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PDF LM675T Data sheet ( Hoja de datos )
| Número de pieza | LM675T | |
| Descripción | Power Operational Amplifier | |
| Fabricantes | National Semiconductor | |
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May 1999
LM675
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 de-
vice overload protection consists of both internal current lim-
iting and thermal shutdown. The amplifier is also internally
compensated for gains of 10 or greater.
Features
n 3A current capability
n AVO typically 90 dB
n 5.5 MHz gain bandwidth product
n 8 V/µs slew rate
n Wide power bandwidth 70 kHz
n 1 mV typical offset voltage
n Short circuit protection
n Thermal protection with parole circuit (100% tested)
n 16V–60V supply range
n Wide common mode range
n Internal output protection diodes
n 90 dB ripple rejection
n Plastic power package TO-220
Applications
n High performance power op amp
n Bridge amplifiers
n Motor speed controls
n Servo amplifiers
n Instrument systems
Connection Diagram
Typical Applications
TO-220 Power Package (T)
Non-Inverting Amplifier
*The tab is internally connected to pin 3 (−VEE)
Front View
Order Number LM675T
See NS Package T05D
DS006739-1
DS006739-2
© 1999 National Semiconductor Corporation DS006739
www.national.com
1 page  
Application Hints
STABILITY
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 volt-
ages on the conductor which can effectively act as signals at
the input, resulting in high frequency oscillation or excessive
distortion. It is advisable to keep the output compensation
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 in-
put, 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 re-
sistance, 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 os-
cillating, but this is not recommended. If highly capacitive
loads are expected, a resistor (at least 1Ω) 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 10Ω resistor in
parallel with a 5 µH inductor.
CURRENT LIMIT AND SAFE OPERATING AREA
(SOA) PROTECTION
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 be-
tween 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.
THERMAL PROTECTION
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 im-
prove the long-term reliability of any power semiconductor.
POWER DISSIPATION AND HEAT SINKING
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 tem-
perature of 97˚C above ambient assuming θjA = 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 TJ will then in-
crease 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 VS is the total power supply voltage across the
LM675, RL is the load resistance and PQ is the quiescent
power dissipation of the amplifier. The above equation is
only an approximation which assumes an “ideal” class B out-
put 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 8Ω, it can develop up to
19W of internal power dissipation. If the die temperature 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 θJC = 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 in-
terface 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 1.6˚C/W lubricated and 3.4˚C/W dry. For this ex-
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