MP6902- FAST TURN-OFF INTELLIGENT CONTROLLER
MP6902 Rev. 1.14 www.MonolithicPower.com 7
6/23/2014 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2014 MPS. All Rights Reserved.
OPERATION
The MP6902 supports operation in DCM and
Quasi-Resonant Flyback converters. The control
circuitry controls the gate in forward mode and
will turn the gate off when the MOSFET current is
fairly low.
Blanking
The control circuitry contains a blanking function.
When it pulls the MOSFET on/off, it makes sure
that the on/off state at least lasts for some time.
The turn on blanking time is ~1.6us, which
determines the minimum on-time. During the turn
on blanking period, the turn off threshold is not
totally blanked, but changes the threshold
voltage to ~+50mV (instead of -30mV). This
assures that the part can always be turned off
even during the turn on blanking period. (Albeit
slower)
VD Clamp
Because V
D
can go as high as 180V, a High-
Voltage JFET is used at the input. To avoid
excessive currents when Vg goes below -0.7V, a
small resistor is recommended between V
D
and
the drain of the external MOSFET.
Under-Voltage Lockout (UVLO)
When the VDD is below UVLO threshold, the part
is in sleep mode and the Vg pin is pulled low by a
10kΩ resistor.
Enable pin
If EN is pulled low, the part is in sleep mode.
Thermal shutdown
If the junction temperature of the chip exceeds
170
o
C, the Vg will be pulled low and the part
stops switching. The part will return to normal
function after the junction temperature has
dropped to 120
o
C.
Thermal Design
If the dissipation of the chip is higher than
100mW due to switching frequencies above
100kHz.
Turn-on Phase
When the synchronous MOSFET is conducting,
current will flow through its body diode which
generates a negative Vds across it. Because this
body diode voltage drop (<-500mV) is much
smaller than the turn on threshold of the control
circuitry (-70mV), which will then pull the gate
driver voltage high to turn on the synchronous
MOSFET after about 150ns turn on delay
(Defined in Figure 2).
As soon as the turn on threshold (-70mV) is
triggered, a blanking time (Minimum on-time:
~1.6us) will be added during which the turn off
threshold will be changed from -30mV to +50mV.
This blanking time can help to avoid error trigger
on turn off threshold caused by the turn on
ringing of the synchronous MOSFET.
V
DS
V
GATE
t
Don
t
Doff
-70mV
-30mV
2V
Total
t
5V
Figure 2—Turn on and Turn off delay
Conducting Phase
When the synchronous MOSFET is turned on,
Vds becomes to rise according to its on
resistance, as soon as Vds rises above the turn
on threshold (-70mV), the control circuitry stops
pulling up the gate driver which leads the gate
voltage is pulled down by the internal pull-down
resistance (10kΩ) to larger the on resistance of
synchronous MOSFET to ease the rise of Vds.
By doing that, Vds is adjusted to be around -
70mV even when the current through the MOS is
fairly small, this function can make the driver
voltage fairly low when the synchronous
MOSFET is turned off to fast the turn off speed
(this function is still active during turn on blanking
time which means the gate driver could still be
turned off even with very small duty of the
synchronous MOSFET).
