LM111J8883 National Semiconductor, LM111J8883 Datasheet

no-image

LM111J8883

Manufacturer Part Number
LM111J8883
Description
CDIP-8
Manufacturer
National Semiconductor
Datasheet

Specifications of LM111J8883

Date_code
01+
© 2002 National Semiconductor Corporation
Fast IC Power Transistor
with Thermal Protection
Introduction
Overload protection is perhaps most necessary in power
circuitry. This is shown by recent trends in power transistor
technology. Safe-area, voltage and current handling capabil-
ity have been increased to limits far in excess of package
power dissipation. In RF transistors, devices are now avail-
able and able to withstand badly mismatched loads without
destruction. However, for anyone working with power tran-
sistors, they are still easily destroyed.
Since power circuitry, in many cases, drives other low level
circuitry — such as a voltage regulator — protection is doubly
important. Overloads that cause power transistor failure can
result in the destruction of the entire circuit. This is because
the common failure mode for power transistors is a short
from collector to emitter — applying full voltage to the load. In
the case of a voltage regulator, the raw supply voltage would
be applied to the low level circuitry.
A new monolithic power transistor provides virtually absolute
protection against any type of overload. Included on the chip
are current limiting, safe area protection and thermal limiting.
Current limiting controls the peak current through the chip to
a safe level below the fuzing current of the aluminum metal-
ization. At high collector to emitter voltage the safe area
limiting reduces the peak current to further protect the power
transistor. If, under prolonged overload, power dissipation
causes chip temperature to rise toward destructive levels,
thermal limiting turns off the device keeping the devices at a
safe temperature. The inclusion of thermal limiting, a feature
not easily available in discrete circuitry makes this device
especially attractive in applications where normal protective
schemes are ineffective.
The device’s high gain and fast response further reduce
requirements of surrounding circuitry. As well as being used
in
transistor-transistor logic or complementary-MOS logic to
power loads without external devices. In fact, the
input-current requirement of 3 microamperes is small
enough for one CMOS gate to drive over 400 LM195’s.
Besides high dc current gain, the IC has low input capaci-
tance so it can be easily driven from high impedance
sources — even at high frequencies. In a standard TO-3
power package, the monolithic structure ties the emitter,
rather
boot-strapping the base-to-package capacitance. Addition-
ally, connecting the emitter to the package is especially
convenient for grounded emitter circuits.
The device is fully protected against any overload condition
when it is used below the maximum voltage rating. The
current-limiting circuitry restricts the power dissipation to 35
watts, 1.8 amperes are available at collector-to-emitter volt-
age of 17V decreasing to about 0.8 amperes at 40V. In
reality, however, like standard transistors, power dissipation
in actual use is limited by the size of the external heat sink.
Switching time is fast also. At 40V 25 Ohm load can be
switched on or off in a relatively fast 500 ns. The internal
planar double diffused monolithic transistors have an f
200 MHz to 400 MHz. The limiting factor on overall speed is
linear
than
applications,
the
collector,
the
to
the
IC
AN007418
case
can
effectively
interface
t
of
National Semiconductor
Application Note 110
April 1998
the protective and biasing circuitry around the output tran-
sistors. An important performance point is that no more than
the normal 3 µA base current is needed for fast switching.
To the designer, the LM195 acts like an ordinary power
transistor, and its operation is almost identical to that of a
standard power device. However, it provides almost absolute
protection against any type of overload. And, since it is
manufactured with standard seven-mask IC technology, the
device is produceable in large quantities at reasonable cost.
Circuit Design
Besides the protective features, the monolithic power tran-
sistor should function as closely to a discrete transistor as
possible. Of course, due to the circuitry on the chip, there will
be some differences.
Figure 1 shows a simplified schematic of the power transis-
tor. A power NPN Darlington is driven by an input PNP. The
PNP and output NPN’s are biased by internal current source
I
in excess of 10
from high impedance sources. Unlike normal power transis-
tors, the base current is negative, flowing out of the PNP.
However, in most cases this is not a problem.
The input PNP transistor is made with standard IC process-
ing and has a reverse base-emitter breakdown voltage in
excess of 40V. This allows the power transistor to be driven
from a stiff voltage source without damage due to excessive
base current. At input voltages in excess of about 1V the
input PNP becomes reverse biased and no current is drawn
from the base lead. In fact it is possible for the base of the
monolithic transistor to be driven with up to 40V even though
the collector to emitter voltage is low. Further, the input PNP
isolates the base drive from the protective circuitry insuring
1
. The composite three transistors yield a total current gain
FIGURE 1. Simplified Circuit of the LM195
6
making it easy to drive the power transistors
www.national.com
00741801

Related parts for LM111J8883

Related keywords