AN4158 datasheet(22/34 Pages) STMICROELECTRONICS

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STMicroelectronics enhanced current control

AN4158

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Figure 16. Minimum on-time threshold and on-time limit relation

At every control cycle the on-time is measured and it is compared to the programmable

threshold tON_MIN. If the measured on-time is longer than the programmed threshold the

current is well controlled and no fast decay is needed, so slow decay is used (Figure 17).

As the slow decay reduces its effectiveness, the on-time is reduced until the programmed

tON_MIN is reached. When tON is shorter than tON_MIN, a fast decay is used in order to

increase the current drop (Figure 17) and to avoid the failure of the current control algorithm

due to the minimum on-time limit (see Section 3.2 on page 17). In the first occurrence of this

event the fast decay duration is 1/8th of the maximum programmed value (tOFF_FAST).

If the next on-time is longer than the tON_MIN threshold, the control system returns using the

slow decay, as shown in Figure 17.

Figure 17. Decay selection according to on-time duration

When two or more minimum on-time violations occur at the same reference current value

(i.e. the electrical position of the motor is not changed), the control system changes the

decay strategy using slow decay followed by a fast decay (Figure 18). The total duration of

the decay phase is equal to the programmed off-time (tOFF) and the fast part is equal to the

last tFAST used.

comm

on,min

on-time limit

of the control system

Target minimum on-time

Reference current

AM15118v1

ON_MIN

on < tON_MIN

on > tON_MIN

OFF

fast

On-time is over the

target minimum value

A slow decay is used

On-time is overthe

target minimum value

A slow decay is used

On-time is below the

target minimum value

A fast decay is used

Reference current

AM15119v1

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