AN4158 datasheet(25/34 Pages) STMICROELECTRONICS

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AN4158

STMicroelectronics enhanced current control

Figure 21. Current control at falling step (reference current decreased)

4.2

Predictive current control(a)

The peak current control with fixed off-time has two cons:



The average current value is not controlled, so the actual rotor position is affected by

some errors (see Section 3.3 on page 19).



The PWM frequency is not fixed but it is determined by the on-time which varies

according to several conditions (current value, BEMF, etc.).

The new predictive current control provided by the L6472, which is based on the peak

current control described in Section 4.1 on page 21, allows these limits to be reduced and

removed in some cases.

The control system operation can be configured through the parameters listed in Table 6.

The tON_MIN, tOFF_FAST and tFAST_STEP parameters act as described in Section 4.1, but tOFF

listed in Table 5 on page 21 is replaced by the tSW and tOFF_MIN combination as described in

Section 4.2.2.

ON_MIN

OFF

fast

OFF

fast

fast,fall

Causes the falling step

a fast decay of

fast,fall

= t

FAST_STEP /4

is used.

on > t ON_MIN

on < tON_MIN

On-time is below the target

minimum value.

A further fast decay is used

with duration of t

fast,fall = 2 x tfast,fall

The maximum value is t

FAST_STEP

On-time is over the target

minimum value.

System returns to normal operation

using the previous decay stategy

(slow only or slow + fast).

Falling step

Reference current

AM15123v1

a. Not available for the L6474.

Table 6. Predictive current control parameters

Symbol

Name

Description

tSW

Target switching time

It sets the target PWM frequency of the control system.

tON_MIN

Target minimum on-time

It defines which is the shortest on-time (i.e.: the time to reach

reference current) which guarantees a correct operation of

the control algorithm (Figure 16).

tOFF_MIN

Minimum off-time

It defines which is the shortest off-time (i.e.: the current decay

duration) which guarantees a correct operation of the control

algorithm.

tOFF_FAST

Maximum fast decay

time

It is the maximum duration of a fast decay during operation of

the current control system.

tFAST_STEP Maximum step fall time

It is the maximum duration of a fast decay during reference

current changing.

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