AN4158 datasheet(26/34 Pages) STMICROELECTRONICS

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STMicroelectronics enhanced current control

AN4158

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DocID023635 Rev 2

4.2.1

Predictive on-time and average current regulation

The classic peak current control limits the maximum current value (peak) but cannot control

the value of the average current.

The predictive current control adds an extra on-time (tpred) after reaching the reference

current which is the two-step average of the last on-time values, as described in Equation 9

(see Figure 22).

Equation 9

Figure 22. Predictive current control basic principle

When the system reaches the stability, ton and tpred are equal and the resulting average

current value is equal to the reference threshold.

In order to avoid instability, all those conditions causing the on-time to be excessively long or

short must be carefully managed. The following cases should be considered:



The motor has been stopped



The bridges have just been enabled (exit from high impedance status)



The current threshold has been changed (both falling and rising steps)



The on-time is below the minimum on-time limit (tON_MIN)



The on-time is over the target switching time (tSW).

In these cases the measured on-time is excluded from the average calculation and the

previous tpred is used. If the next on-time does not fall under the cases listed above, its value

is used in the tpred calculation.

4.2.2

Off-time regulation and target switching frequency

In the control system described in Section 4.1 on page 21, the decay phase of the current

(namely, off-time) is constant and it is set through the tOFF parameter. Because of this, the

PWM frequency of the control system may vary significantly depending on situations

(current value, BEMF, etc.).

The predictive current control automatically adjusts the off-time duration in order to obtain

a PWM frequency near to the target value. This function is configured through the tSW and

tOFF_MIN parameters.

At every current reference change the off-time of the control system is recalculated

according to Equation 10, where tpred(n-1) is the last predictive on-time used. If the new

value is lower than the tOFF_MIN parameter, tOFF_MIN is used instead.

tpred n



ton n

 t

on n1

–



-----------------------------------------------

Average current

on(1)

on(2)

on(n)

pred(1)

pred(2)

pred(n)

Reference current

First system turns on

pred(0) and t on(0) are zero.

pred(1) =

on(1) + ton(0)

pred(2) =

on(2) + ton(1)

pred(n) =

= t

on(n)

on(n) + ton(n-1)

AM15124v1

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