LM1875
20W Audio Power Amplifier
General Description
The LM1875 is a monolithic power amplifier offering very low
distortion and high quality performance for consumer audio
applications.
The LM1875 delivers 20 watts into a 4Ωor 8Ωload on ±25V
supplies. Using an 8Ωload and ±30V supplies, over 30
watts of power may be delivered. The amplifier is designed
to operate with a minimum of external components. Device
overload protection consists of both internal current limit and
thermal shutdown.
The LM1875 design takes advantage of advanced circuit
techniques and processing to achieve extremely low distor-
tion levels even at high output power levels. Other outstand-
ing features include high gain, fast slew rate and a wide
power bandwidth, large output voltage swing, high current
capability, and a very wide supply range. The amplifier is
internally compensated and stable for gains of 10 or greater.
Features
nUp to 30 watts output power
nA
VO
typically 90 dB
nLow distortion: 0.015%, 1 kHz, 20 W
nWide power bandwidth: 70 kHz
nProtection for AC and DC short circuits to ground
nThermal protection with parole circuit
nHigh current capability: 4A
nWide supply range 16V-60V
nInternal output protection diodes
n94 dB ripple rejection
nPlastic power package TO-220
Applications
nHigh performance audio systems
nBridge amplifiers
nStereo phonographs
nServo amplifiers
nInstrument systems
Connection Diagram
00503001
Front View
Package Ordering Info NSC Package
Number
For Straight Leads LM1875T
SL108949
T05A
For Stagger Bend LM1875T
LB03
T05D
For 90˚ Stagger
Bend
LM1875T
LB05
T05E
For 90˚ Stagger
Bend
LM1875T
LB02
TA05B
Typical Applications
00503002
July 2002
LM1875 20W Audio Power Amplifier
© 2002 National Semiconductor Corporation DS005030 www.national.com
Absolute Maximum Ratings (Note 1)
Supply Voltage 60V
Input Voltage −V
EE
to V
CC
Storage Temperature −65˚C to + 150˚C
Junction Temperature 150˚C
Lead Temperature
(Soldering, 10 seconds) 260˚C
θ
JC
3˚C
θ
JA
73˚C
Electrical Characteristics
V
CC
=+25V, −V
EE
=−25V, T
AMBIENT
=25˚C, R
L
=8Ω,A
V
=20 (26 dB), f
o
=1 kHz, unless otherwise specified.
Parameter Conditions Typical Tested Limits Units
Supply Current P
OUT
=0W 70 100 mA
Output Power (Note 2) THD=1% 25 W
THD (Note 2) P
OUT
=20W, f
o
=1 kHz 0.015 %
P
OUT
=20W, f
o
=20 kHz 0.05 0.4 %
P
OUT
=20W, R
L
=4Ω,f
o
=1 kHz 0.022 %
P
OUT
=20W, R
L
=4Ω,f
o
=20 kHz 0.07 0.6 %
Offset Voltage ±1±15 mV
Input Bias Current ±0.2 ±2µA
Input Offset Current 0 ±0.5 µA
Gain-Bandwidth Product f
o
=20 kHz 5.5 MHz
Open Loop Gain DC 90 dB
PSRR V
CC
, 1 kHz, 1 Vrms 95 52 dB
V
EE
, 1 kHz, 1 Vrms 83 52 dB
Max Slew Rate 20W, 8Ω, 70 kHz BW 8 V/µs
Current Limit V
OUT
=V
SUPPLY
−10V 4 3 A
Equivalent Input Noise Voltage R
S
=600Ω, CCIR 3 µVrms
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.
Note 2: Assumes the use of a heat sink having a thermal resistance of 1˚C/W and no insulator with an ambient temperature of 25˚C. Because the output limiting
circuitry has a negative temperature coefficient, the maximum output power delivered to a 4Ωload may be slightly reduced when the tab temperature exceeds 55˚C.
Typical Applications
Typical Single Supply Operation
00503003
LM1875
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Typical Performance Characteristics
THD vs Power Output THD vs Frequency
00503009 00503010
Power Output vs Supply
Voltage
Supply Current vs Supply
Voltage
00503011 00503012
PSRR vs Frequency
Device Dissipation vs
Ambient Temperature†
00503013 00503014
†φINTERFACE = 1˚C/W.
See Application Hints.
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Typical Performance Characteristics (Continued)
Power Dissipation vs
Power Output
Power Dissipation vs
Power Output
00503015 00503016
I
OUT
vs V
OUT
-Current Limit/
Safe Operating Area Boundary
Open Loop Gain and
Phase vs Frequency
00503017 00503018
Note 3: Thermal shutdown with infinite heat sink
Note 4: Thermal shutdown with 1˚C/W heat sink
Input Bias Current
vs Supply Voltage
00503019
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Schematic Diagram
00503005
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Application Hints
STABILITY
The LM1875 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 LM1875 can be made to oscillate
under certain conditions. These usually involve printed cir-
cuit board layout or output/input coupling.
Proper layout of the printed circuit board is very important.
While the LM1875 will be stable when installed in a board
similar to the ones shown in this data sheet, it is sometimes
necessary to modify the layout somewhat to suit the physical
requirements of a particular application. When designing a
different 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 signals at the input, resulting in
high frequency oscillation or excessive distortion. It is advis-
able to keep the output compensation components and the
0.1 µF supply decoupling capacitors as close as possible to
the LM1875 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 LM1875 is no exception. If the output of the
LM1875 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 1Ω) should be placed
in series with the output of the LM1875. A method commonly
employed to protect amplifiers from low impedances at high
frequencies is to couple to the load through a 10Ωresistor in
parallel witha5µHinductor.
DISTORTION
The preceding suggestions regarding circuit board ground-
ing techniques will also help to prevent excessive distortion
levels in audio applications. For low THD, it is also neces-
sary to keep the power supply traces and wires separated
from the traces and wires connected to the inputs of the
LM1875. This prevents the power supply currents, which are
large and nonlinear, from inductively coupling to the LM1875
inputs. Power supply wires should be twisted together and
separated from the circuit board. Where these wires are
soldered to the board, they should be perpendicular to the
plane of the board at least to a distance of a couple of
inches. With a proper physical layout, THD levels at 20 kHz
with 10W output to an 8Ωload should be less than 0.05%,
and less than 0.02% at 1 kHz.
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 audio 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 LM1875 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 LM1875, and
needn’t be added externally when standard reactive loads
are driven.
THERMAL PROTECTION
The LM1875 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 LM1875 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.
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 operation. 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
device.
POWER DISSIPATION AND HEAT SINKING
The LM1875 must always be operated with a heat sink, even
when it is not required to drive a load. The maximum idling
current of the device is 100 mA, so that on a 60V power
supply an unloaded LM1875 must dissipate 6W of power.
The 54˚C/W junction-to-ambient thermal resistance of a
TO-220 package would cause the die temperature to rise
324˚C above ambient, so the thermal protection circuitry will
shut the amplifier down if operation without a heat sink is
attempted.
In order to determine the appropriate heat sink for a given
application, the power dissipation of the LM1875 in that
application must be known. When the load is resistive, the
maximum average power that the IC will be required to
dissipate is approximately:
where V
S
is the total power supply voltage across the
LM1875, R
L
is the load resistance, and P
Q
is the quiescent
power dissipation of the amplifier. The above equation is
only an approximation which assumes an “ideal” class B
LM1875
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Application Hints (Continued)
output stage and constant power dissipation in all other parts
of the circuit. The curves of “Power Dissipation vs Power
Output” give a better representation of the behavior of the
LM1875 with various power supply voltages and resistive
loads. As an example, if the LM1875 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 θ
JC
=2˚C/W, the sum of the case-to-heat-sink interface
thermal resistance and the heat-sink-to-ambient thermal re-
sistance 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 1.6˚C/W lubricated and 3.4˚C/W dry. For this example,
we assume a lubricated mica insulator between the LM1875
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.
[EM00001]The maximum ambient operating temperature
can be reduced 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.
Note: When using a single supply, maximum transfer of heat away from the
LM1875 can be achieved by mounting the device directly to the heat
sink (tab is at ground potential); this avoids the use of a mica or other
type insulator.
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 (usually considered to be a worst-case
loudspeaker load) will be roughly that of the same amplifier
driving the resistive part of that load. For example, a loud-
speaker may at some frequency have an impedance with a
magnitude of 8Ωand a phase angle of 60˚. The real part of
this load will then be 4Ω, and the amplifier power dissipation
will roughly follow the curve of power dissipation with a 4Ω
load.
LM1875
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Component Layouts
Split Supply
00503006
Single Supply
00503007
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Physical Dimensions inches (millimeters) unless otherwise noted
TO-220 Power Package (T)
Order Number LM1875T
NS Package Number T05D
Order Number LM1875T SL108949
NS Package Number T05A
LM1875
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Physical Dimensions inches (millimeters) unless otherwise noted (Continued)
Order Number LM1875T LB05
NS Package Number T05E
LM1875
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Physical Dimensions inches (millimeters) unless otherwise noted (Continued)
Order Number LM1875T LB02
NS Package Number TA05B
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whose failure to perform when properly used in
accordance with instructions for use provided in the
labeling, can be reasonably expected to result in a
significant injury to the user.
2. A critical component is any component of a life
support device or system whose failure to perform
can be reasonably expected to cause the failure of
the life support device or system, or to affect its
safety or effectiveness.
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Email: support@nsc.com
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LM1875 20W Audio Power Amplifier
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.
