LM1875 20W Audio Power Amplifier
General Description

Features

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 4X or 8X load on
g 25V supplies. Using an 8X load and g 30V supplies, over30 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 distortion levels even at high output power levels. Other outstanding features include high gain, fast slew rate and a widepower bandwidth, large output voltage swing, high current
capability, and a very wide supply range. The amplifier isinternally compensated and stable for gains of 10 or greater.

YConnection Diagram

Typical Applications

Up to 30 watts output power
AVO typically 90 dB
Y Low distortion: 0.015%, 1 kHz, 20 W
Y Wide power bandwidth: 70 kHz
Y Protection for AC and DC short circuits to groundY Thermal protection with parole circuit
Y High current capability: 4A
Y Wide supply range 16V-60V
Y Internal output protection diodes
Y 94 dB ripple rejection
Y Plastic power package TO-220
YApplications
High performance audio systems
Bridge amplifiers
Y Stereo phonographs
Y Servo amplifiers
Y Instrument systems
YY

TL/H/5030 – 1

Front View

Order Number LM1875T
See NS Package Number T05B

TL/H/5030 – 2

C1995 National Semiconductor Corporation

TL/H/5030

RRD-B30M75/Printed in U. S. A.

LM1875 20W Audio Power Amplifier

March 1995

Absolute Maximum RatingsStorage Temperature

If Military/Aerospace specified devices are required,please contact the National Semiconductor Sales
Office/Distributors for availability and specifications.
Supply Voltage
Input Voltage

60Vb VEE to VCC

b 65§ C to a 150§ C

Junction Temperature150§ C

Lead Temperature (Soldering, 10 seconds)
iJC
iJA

260§ C
3§ C
73§ C

Electrical Characteristics
VCC e a 25V, bVEE eb25V, TAMBIENT e 25§ C, RL e 8X, AV e 20 (26 dB), fo e 1 kHz, unless otherwise specified.
TypicalTested Limits

Units

Supply Current

Parameter

POUT e 0W

70100mA

Output Power (Note 1)

THD e 1%25W

THD (Note 1)

POUT e 20W, fo e 1 kHz
POUT e 20W, fo e 20 kHz
POUT e 20W, RL e 4X, fo e 1 kHz
POUT e 20W, RL e 4X, fo e 20 kHz

0.015
0.05
0.022
0.07

0.6

%
%%
%

Offset Voltage

g1g 15mV

Input Bias Current

g 0.2

g2mA

Input Offset Current

0g 0.5mA

Gain-Bandwidth ProductConditions

fo e 20 kHz

5.5

Open Loop Gain

DC90

PSRR

VCC, 1 kHz, 1 Vrms
VEE, 1 kHz, 1 Vrms

9583

Max Slew Rate

20W, 8X, 70 kHz BW

8Current Limit

VOUT e VSUPPLY b10V

4

Equivalent Input Noise Voltage

RS e 600X, CCIR30.4

MHzdB
5252dBdB
V/ms3AmVrms

Note 1: 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 4X load may be slightly reduced when the tab temperature exceeds
55§ C.

Typical Applications (Continued)
Typical Single Supply Operation

TL/H/5030 – 32

Typical Performance Characteristics
THD vs Power Output

THD vs Frequency

Power Output vs Supply
Voltage

Supply Current vs Supply
Voltage

PSRR vs Frequency

Device Dissipation vsAmbient Temperature ²² wINTERFACE e 1§ C/W.

See Application Hints.

Power Dissipation vsPower Output

Power Dissipation vsPower Output

Open Loop Gain andPhase vs Frequency

IOUT vs VOUT-Current Limit/
Safe Operating Area Boundary

Input Bias Current
vs Supply Voltage

TL/H/5030 – 4*Thermal shutdown with infinite heat sink**Thermal shutdown with 1§ C/W heat sink3

TL/H/5030 – 5

Schematic Diagram

4

Application Hints
CURRENT LIMIT AND SAFE OPERATING AREA (SOA)
PROTECTIONSTABILITY
The LM1875 is designed to be stable when operated at a
closed-loop gain of 10 or greater, but, as with any otherhigh-current amplifier, the LM1875 can be made to oscillate
under certain conditions. These usually involve printed circuit 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 designinga 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 advisable to keep the output compensation components and the 0.1 mF supply decoupling capacitors as close
as possible to the LM1875 to reduce the effects of PCB
trace resistance and inductance. For the same reason, theground return paths for these components should be as
short as possible.
Occasionally, current in the output leads (which function asantennas) can be coupled through the air to the amplifier
input, resulting in high-frequency oscillation. This normallyhappens when the source impedance is high or the input
leads are long. The problem can be eliminated by placing asmall capacitor (on the order of 50 pF to 500 pF) across thecircuit input.
Most power amplifiers do not drive highly capacitive loads
well, and the LM1875 is no exception. If the output of theLM1875 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 mF. The amplifier
can typically drive load capacitances up to 2 mF or so without oscillating, but this is not recommended. If highly capacitive loads are expected, a resistor (at least 1X) should beplaced in series with the output of the LM1875. A methodcommonly employed to protect amplifiers from low impedances at high frequencies is to couple to the load through a
10X resistor in parallel with a 5 mH inductor.

A power amplifier’s output transistors can be damaged byexcessive applied voltage, current flow, or power dissipation. The voltage applied to the amplifier is limited by thedesign of the external power supply, while the maximum
current passed by the output devices is usually limited byinternal 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 reactiveloads, 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 acrossit.
When driving nonlinear reactive loads such as motors or
loudspeakers with built-in protection relays, there is a possibility that an amplifier output will be connected to a load
whose terminal voltage may attempt to swing beyond thepower supply voltages applied to the amplifier. This cancause degradation of the output transistors or catastrophic
failure of the whole circuit. The standard protection for thistype of failure mechanism is a pair of diodes connected between the output of the amplifier and the supply rails. Theseare part of the internal circuitry of the LM1875, and needn’t
be added externally when standard reactive loads are driven.
THERMAL PROTECTIONThe LM1875 has a sophisticated thermal protection scheme
to prevent long-term thermal stress to the device. When thetemperature on the die reaches 170§ C, the LM1875 shuts
down. It starts operating again when the die temperaturedrops to about 145§ C, but if the temperature again begins torise, shutdown will occur at only 150§ C. Therefore, the device 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 theheat sink, the heat sink should be chosen for thermal resistance low enough that thermal shutdown will not be reachedduring 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.

DISTORTION
The preceding suggestions regarding circuit board grounding techniques will also help to prevent excessive distortionlevels in audio applications. For low THD, it is also necessary to keep the power supply traces and wires separated
from the traces and wires connected to the inputs of theLM1875. This prevents the power supply currents, which
are large and nonlinear, from inductively coupling to theLM1875 inputs. Power supply wires should be twisted together and separated from the circuit board. Where thesewires are soldered to the board, they should be perpendicular to the plane of the board at least to a distance of acouple of inches. With a proper physical layout, THD levels
at 20 kHz with 10W output to an 8X load should be lessthan 0.05%, and less than 0.02% at 1 kHz.

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 ofpower. The 54§ C/W junction-to-ambient thermal resistanceof a TO-220 package would cause the die temperature torise 324§ C above ambient, so the thermal protection circuitry will shut the amplifier down if operation without a heatsink is attempted.

5Application Hints (Continued)
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 theLM1875 and the heat sink. The heat sink thermal resistancemust then be less than
4.2§ C/Wb2§ C/Wb1.6§ C/W e 0.6§ C/W.

In order to determine the appropriate heat sink for a givenapplication, the power dissipation of the LM1875 in that application must be known. When the load is resistive, themaximum average power that the IC will be required to dissipate is approximately:
VS2a PQ
2q2RL
where VS is the total power supply voltage across theLM1875, RL is the load resistance, and PQ is the quiescentpower 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 otherparts of the circuit. The curves of ‘‘Power Dissipation vsPower Output’’ give a better representation of the behaviorof the LM1875 with various power supply voltages and resistive loads. As an example, if the LM1875 is operated on a50V power supply with a resistive load of 8X, 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 resistancemust be less thanPD(MAX) &This is a rather large heat sink and may not be practical insome 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 reduced to50§ C (122§ F), resulting in a 1.6§ C/W heat sink, or the heatsink 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 theLM1875 can be achieved by mounting the device directly to the heatsink (tab is at ground potential); this avoids the use of a mica or othertype insulator.

The thermal requirements can become more difficult when
an amplifier is driving a reactive load. For a given magnitudeof load impedance, a higher degree of reactance will cause
a higher level of power dissipation within the amplifier. As ageneral rule, the power dissipation of an amplifier driving a60§ reactive load (usually considered to be a worst-caseloudspeaker load) will be roughly that of the same amplifier
driving the resistive part of that load. For example, a loudspeaker may at some frequency have an impedance with amagnitude of 8X and a phase angle of 60§ . The real part ofthis load will then be 4X, and the amplifier power dissipation
will roughly follow the curve of power dissipation with a 4Xload.

150§ Cb70§ Ce 4.2§ C/W.
19W
Using iJC e 2§ C/W, the sum of the case-to-heat-sink interface thermal resistance and the heat-sink-to-ambient thermal resistance must be less than 2.2§ C/W. The case-toheat-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.

Component Layouts
Split Supply

Single Supply

TL/H/5030 – 7

TL/H/5030–667LM1875 20W Audio Power Amplifier

Physical Dimensions inches (millimeters)

TO-220 Power Package (T)Order Number LM1875T
NS Package Number T05B
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