Datasheet

LM9061
www.ti.com SNOS738F –APRIL 1995–REVISED APRIL 1995
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 capacitances 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
GD
, and from gate to source, C
GS
. Figure 16 details the turn ON and turn OFF intervals in a typical
application. An inductive load is assumed to illustrate the output transient voltage to be expected. 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
CC
supply of the LM9061. The source current from pin 4 is typically 30 mA which quickly charges
C
GD
and C
GS
. As soon as the gate reaches the V
GS(ON)
threshold of the MOSFET, the switch turns ON and the
source voltage starts rising towards V
CC
. V
GS
remains equal to the threshold voltage until the source reaches
V
CC
. While V
GS
is constant only C
GD
is charging. When the source voltage reaches V
CC
, at time t2, the charge
pump 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
CC
supply in typical applications. The gate voltage rises above V
CC
in an exponential
fashion with a time constant dependent upon the sum of C
GD
and C
GS
. At this time however 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 µA current sink begins to discharge the gate input
capacitances to ground. The discharge rate (ΔV/ΔT) is equal to 110 µA/ (C
GD
+ C
GS
).
The load is still fully energized until time t5 when the gate voltage has reached a potential of the source voltage
(V
CC
) plus the V
GS(ON)
threshold voltage of the MOSFET. Between time t5 and t6, the V
GS
voltage remains
constant and the source voltage follows the gate voltage. With the voltage on C
GD
held constant the discharge
rate now becomes 110 µA/C
GD
.
At time t6 the source voltage reaches 0V. As the gate moves below the V
GS(ON)
threshold the MOSFET tries to
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 transient (flyback) across the load. The negative transient
will be clamped to −V
GS(ON)
because the MOSFET must turn 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 excessive 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 typically requires a small valued power resistor in series with the load, the LM9061 monitors the voltage
drop from drain to source, V
DS
, across the MOSFET. This “lossless” technique 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 particular power device for an application will minimize this concern. Another benefit of this
technique is that all applications use only standard inexpensive ¼W or less resistors.
To utilize this lossless protection technique requires knowledge 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.
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