improving dramatically the accuracy of the current control system: in fact
the average value of a triangular wave is exactly equal to the middle point of each of its
segments and at steady-state the predictive current control tends to equalize the duration of
the t
SENSE
and the t
PRED
time.
Furthermore, the t
OFF
value is recalculated each time a new current value is requested
(microstep change) in order to keep the PWM frequency as near as possible to the
programmed one (TSW parameter in the CONFIG register).
The device can be forced to work using a classic peak current control setting the PRED_EN
bit in the CONFIG register low (default condition). In this case, after the sense phase
(t
SENSE
) the power stage is set in the OFF state, as shown in
Figure 15. Non-predictive current control
7.2
Auto-adjusted decay mode
During the current control, the device automatically selects the better decay mode in order
to follow the current profile reducing the current ripple.
At reset, the OFF time is performed by turning on both the low-side MOSFETs of the power
stage and the current recirculates in the lower half of the bridge (slow decay).
If, during a PWM cycle, the target current threshold is reached in a time shorter than the
TON_MIN value, a fast decay of TOFF_FAST/8 (T_FAST register) is immediately performed
turning on the opposite MOS of both half-bridges and the current recirculates back to the
supply bus.
After this time, the bridge returns to the ON state: if the time needed to reach the target
current value is still less than TON_MIN, a new fast decay is performed with a period twice
the previous one. Otherwise, the normal control sequence is followed as described in
. The maximum fast decay duration is set by the TOFF_FAST value.
