lm9061mx National Semiconductor Corporation, lm9061mx Datasheet - Page 10

• Manufacturer Part Number: lm9061mx
• Description: Power Mosfet Driver With Lossless Protection
• Manufacturer: National Semiconductor Corporation

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Application Information
BASIC OPERATION
The LM9061 contains a charge pump circuit that generates a
voltage in excess of the applied supply voltage to provide
gate drive voltage to power MOSFET transistors. Any size of
N-channel power MOSFET, including multiple parallel con-
nected MOSFETs for very high current applications, can be
used to apply power to a ground referenced load circuit in
what is referred to as “high side drive” applications. Figure 1
shows the basic application of the LM9061.
When commanded ON by a logic “1” input to pin 7 the gate
drive output, pin 4, rises quickly to the V
at pin 5. Once the gate voltage exceeds the gate-source
threshold voltage of the MOSFET, V
connected to ground through the load) the MOSFET turns
ON and connects the supply voltage to the load. With the
source at near the supply potential, the charge pump con-
tinues to provide a gate voltage greater than the supply to
keep the MOSFET turned ON. To protect the gate of the
MOSFET, the output voltage of the LM9061 is clamped to
limit the maximum V
It is important to remember that during the Turn-ON of the
MOSFET the output current to the Gate is drawn from the
V
capacitor with a value of at least ten times the Gate capaci-
tance, and no less than 0.1 µF. The output current into the
Gate will typically be 30 mA with V
0V. As the Gate voltage rises to V
decrease. When the Gate voltage reaches V
current will typically be 1 mA with V
A logic “0” on pin 7 turns the MOSFET OFF. When com-
manded OFF a 110 µA current sink is connected to the
output pin. This current discharges the gate capacitances of
the MOSFET linearly. When the gate voltage equals the
source voltage (which is near the supply voltage) plus the
CC
supply pin. The V
FIGURE 1. Basic Application Circuit
GS
CC
to 15V.
pin should be bypassed with a
CC
CC
CC
, the output current will
at 14V and the Gate at
GS(ON)
at 14V.
CC
supply potential
, (the source is
CC
, the output
01231708
10
V
following the gate voltage and ramps toward ground. Even-
tually the source voltage equals 0V and the gate continues to
ramp to zero thus turning OFF the power device. This
gradual Turn-OFF characteristic, instead of an abrupt re-
moval of the gate drive, can, in some applications, minimize
the power dissipation in the MOSFET or reduce the duration
of negative transients, as is the case when driving inductive
loads. In the event of an overstress condition on the power
device, the turn OFF characteristic is even more gradual as
the output sinking current is only 10 µA (see Protection
Circuitry Section).
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 ex-
pected. 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
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 µA current
sink begins to discharge the gate input capacitances to
ground. The discharge rate (∆V/∆T) is equal to 110 µA/ (C
+ C
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
time t5 and t6, the V
source voltage follows the gate voltage. With the voltage on
C
110 µA/C
At time t6 the source voltage reaches 0V. As the gate moves
below the V
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 transient (flyback)
across the load. The negative transient will be clamped to
GS(ON)
GS(ON)
CC
GD
GS
in an exponential fashion with a time constant depen-
held constant the discharge rate now becomes
).
threshold of the MOSFET, the source voltage starts
GS
threshold of the MOSFET, the switch turns ON and
GD
GS(ON)
GD
GS(ON)
.
is constant only C
GS
and C
. Figure 2 details the turn ON and turn OFF
threshold voltage of the MOSFET. Between
threshold the MOSFET tries to turn OFF.
GS
GS
GD
. As soon as the gate reaches the
voltage remains constant and the
and C
CC
CC
, at time t2, the charge pump
supply of the LM9061. The
GD
GS
. At this time however the
is charging. When the
GD
CC
, and from gate
. V
GS
remains
CC
CC
CC
GD
)
.