LM9061N National Semiconductor, LM9061N Datasheet - Page 8

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

Figure 3 illustrates how the threshold voltage for the internal
Application Hints
To protect the MOSFET from exceeding its maximum junc-
tion temperature rating the power dissipation needs to be
limited The maximum power dissipation allowed (derated
for temperature) and the maximum drain to source ON re-
sistance R
bient temperature needs to be determined When switched
ON the power dissipation in the MOSFET will be
The V
therefore
With this restriction the actual load current and power dissi-
pation obtained will be a direct function of the actual
R
ture but the junction temperature of the power device will
never exceed its rated maximum
To limit the maximum load current requires an estimate of
the minimum R
R
required operating temperature range
The maximum current to the load will be
The maximum junction temperature of the MOSFET and or
the maximum current to the load can be limited by monitor-
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
device
protection comparator is established
DS(ON)
DS(ON)
DS
of the MOSFET at any particular ambient tempera-
of discrete MOSFETs is rarely specified) over the
voltage to limit the maximum power dissipation is
V
DS(ON)
DS (MAX)
I
LOAD (MAX)
DS(ON)
with both at the maximum operating am-
e
P
DISS
DS
A to gradually turn OFF the power
P
D (MAX) c
of the MOSFET (the minimum
In addition in the event that the
e
e
(Continued)
R
R
V
DS(ON)
DS(ON) (MIN)
DS
V
R
2
DS
DS(ON) (MAX)
FIGURE 3 Protection Comparator Biasing
8
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 re-
sistor 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 ref-
erence current is set by
The reference current sink output is internally connected to
the threshold pin I
through R
R
of V
tor trips
It is important to note that the programmed reference cur-
rent 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
set with a 15 4 k
guaranteed performance characteristics it is recommended
that a 15 4 k
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
THRESHOLD
DS
across the MOSFET before the protection compara-
THRESHOLD
THRESHOLD
is approximately equal to the maximum value
resistor be used for R
REF
REF
g
A reference current is fixed by a resis-
pin 6 To precisely regulate the refer-
1% resistor for R
I
REF
The fixed voltage drop across
then flows from the load supply
DS
e
allowed across the MOSFET
V
R
REF
REF
REF
REF
TL H 12317– 11
To obtain the
A