LM675 Power Operational Amplifier
Literature Number: SNOSBP3D
Power Operational Amplifier
General Description
The LM675 is a monolithic power operational amplifier fea-
turing wide bandwidth and low input offset voltage, making it
equally suitable for AC and DC applications.
The LM675 is capable of delivering output currents in excess
of 3 amps, operating at supply voltages of up to 60V. The
device overload protection consists of both internal current
limiting and thermal shutdown. The amplifier is also internally
compensated for gains of 10 or greater.
n3A current capability
typically 90 dB
n5.5 MHz gain bandwidth product
n8 V/µs slew rate
nWide power bandwidth 70 kHz
n1 mV typical offset voltage
nShort circuit protection
nThermal protection with parole circuit (100% tested)
n16V–60V supply range
nWide common mode range
nInternal output protection diodes
n90 dB ripple rejection
nPlastic power package TO-220
nHigh performance power op amp
nBridge amplifiers
nMotor speed controls
nServo amplifiers
nInstrument systems
Connection Diagram
TO-220 Power Package (T)
*The tab is internally connected to pin 3 (−VEE)
Front View
Order Number LM675T
See NS Package T05D
Typical Applications
Non-Inverting Amplifier
May 1999
LM675 Power Operational Amplifier
© 2004 National Semiconductor Corporation DS006739 www.national.com
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage ±30V
Input Voltage −V
to V
Operating Temperature 0˚C to +70˚C
Storage Temperature −65˚C to +150˚C
Junction Temperature 150˚C
Power Dissipation (Note 2) 30W
Lead Temperature
(Soldering, 10 seconds) 260˚C
ESD rating to be determined.
Electrical Characteristics
=±25V, T
=25˚C unless otherwise specified.
Parameter Conditions Typical Tested Limit Units
Supply Current P
= 0W 18 50 (max) mA
Input Offset Voltage V
= 0V 1 10 (max) mV
Input Bias Current V
= 0V 0.2 2 (max) µA
Input Offset Current V
= 0V 50 500 (max) nA
Open Loop Gain R
=90 70 (min) dB
=±5V 90 70 (min) dB
=±20V 90 70 (min) dB
Output Voltage Swing R
=8±21 ±18 (min) V
Offset Voltage Drift Versus Temperature R
<100 k25 µV/˚C
Offset Voltage Drift Versus Output Power 25 µV/W
Output Power THD = 1%, f
= 1 kHz, R
=825 20 W
Gain Bandwidth Product f
= 20 kHz, A
= 1000 5.5 MHz
Max Slew Rate 8 V/µs
Input Common Mode Range ±22 ±20 (min) V
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is
functional, but do not guarantee specific performance limits. Electrical Characteristics state DC and AC electrical specifications under particular test conditions which
guarantee specific performance limits. This assumes that the device is within the Operating Ratings. Specifications are not guaranteed for parameters where no limit
is given, however, the typical value is a good indication of device performance.
Note 2: Assumes TAequal to 70˚C. For operation at higher tab temperatures, the LM675 must be derated based on a maximum junction temperature of 150˚C.
Typical Applications
Generating a Split Supply From a Single Supply
VS=±8V ±30V
www.national.com 2
Typical Performance Characteristics
THD vs Power Output
Input Common Mode
Range vs Supply Voltage
00673910 00673911
Supply Current vs
Supply Voltage PSRR vs Frequency
00673912 00673913
Device Dissipation vs
Ambient Temperature
Current Limit vs
Output Voltage*
See Application Hints.
Typical Performance Characteristics (Continued)
vs Supply Voltage
Output Voltage
Swing vs Supply Voltage
www.national.com 4
Schematic Diagram
Application Hints
The LM675 is designed to be stable when operated at a
closed-loop gain of 10 or greater, but, as with any other
high-current amplifier, the LM675 can be made to oscillate
under certain conditions. These usually involve printed cir-
cuit board layout or output/input coupling.
When designing a printed circuit board layout, it is important
to return the load ground, the output compensation ground,
and the low level (feedback and input) grounds to the circuit
board ground point through separate paths. Otherwise, large
currents flowing along a ground conductor will generate
voltages on the conductor which can effectively act as sig-
nals at the input, resulting in high frequency oscillation or
excessive distortion. It is advisable to keep the output com-
pensation components and the 0.1 µF supply decoupling
capacitors as close as possible to the LM675 to reduce the
effects of PCB trace resistance and inductance. For the
same reason, the ground return paths for these components
should be as short as possible.
Occasionally, current in the output leads (which function as
antennas) can be coupled through the air to the amplifier
input, resulting in high-frequency oscillation. This normally
happens when the source impedance is high or the input
leads are long. The problem can be eliminated by placing a
small capacitor (on the order of 50 pF to 500 pF) across the
circuit input.
Most power amplifiers do not drive highly capacitive loads
well, and the LM675 is no exception. If the output of the
LM675 is connected directly to a capacitor with no series
resistance, the square wave response will exhibit ringing if
the capacitance is greater than about 0.1 µF. The amplifier
can typically drive load capacitances up to 2 µF or so without
oscillating, but this is not recommended. If highly capacitive
loads are expected, a resistor (at least 1) should be placed
in series with the output of the LM675. A method commonly
employed to protect amplifiers from low impedances at high
frequencies is to couple to the load through a 10resistor in
parallel witha5µHinductor.
A power amplifier’s output transistors can be damaged by
excessive applied voltage, current flow, or power dissipation.
The voltage applied to the amplifier is limited by the design of
the external power supply, while the maximum current
passed by the output devices is usually limited by internal
circuitry to some fixed value. Short-term power dissipation is
usually not limited in monolithic operational power amplifiers,
and this can be a problem when driving reactive loads, which
may draw large currents while high voltages appear on the
output transistors. The LM675 not only limits current to
around 4A, but also reduces the value of the limit current
when an output transistor has a high voltage across it.
When driving nonlinear reactive loads such as motors or
loudspeakers with built-in protection relays, there is a possi-
bility that an amplifier output will be connected to a load
whose terminal voltage may attempt to swing beyond the
power supply voltages applied to the amplifier. This can
cause degradation of the output transistors or catastrophic
failure of the whole circuit. The standard protection for this
type of failure mechanism is a pair of diodes connected
between the output of the amplifier and the supply rails.
These are part of the internal circuitry of the LM675, and
needn’t be added externally when standard reactive loads
are driven.
The LM675 has a sophisticated thermal protection scheme
to prevent long-term thermal stress to the device. When the
temperature on the die reaches 170˚C, the LM675 shuts
down. It starts operating again when the die temperature
drops to about 145˚C, but if the temperature again begins to
rise, shutdown will occur at only 150˚C. Therefore, the de-
vice is allowed to heat up to a relatively high temperature if
the fault condition is temporary, but a sustained fault will limit
the maximum die temperature to a lower value. This greatly
reduces the stresses imposed on the IC by thermal cycling,
which in turn improves its reliability under sustained fault
conditions. This circuitry is 100% tested without a heat sink.
Since the die temperature is directly dependent upon the
heat sink, the heat sink should be chosen for thermal resis-
tance low enough that thermal shutdown will not be reached
during normal operaton. Using the best heat sink possible
within the cost and space constraints of the system will
improve the long-term reliability of any power semiconductor.
The LM675 should always be operated with a heat sink,
even though at idle worst case power dissipation will be only
1.8W (30 mA x 60V) which corresponds to a rise in die
temperature of 97˚C above ambient assuming θ
= 54˚C/W
for a TO-220 package. This in itself will not cause the thermal
protection circuitry to shut down the amplifier when operating
at room temperature, but a mere 0.9W of additional power
dissipation will shut the amplifier down since T
will then
increase from 122˚C (97˚C + 25˚C) to 170˚C.
In order to determine the appropriate heat sink for a given
application, the power dissipation of the LM675 in that appli-
cation must be known. When the load is resistive, the maxi-
mum average power that the IC will be required to dissipate
is approximately:
where V
is the total power supply voltage across the
LM675, R
is the load resistance and P
is the quiescent
power dissipation of the amplifier. The above equation is
only an approximation which assumes an “ideal” class B
output stage and constant power dissipation in all other parts
of the circuit. As an example, if the LM675 is operated on a
50V power supply with a resistive load of 8, it can develop
up to 19W of internal power dissipation. If the die tempera-
ture is to remain below 150˚C for ambient temperatures up to
70˚C, the total junction-to-ambient thermal resistance must
be less than
Using θ
= 2˚C/W, the sum of the case-to-heat sink inter-
face thermal resistance and the heat-sink-to-ambient ther-
mal resistance must be less than 2.2˚C/W. The case-to-heat-
sink thermal resistance of the TO-220 package varies with
the mounting method used. A metal-to-metal interface will be
about 1˚C/W if lubricated, and about 1.2˚C/W if dry. If a mica
insulator is used, the thermal resistance will be about
www.national.com 6
Application Hints (Continued)
1.6˚C/W lubricated and 3.4˚C/W dry. For this example, we
assume a lubricated mica insulator between the LM675 and
the heat sink. The heat sink thermal resistance must then be
less than
4.2˚C/W 2˚C/W 1.6˚C/W = 0.6˚C/W.
This is a rather large heat sink and may not be practical in
some applications. If a smaller heat sink is required for
reasons of size or cost, there are two alternatives. The
maximum ambient operating temperature can be restricted
to 50˚C (122˚F), resulting in a 1.6˚C/W heat sink, or the heat
sink can be isolated from the chassis so the mica washer is
not needed. This will change the required heat sink to a
1.2˚C/W unit if the case-to-heat-sink interface is lubricated.
The thermal requirements can become more difficult when
an amplifier is driving a reactive load. For a given magnitude
of load impedance, a higher degree of reactance will cause
a higher level of power dissipation within the amplifier. As a
general rule, the power dissipation of an amplifier driving a
60˚ reactive load will be roughly that of the same amplifier
driving the resistive part of that load. For example, some
reactive loads may at some frequency have an impedance
with a magnitude of 8and a phase angle of 60˚. The real
part of this load will then be 8x cos 60˚ or 4, and the
amplifier power dissipation will roughly follow the curve of
power dissipation with a 4load.
Typical Applications
Non-Inverting Unity Gain Operation
Inverting Unity Gain Operation
Typical Applications (Continued)
Servo Motor Control
High Current Source/Sink
IN x 2.5 amps/volt
i.e. IOUT = 1A when VIN = 400 mV
Trim pot for max ROUT
www.national.com 8
Physical Dimensions inches (millimeters) unless otherwise noted
TO-220 Power Package (T)
Order Number LM675T
NS Package T05D
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves
the right at any time without notice to change said circuitry and specifications.
For the most current product information visit us at www.national.com.
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LM675 Power Operational Amplifier
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