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STMicroelectronics enhanced current control

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STMicroelectronics enhanced current control

The new L6472 and L6474 devices implement a new advanced current control algorithm

that increases the system performance in both full step and microstepping driving.

As discussed in previous paragraphs, the decay mode of a peak current control algorithm is

a critical choice concerning performance and stability of the system.

In particular, the control system must choose between slow and fast decay considering the

following trade-off:



Slow decay allows lower current ripples (more accurate average current, lower power

dissipation, lower noise), but it may fail to reduce the current bringing the system into

a minimum on-time limit condition (see Section 3.2 on page 17).



Fast decay increases the robustness to the minimum on-time limit, but it implies

a higher current ripple (higher average current error, higher power dissipation).

The best solution is a compromise between the two methods, where the fast decay is used

only when needed and for the shortest time possible. The auto adjusted decay system

addresses this balance.

4.1

Peak current control with auto adjusted decay

The new current control algorithm is based on the parameters listed in Table 5.

The first three parameters are used when the system keeps the phase current constant; the

fourth parameter is used when the reference current is decreased only (see Section 4.1.2).

Table 5. Peak current control parameters

Symbol

Name

Description

Off-time

It sets the current decay duration of the control cycle (fixed

off-time current control).

Target minimum on-time

It defines which is the shortest on-time (i.e. the time to reach

reference current) which guarantees a correct operation of

the control algorithm (Figure 16). This parameter should be

set to a value higher than the intrinsic minimum on-time of the

system.

Maximum fast decay

time

It is the maximum duration of a fast decay during operation of

the current control system.

It is the maximum duration of a fast decay during reference

current changing.