STMicroelectronics Stepper motor driver mounting the L6474 in high power PowerSO package EVAL6474PD EVAL6474PD Ficha De Dados

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Doc ID 022529 Rev 3

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.

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 
(t

OFF

 is split into t

OFF,SLOW

 and t

OFF,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 t

FAST

 value is halved.

Reaching the current sinewave zero crossing causes the current control system to return to 
the reset state.

REFERENCE



ST

4FAST



ND

4FAST

 .OTE

ND

FAST



RD

4FAST

4ON

4IME