Operation description

SRK2001A

12/20

DocID029495 Rev 1

7.3

Adaptive turn-off

The SR MOSFET turn-off may be triggered by two different mechanisms: by the adaptive

turn-off mechanism (two-slope turn-off) or by the ZCD_OFF comparator (fast turn-off, see

Section 7.4).

stray inductance), the sensed drain-source signal is not really equal to the voltage drop

zero, causing a premature MOSFET turn-off.

To overcome this problem (without adding any stray inductance compensation circuit), the

device uses a turn-off mechanism based on an adaptive algorithm. This consists in turning

off the SR MOSFET with a certain delay after the sensed drain-source voltage has reached

zero and adapting progressively this delay in order to maximize the conduction period and

Figure 8 shows this adaptive algorithm: cycle-by-cycle the conduction time is maximized

allowing in a steady-state the maximum converter efficiency.

Figure 8. Adaptive turn-off

After the turn-on, a blanking time (equal to 50% of the clock period - refer to Figure 6) masks

the adaptive turn-off mechanism in order to avoid an undesired turn-off due to the drain-

source voltage drop, consequent to the MOSFET switch-on (the flowing current passes from

the body diode to MOSFET channel resistance) or due to the ringing generated at the

MOSFET turn-on.

During the startup and on the low consumption mode exiting, the control circuit turns off the

SR MOSFET at 50% of the clock cycle and progressively adapts this delay in order to

maximize the SR MOSFET conduction time. This helps reducing system perturbations.