AN4158 Datasheet, PDF (17/34 Pages) STMicroelectronics – Peak current control with automatic decay adjustment

AN4158
Peak current control with PWM and constant off-time
Figure 11. Constant off-time control using fast decay
V_BUS
V_BUS
V_BUS
I_Ph
I_Ph
I_Ph
V_Sense
R_SHUNT
V_Sense
R_SHUNT
V_Sense
R_SHUNT
I_Peak
V_Ref
t _OFF
AM15113v1
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
increase related to the t_comm can be estimated through Equation 8 (back electromotive force
has not been taken into account for simplicity).
DocID023635 Rev 2
17/34
34