LM1875T/LB02 - NATIONAL SEMICONDUCTOR

LM1875

LM1875 20W Audio Power Amplifier

Literature Number: SNAS524A

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

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

Absolute Maximum Ratings (Note 1)

Supply Voltage 60V

Input Voltage −V

to V

Storage Temperature −65˚C to + 150˚C

Junction Temperature 150˚C

Lead Temperature

(Soldering, 10 seconds) 260˚C

3˚C

73˚C

Electrical Characteristics

=+25V, −V

=−25V, T

AMBIENT

=25˚C, R

=8Ω,A

=20 (26 dB), f

=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

=1 kHz 0.015 %

OUT

=20W, f

=20 kHz 0.05 0.4 %

OUT

=20W, R

=4Ω,f

=1 kHz 0.022 %

OUT

=20W, R

=4Ω,f

=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

=20 kHz 5.5 MHz

Open Loop Gain DC 90 dB

PSRR V

, 1 kHz, 1 Vrms 95 52 dB

, 1 kHz, 1 Vrms 83 52 dB

Max Slew Rate 20W, 8Ω, 70 kHz BW 8 V/µs

Current Limit V

OUT

SUPPLY

−10V 4 3 A

Equivalent Input Noise Voltage R

=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.

LM1875

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Typical Performance Characteristics (Continued)

Power Dissipation vs

Power Output

Power Dissipation vs

Power Output

00503015 00503016

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

LM1875

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Schematic Diagram

00503005

LM1875

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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

is the total power supply voltage across the

LM1875, 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

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

=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

LM1875

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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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www.national.com

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.

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