Phase current control: current mode
powerSTEP01
described in Section 9.2.
As shown in Figure 19, 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
segment and at steady-state the predictive current control tends to equalize the duration of
the tSENSE and the tPRED time.
Furthermore, the tOFF 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 classic peak current control setting low the
PRED_EN bit in the CONFIG register (default condition). In this case, after the sense phase
(tSENSE) the power stage is set in OFF state, as shown in Figure 20.
Figure 20. Non-predictive current control
I
out
sense ON
state
OFF
state
I
ref
t
OFF
t
OFF
AM15049v1
9.2
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 turning on both the low-side MOS 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 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
Section 9.1. The maximum fast decay duration is set by the TOFF_FAST value.
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