DocID023635 Rev 2

17/34

AN4158

Peak current control with PWM and constant off-time

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Both decay methods have pros and cons: the slow decay allows a lower current ripple to be

obtained, but at low current values or at high speed (significant BEMF value) the current

control may fail. The fast decay, on the other hand, keeps the current under control at the

cost of a higher ripple.

3.2

Minimum on-time limit

The control system, according to its structure, is usually not able to check the phase current

immediately after the beginning of the on-phase (usually a blanking time is adopted to avoid

spurious triggering of the current comparator), as shown in Figure 12. This limits the

minimum on-time the power bridge must remain in the on-phase. The minimum current

has not been taken into account for simplicity).

Figure 11. Constant off-time control using fast decay

V

BUS

V

Sense

I

Ph

R

SHUNT

R

SHUNT

V

BUS

V

Sense

I

Ph

R

SHUNT

V

BUS

V

Sense

I

Ph

V

Ref

I

Peak

t

OFF

AM15113v1