lm9061mx National Semiconductor Corporation, lm9061mx Datasheet - Page 12

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

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Application Information
ing and setting a maximum operational value for the drain to
source voltage drop, V
load is inadvertently shorted to ground, the power device will
automatically be turned-OFF.
In all cases, should the MOSFET be switched OFF by the
built in protection comparator, the output sink current is
switched to only 10 µA to gradually turn OFF the power
device.
Figure 3 illustrates how the threshold voltage for the internal
protection comparator is established.
Two resistors connect the drain and source of the MOSFET
to the LM9061. The Sense input, pin 1, monitors the source
voltage while the Threshold input, pin 2, is connected to the
drain, which is also connected to the constant load power
supply. Both of these inputs are the two inputs to the protec-
tion comparator. Should the voltage at the sense input ever
drop below the voltage at the threshold input, the protection
comparator output goes high and initiates an automatic
latch-OFF function to protect the power device. Therefore
the switching threshold voltage of the comparator directly
controls the maximum V
while conducting load current.
The threshold voltage is set by the voltage drop across
resistor R
tor to ground at I
ence current over temperature, a stable band gap reference
voltage is provided to bias a constant current sink. The
reference current is set by:
The reference current sink output is internally connected to
the threshold pin. I
through R
THRESHOLD
THRESHOLD
REF
REF
. A reference current is fixed by a resis-
, pin 6. To precisely regulate the refer-
. The fixed voltage drop across
DS
then flows from the load supply
. In addition, in the event that the
DS
allowed across the MOSFET
(Continued)
12
R
of V
tor trips.
It is important to note that the programmed reference current
serves a multiple purpose as it is used internally for biasing
and also has a direct effect on the internal charge pump
switching frequency. The design of the LM9061 is optimized
for a reference current of approximately 80 µA, set with a
15.4 kΩ
performance characteristics it is recommended that a
15.4 kΩ resistor be used for R
The protection comparator is configured such that during
normal operation, when the output of the comparator is low,
the differential input stage of the comparator is switched in a
manner that there is virtually no current flowing into the
non-inverting input of the comparator. Therefore, only I
flows through resistor R
current, 20 µA maximum, for the comparator input stage
(twice the I
for equal potentials on each of the comparator inputs) how-
ever flows into the inverting input through resistor R
the comparator threshold, the current through R
no more than the I
THRESHOLD
DS
across the MOSFET before the protection compara-
±
1% resistor for R
SENSE
is approximately equal to the maximum value
specification of 10 µA maximum, defined
SENSE
THRESHOLD
specification of 10 µA.
REF
REF
. To obtain the guaranteed
.
. All of the input bias
SENSE
SENSE
will be
REF
. At