MP6922—DUAL FAST TURN-OFF INTELLIGENT CONTROLLER
MP6922 Rev. 1.25 www.MonolithicPower.com 9
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stops switching. The part will return to normal
operation after the junction temperature has
dropped to 120
o
C.
Turn-on Phase
When the switch current flows through the body
diode of the MOSFET, there will be a negative
V
DS
(V
D
-V
SS
) across it (<-500mV), the V
DS
is
much lower than the turn on threshold of the
control circuitry (-70mV), which then turns on the
MOSFET after 200ns turn-on delay (defined in
Fig.2).
As soon as the turn on threshold (-70mV) is
triggered, a blanking time (Minimum on-time:
~1us) will be added during which the turn off
threshold will be changed from 0mV to +50mV.
This blanking time can help to avoid error trigger
on turn off threshold caused by the turn on
ringing of the synchronous power switch.
+30mV
-70mV
2V
V
DS
V
GS
Flag
Turn On
Delay
Turn Off
Delay
Turn On
Blanking
Driver begin to
be pulled down
~1us
Driver turn off
Off threshold
changes to
+100mV during
blanking time
+100mV
Figure 2—Turn on and Turn off delay
Conducting Phase
When the MOSFET is turned on, V
DS
(-I
SD
X
r
DS(ON)
) becomes to rise according to the drop of
the switch current (I
SD
) , as soon as V
DS
rises
above the turn on threshold (-70mV), the control
circuitry stops pulling up the gate driver and the
driver voltage of the MOSFET dropped, which
makes the on resistance r
DS(ON)
of the MOSFET
becomes larger. By doing that, V
DS
(-I
SD
x r
DS(ON)
)
is adjusted to be around -70mV even when the
switch current I
SD
is fairly small, this function can
make the turn off threshold (0mV) of the internal
driver never triggered until the current through
the MOSFET has dropped to near zero.
Turn-off Phase
When V
DS
rises to trigger the turn off threshold
(30mV), the driver voltage of the switch is pulled
to
low after about 20ns turn off delay (defined in
Fig.4) by the control circuitry. Similar with turn-on
phase, a 1.6us blanking time is added after the
switch is turned off, during which the MOSFET is
never turned on to avoid error trigger.
Fig.3 show the MP6922 operation at heavy load
condition. Due to the high current, the driver
voltage will be saturated at first. After V
DS
goes to
above -70mV, driver voltage decreases to adjust
the V
DS
to typical -70mV.
Fig.4 show the MP6922 operation at light load
condition. Due to the low current, the driver
voltage never saturates but begins to decrease
as soon as the synchronous power switch is
turned on and adjust the V
DS
.
Figure 3—Synchronous Rectification
Operation at heavy load
Figure 4—Synchronous Rectification
Operation at light load
