LM9061N National Semiconductor, LM9061N Datasheet - Page 9

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

Application Hints
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
(twice the I
for equal potentials on each of the comparator inputs) how-
ever flows into the inverting input through resistor R
At the comparator threshold the current through R
will be no more than the I
To tailor the V
tion the resistor R
lowing formula
V
where R
the protection comparator R
ed to pin 1 and V
comparator (typically in the range of
The resistor R
ed to provide transient protection for the Sense pin espe-
cially when driving inductive type loads A minimum value of
1 k
to
resistor used for R
larger than R
output goes high the total bias current for the input stage
transfers from the Sense pin to the Threshold pin thereby
changing the voltages present at the inputs to the compara-
tor For consistent switching of the comparator right at the
desired threshold point the voltage drop that occurs at the
non-inverting input (Threshold) should equal or exceed the
rise in voltage at the inverting input (Sense)
In automotive applications the load supply may be the bat-
tery of the vehicle whereas the V
is a switched ignition supply When the V
switched OFF there is always a concern for the amount of
current drained from the battery The only current drain un-
der this condition is a leakage current into the Threshold pin
which is less than 10 A
A bypass capacitor across R
help keep the reference voltage constant in applications
where the V
noise This bypass capacitor should be no larger than
0 1 F and is not needed for most applications
DS (MAX)
a
60V This resistor should be equal to or less than the
will protect the pin from transients ranging from
REF
e
SENSE
CC
e
V
THRESHOLD
A maximum for the comparator input stage
SENSE
DS (MAX)
REF c
supply is subject to high levels of transient
15 4 k
R
OS
specification of 10 A maximum defined
THRESHOLD
THRESHOLD
REF
is optional but is strongly recommend-
is the offset voltage of the protection
R
THR
threshold for any particular applica-
THRESHOLD
I
SENSE
SENSE
When the protection comparator
b
SENSE
REF
(Continued)
(I
Never set R
can be selected per the fol-
SENSE c
specification of 10 A
is the input bias current to
CC
is optional and is used to
is the resistor connect-
supply for the LM9061
g
All of the input bias
10 mV)
R
SENSE
SENSE
CC
)
to a value
supply is
a
FIGURE 4 Delay Timer
SENSE
SENSE
V
b
OS
25V
REF
9
DELAY TIMER
To allow the MOSFET to conduct currents beyond the pro-
tection threshold for a brief period of time a delay timer
function is provided This timer delays the actual latching
OFF of the MOSFET for a programmable interval This fea-
ture is important to drive loads which require a surge of
current in excess of the normal ON current upon start up or
at any point in time such as lamps and motors Figure 4
details the delay timer circuitry A capacitor connected from
the Delay pin 8 to ground sets the delay time interval With
the MOSFET turned ON and all conditions normal the out-
put of the protection comparator is low and this keeps the
discharge transistor ON This transistor keeps the delay ca-
pacitor discharged Should a surge of load current trip the
protection comparator high the discharge transistor turns
OFF and an internal 10
charging the delay capacitor
If the surge current with excessive V
enough for the capacitor to charge to the timing comparator
threshold of typically 5 5V the output of the comparator will
go high to set a flip-flop and immediately latch the MOSFET
OFF It will not re-start until the ON OFF Input is toggled
low then high
The delay time interval is set by the selection of C
can be found from
where typically V
Charging of the delay capacitor is clamped at approximately
7 5V which is the internal bias voltage for the 10 A current
source
MINIMUM DELAY TIME
A minimum delay time interval is required in all applications
due to the nature of the protection circuitry At the instant
the MOSFET is commanded ON the voltage across the
MOSFET V
cause the source is held at ground by the load This condi-
tion will immediately trip the protection comparator Without
a minimum delay time set the timing comparator will trip
and force the MOSFET to latch OFF thereby never allowing
the load to be energized
DS
T
DELAY
is equal to the full load supply voltage be-
TIMER
e
e
(V
5 5V and I
A current source begins linearly
TIMER c
I
DELAY
C
DELAY
DS
DELAY
voltage lasts long
TL H 12317 – 12
e
)
10 A
DELAY
and