NCP43080 Datasheet, PDF (15/36 Pages) ON Semiconductor – Synchronous Rectifier Controller

NCP43080
Figure 37. Current Sensing Circuitry Functionality
The SR MOSFET is turned-off as soon as the voltage on
the CS pin is higher than V_{TH_CS_OFF} (typically −0.5 mV
minus any voltage dropped on the optional R_{SHIFT_CS}). For
the same ringing reason, a minimum off-time timer is
asserted once the V_CS goes above V_{TH_CS_RESET}. The
minimum off-time can be externally adjusted using
R_{MIN_TOFF} resistor. The minimum off−time generator can
be re−triggered by MIN_TOFF reset comparator if some
spurious ringing occurs on the CS input after SR MOSFET
turn−off event. This feature significantly simplifies SR
system implementation in flyback converters.
In an LLC converter the SR MOSFET M1 channel
conducts while secondary side current is decreasing (refer to
Figure 38). Therefore the turn−off current depends on
MOSFET R_DSON. The −0.5 mV threshold provides an
optimum switching period usage while keeping enough time
margin for the gate turn-off. The R_{SHIFT_CS} resistor
provides the designer with the possibility to modify
(increase) the actual turn−on and turn−off secondary current
thresholds. To ensure proper switching, the min_t_OFF timer
is reset, when the V_DS of the MOSFET rings and falls down
past the V_{TH_CS_RESET}. The minimum off−time needs to
expire before another drive pulse can be initiated. Minimum
off−time timer is started again when V_DS rises above
V_{TH_CS_RESET}.
www.onsemi.com
15