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).
Due to the stray inductance in series with the SR MOSFET R
DS(on)
(mainly the package
stray inductance), the sensed drain-source signal is not really equal to the voltage drop
across the MOSFET R
DS(on)
, but it anticipates the time instant where the current reaches
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
get the target residual conduction (T
diode
) of the MOSFET body diode after the turn-off.
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.
