AN4158 datasheet(24/34 Pages) STMICROELECTRONICS

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STMicroelectronics enhanced current control

AN4158

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DocID023635 Rev 2

Figure 20 shows the system behavior in the case of a rising step.

Figure 20. Current control at rising step (reference current increased)

In the case of falling step a sequence of fast decays is immediately used regardless of the

control cycle conditions. The decay strategy and the respective tFAST value are unchanged.

The system behavior in the case of a falling step is described in detail in Section 4.1.2.

When the reference current is set to zero (zero crossing of the current sinewave) or when

the motor is stopped, the control system is reset to the default: slow decay only and tFAST

equal to 1/8th of tOFF_FAST.

4.1.2

Auto adjusted duration of fast decay at falling steps

When the reference current is reduced by a microstep change (falling step), the control

system immediately uses a fast decay in order to reach the new target value as soon as

possible. The fast decay duration is adjusted using an algorithm similar to the one used

during normal operation (see Section 4.1).

At first occurrence of this event, the fast decay length is set to a quarter of the programmed

maximum step fall time (tFAST_STEP). At the end of the decay, the phase is turned on and the

on-time is measured; if its duration is below the programmed minimum threshold (tON_MIN)

a further fast decay is performed with double the length of the previous. The maximum fast

decay duration is imposed by the tFAST_STEP parameter.

The fast decay sequence continues until the on-time exceeds the programmed minimum

limit (see Figure 21).

At the next falling step occurrence the last fast decay value is used. When the reference

current is set to zero (zero crossing of the current sinewave) or the motor is stopped, the

fast decay time is reset to the starting value of tFAST_STEP /4.

Note:

The fast decay time used during falling step sequence is totally independent from the one

used during the current control.

ON_MIN

The rising step causes the system

to switch to slow decay only strategy.

The t

fast value is halved.

(lower limit is tOFF_FAST /8)

OFF

fast

OFF

on < tON_MIN

the fast decay duration is half of

the one used during the control of

the previous current reference level.

Rising step

Reference current

At first minimum on-time violation

AM15122v1

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