LM9061N National Semiconductor, LM9061N Datasheet - Page 7

• Manufacturer Part Number: LM9061N
• Description: Power MOSFET Driver with Lossless Protection
• Manufacturer: National Semiconductor

Application Hints
TURN ON AND TURN OFF CHARACTERISTICS
The actual rate of change of the voltage applied to the gate
of the power device is directly dependent on the input ca-
pacitances of the MOSFET used These times are important
to know if the power to the load is to be applied repetitively
as is the case with pulse width modulation drive Of concern
are the capacitances from gate to drain C
to source C
intervals in a typical application An inductive load is as-
sumed to illustrate the output transient voltage to be expect-
ed At time t1 the ON OFF input goes high The output
which drives the gate of the MOSFET immediately pulls the
gate voltage towards the V
source current from pin 4 is typically 30 mA which quickly
charges C
V
the source voltage starts rising towards V
equal to the threshold voltage until the source reaches V
While V
source voltage reaches V
takes over the drive of the gate to ensure that the MOSFET
remains ON
The charge pump is basically a small internal capacitor that
acquires and transfers charge to the output pin The clock
rate is set internally at typically 300 kHz In effect the charge
pump acts as a switched capacitor resistor (approximately
67k) connected to a voltage that is clamped at 13V above
the Sense input pin of the LM9061 which is equal to the V
supply in typical applications The gate voltage rises above
V
dent upon the sum of C
the load is fully energized At time t3 the charge pump
reaches its maximum potential and the switch remains ON
At time t4 the ON OFF input goes low to turn OFF the
MOSFET and remove power from the load At this time the
charge pump is disconnected and an internal 110
rent sink begins to discharge the gate input capacitances to
ground The discharge rate ( V
(C
GS(ON)
CC
GD
in an exponential fashion with a time constant depen-
a
GS
threshold of the MOSFET the switch turns ON and
C
GD
GS
is constant only C
GS
)
and C
Figure 2 details the turn ON and turn OFF
GS
GD
As soon as the gate reaches the
CC
CC
and C
(Continued)
at time t2 the charge pump
supply of the LM9061 The
GD
GS
T) is equal to 110
is charging When the
At this time however
FIGURE 2 Turn ON and Turn OFF Waveforms
GD
CC
and from gate
V
GS
remains
A cur-
CC
CC
A
7
The load is still fully energized until time t5 when the gate
voltage has reached a potential of the source voltage (V
plus the V
tween time t5 and t6 the V
the source voltage follows the gate voltage With the volt-
age on C
110 A C
At time t6 the source voltage reaches 0V As the gate
moves below the V
turn OFF With an inductive load if the current in the load
has not collapsed to zero by time t6 the action of the
MOSFET turning OFF will create a negative voltage tran-
sient (flyback) across the load The negative transient will
be clamped to
itself back ON to continue conducting the load current until
the energy in the inductance has been dissipated (at time
t7)
MOSFET PROTECTION CIRCUITRY
A unique feature of the LM9061 is the ability to sense ex-
cessive power dissipation in the MOSFET and latch it OFF
to prevent permanent failure Instead of sensing the actual
current flowing through the MOSFET to the load which typi-
cally requires a small valued power resistor in series with
the load the LM9061 monitors the voltage drop from drain
to source V
nique allows all of the energy available from the supply to be
conducted to the load as required The only power loss is
that of the MOSFET itself and proper selection of a particu-
lar power device for an application will minimize this con-
cern Another benefit of this technique is that all applica-
tions use only standard inexpensive
To utilize this lossless protection technique requires knowl-
edge of key characteristics of the power MOSFET used In
any application the emphasis for protection can be placed
on either the power MOSFET or on the amount of current
delivered to the load with the assumption that the selected
MOSFET can safely handle the maximum load current
GD
GD
GS(ON)
DS
held constant the discharge rate now becomes
b
across the MOSFET This ‘‘lossless’’ tech-
V
threshold voltage of the MOSFET Be-
GS(ON)
GS(ON)
GS
because the MOSFET must turn
threshold the MOSFET tries to
voltage remains constant and
TL H 12317 – 9
W or less resistors
CC
)