L6474 datasheet(27/51 Pages) STMICROELECTRONICS

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L6474

Phase current control

Doc ID 022529 Rev 2

27/51

7.2

Auto-adjusted decay mode

During the current control, the device automatically selects the best decay mode in order to

follow the current profile reducing the current ripple.

At reset, the OFF time is performed by turning on both the low side MOS of the power stage

and the current recirculates in the lower half of the bridge (slow decay).

If, during a PWM cycle, the target current threshold is reached in a time shorter than the

TON_MIN value, a fast decay of TOFF_FAST/8 (T_FAST register) is immediately

performed, turning on the opposite MOS of both half bridges and the current recirculates

back to the supply bus.

After this time, the bridge returns to the ON state: if the time needed to reach the target

current value is still less than TON_MIN, a new fast decay is performed with a period twice

the previous one. Otherwise, the normal control sequence is followed as described in

section 7.1. The maximum fast decay duration is set by TOFF_FAST value.

Figure 10.

Adaptive decay - fast decay tuning

When two or more fast decays are performed with present target current, the control system

adds a fast decay at the end of every OFF time, keeping the OFF state duration constant

(tOFF is split into tOFF,SLOW and tOFF,FAST). When the current threshold is increased by a

micro-step change (rising step), the system returns to normal decay mode (slow decay only)

and the tFAST value is halved.

Reaching the current sinewave zero crossing causes the current control system to return to

the reset state.

L6474 Datasheet(PDF) 27 Page - STMicroelectronics