L6474 datasheet(28/51 Pages) STMICROELECTRONICS

28 / 51 page

Phase current control

L6474

28/51

Doc ID 022529 Rev 2

7.3

Auto-adjusted fast decay during the falling steps

When the target current is decreased by a micro-step change (falling step), the device

performs a fast decay in order to reach the new value as fast as possible. Anyway,

exceeding the fast duration could cause a strong ripple on the step change. The L6474

automatically adjusts these fast decays reducing the current ripple.

At reset, the fast decay value (tFALL) is set to FALL_STEP/4 (T_FAST register). The tFALL

value is doubled every time, within the same falling step, an extra fast decay is necessary to

obtain an ON time greater than TON_MIN. The maximum tFALL value is equal to

FALL_STEP.

At the next falling step, the system uses the last tFALL value of the previous falling step.

Stopping the motor or reaching the current sinewave zero crossing causes the current

control system to return to the reset state.

Figure 11.

Adaptive decay - switch from normal to slow+fast decay mode and vice

versa

Time

reference current

1st fast decay

Target current is increased (raising step)

nd fast decay

system returns to slow decay mode and t

FAST value is halved

switch to fast + slow decay mode

FAST

OFF,FAST

OFF,SLOW

OFF

reference current

L6474 Datasheet(PDF) 28 Page - STMicroelectronics