AN4158 datasheet(23/34 Pages) STMICROELECTRONICS

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AN4158

STMicroelectronics enhanced current control

Figure 18. Decay strategy change due to second on-time threshold violation

The tFAST value is regulated according to the following rules:



At the first occurrence of the minimum on-time violation, the fast decay duration is set

to tOFF_FAST /8.



If a further minimum on-time violation occurs, a new fast decay is used with double the

length of the previous one (tFAST = 2 x tFAST).



The maximum tFAST value is set by the tOFF_FAST parameter.

At the next minimum on-time violation the sequence starts from the last tFAST value used

(except if a current reference change occurred, see Section 4.1.1).

The tFAST value obtained using this sequence is used by the control system during the

decay phase when the slow and fast decay combination is used. Figure 19 shows an

example of the system operation.

Figure 19. Fast decay time regulation

4.1.1

Rising and falling steps (reference current changes)

In microstepping operation the current reference changes according to the point of the

current sinewave which must be obtained.

When the reference value is changed, there are two cases:

The reference current is increased. This event is called “rising step”.

The reference current is reduced. This event is called “falling step”.

These events are managed by the L6472 and L6474 current control, as follows.

In the case of rising step the phase is immediately turned on regardless of the control cycle

conditions and the system switches to slow decay strategy. If the tFAST value has been

increased during the previous microstep (see Section 4.1), it is halved (tFAST = tFAST / 2).

ON_MIN

on < tON_MIN

on > tON_MIN

fast

on > tON_MIN

The second violation causes the control system

to change the decay strategy.

A slow decay followed by a fast decay are used

from now, until the change of the reference current.

First minimum on-time violation.

Fast decay is used.

ON_MIN

OFF

fast

System returns to normal

operation (slow decay)

on < t ON_MIN

Second minimum

on-time violation.

Fast decay is used.

Reference current

AM15120v1

ON_MIN

on < tON_MIN

fast

on > tON_MIN

slow + fast decay strategy

with t

fast = 1/4 x tOFF_FAST

1st fast decay

fast = 1/8 x t OFF_FAST

2nd fast decay

fast = 1/4 x t OFF_FAST

Reference current

AM15121v1

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