Datasheet LM4871M, LM4871N, LM4871MX Datasheet (NSC)

LM4871

1.1W Audio Power Amplifier with Shutdown Mode

General Description

The LM4871 is a bridge-connected audio power amplifier ca­pable of delivering typically 1.1W of continuous average
power to an 8Ω load with 0.5%(THD) from a 5V power sup­ply.

Boomer audio power amplifiers were designed specifically to
provide high quality output power with a minimal amount of
external components. Since the LM4871 does not require
output coupling capacitors, bootstrap capacitors, or snubber
networks, it is optionally suited for low-power portable sys­tems.

The LM4871 features an externally controlled, low-power
consumption shutdown mode, as well as an internal thermal
shutdown protection mechanism.

The unity-gain stable LM4871 can be configured by external
gain-setting resistors.

Key Specifications

n THD at 1 kHz at 1W continuous

average output power into 8Ω 0.5%(max)

n Output power at 10%THD+N

at 1 kHz into 8Ω

1.5W (typ)

n Shutdown Current 0.6 µA (typ)

Features

n No output coupling capacitors, bootstrap capacitors, or

snubber circuits are necessary

n Small Outline or DIP packaging
n Unity-gain stable
n External gain configuration capability
n Pin compatible with LM4861

Applications

n Portable Computers
n Desktop Computers
n Low Voltage Audio Systems

Typical Application Connection Diagram

Boomer®is a registered trademark of National Semiconductor Corporation.

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FIGURE 1. Typical Audio Amplifier Application Circuit

Small Outline and DIP Package

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Top View

Order Number LM4871M or LM4871N

See NS Package Number M08A or N08E

February 2000

LM4871 1.1W Audio Power Amplifier with Shutdown Mode

© 2000 National Semiconductor Corporation DS100008 www.national.com

Absolute Maximum Ratings (Note 2)

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

Supply Voltage 6.0V
Supply Temperature −65˚C to +150˚C
Input Voltage −0.3V to V

DD

to +0.3V
Power Dissipation (Note 3) Internally Limited
ESD Susceptibility (Note 4) 5000V
ESD Susceptibility (Note 5) 250V
Junction Temperature 150˚C
Soldering Information

Small Outline Package
Vapor Phase (60 sec.) 215˚C

Infrared (15 sec.) 220˚C

See AN-450 ″Surface Mounting and their Effects on
Product Reliability″ for other methods of
soldering surface mount devices.

θ

JC

(typ)— M08A 35˚C/W

θ

JA

(typ)— M08A 140˚C/W

θ

JC

(typ)— N08E 37˚C/W

θ

JA

(typ)— N08E 107˚C/W

Operating Ratings

Temperature Range

T

MIN

≤ TA≤ T

MAX

−40˚C ≤ TA≤ 85˚C

Supply Voltage 2.0V ≤ V

DD

≤ 5.5V

Electrical Characteristics (Notes 1, 2)

The following specifications apply for V

DD

= 5V unless otherwise specified. Limits apply for TA= 25˚C.

Symbol Parameter Conditions

LM4871

Units

(Limits)

Typical Limit

(Note 6) (Note 7)

V

DD

Supply Voltage 2.0 V (min)

5.5 V (max)

I

DD

Quiescent Power Supply Current VIN= 0V, Io= 0A 6.5 10.0 mA (max)

I

SD

Shutdown Current V

PIN1=VDD

0.6 2 µA (max)

V

OS

Output Offset Voltage VIN= 0V 5 50 mV (max)

P

o

Output Power THD = 0.5%(max);f=1kHz 1.10 1.0 W (min)

THD+N = 10%;f=1kHz 1.5 W

THD+N Total Harmonic Distortion+Noise P

o

= 1 Wrms; AVD=2;20Hz≤f

≤20 kHz

0.25

%

PSRR Power Supply Rejection Ratio V

DD

= 4.9V to 5.1V 65 dB

Note 1: All voltages are measured with respect to the ground pin, unless otherwise specified.
Note 2:

Absolute Maximum Ratings

indicate limits beyond which damage to the device may occur.

Operating Ratings

indicate conditions for which the device is func-

tional, but do not guarantee specific performance limits.

Electrical Characteristics

state DC andAC 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 3: The maximum power dissipation must be derated at elevated temperatures and is dictated by T

JMAX

, θJA, and the ambient temperature TA. The maximum

allowable power dissipation is P

DMAX

=(T

JMAX–TA

)/θJAor the number given in Absolute Maximum Ratings, whichever is lower. For the LM4871, T

JMAX

= 150˚C. The

typical junction-to-ambient thermal resistance is 140˚C/W for package number M08A and is 107˚C/W for package number N08E.

Note 4: Human body model, 100 pF discharged through a 1.5 kΩ resistor.
Note 5: Machine Model, 220 pF–240 pF discharged through all pins.
Note 6: Typicals are measured at 25˚C and represent the parametric norm.
Note 7: Limits are guaranteed to National’s AOQL (Average Outgoing Quality Level).

LM4871

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External Components Description (

Figure 1

)

Components Functional Description

1. R

i

Inverting input resistance which sets the closed-loop gain in conjunction with Rf. This resistor also forms a
high pass filter with C

i

at fC= 1/(2π RiCi).

2. C

i

Input coupling capacitor which blocks the DC voltage at the amplifiers input terminals. Also creates a
highpass filter with R

i

at fc= 1/(2π RiCi). Refer to the section, Proper Selection of External Components,

for an explanation of how to determine the value of C

i

.

3. R

f

Feedback resistance which sets the closed-loop gain in conjunction with Ri.

4. C

S

Supply bypass capacitor which provides power supply filtering. Refer to the Power Supply Bypassing
section for information concerning proper placement and selection of the supply bypass capacitor.

5. C

B

Bypass pin capacitor which provides half-supply filtering. Refer to the section, Proper Selection of External
Components, for information concerning proper placement and selection of C

B

.

Typical Performance Characteristics

THD+N vs Frequency

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THD+N vs Frequency

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THD+N vs Frequency

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THD+N vs Output Power

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THD+N vs Output Power

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THD+N vs Output Power

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Output Power vs
Supply Voltage

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Output Power vs
Supply Voltage

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Output Power vs
Supply Voltage

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LM4871

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Typical Performance Characteristics (Continued)

Output Power vs
Load Resistance

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Power Dissipation vs
Output Power

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Power Derating Curve

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Clipping Voltage vs
Supply Voltage

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Noise Floor

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Frequency Response vs
Input Capacitor Size

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Power Supply
Rejection Ratio

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Open Loop
Frequency Response

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Supply Current vs
Supply Voltage

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LM4871

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Application Information

BRIDGE CONFIGURATION EXPLANATION

As shown in

Figure 1

, the LM4871 has two operational am­plifiers internally, allowing for a few different amplifier con­figurations. The first amplifier’s gain is externally config­urable, while the second amplifier is internally fixed in a
unity-gain, inverting configuration. The closed-loop gain of
the first amplifier is set by selecting the ratio of R

f

to Riwhile
the second amplifier’s gain is fixed by the two internal 40 kΩ
resistors.

Figure 1

shows that the output of amplifier one
serves as the input to amplifier two which results in both am­plifiers producing signals identical in magnitude, but out of
phase 180˚. Consequently, the differential gain for the IC is

A

VD

= 2 *(Rf/Ri)

By driving the load differentially through outputs Vo1 and
Vo2, an amplifier configuration commonly referred to as
“bridged mode” is established. Bridged mode operation is
different from the classical single-ended amplifier configura­tion where one side of its load is connected to ground.

A bridge amplifier design has a few distinct advantages over
the single-ended configuration, as it provides differential
drive to the load, thus doubling output swing for a specified
supply voltage. Four times the output power is possible as
compared to a single-ended amplifier under the same condi­tions. This increase in attainable output power assumes that
the amplifier is not current limited or clipped. In order to
choose an amplifier’s closed-loop gain without causing ex­cessive clipping, please refer to the Audio Power Amplifier
Design section.

A bridge configuration, such as the one used in LM4871,
also creates a second advantage over single-ended amplifi­ers. Since the differential outputs, Vo1 and Vo2, are biased
at half-supply,no net DC voltage exists across the load. This
eliminates the need for an output coupling capacitor which is
required in a single supply, single-ended amplifier configura­tion. Without an output coupling capacitor, the half-supply
bias across the load would result in both increased internal
IC power dissipation and also possible loudspeaker damage.

POWER DISSIPATION

Power dissipation is a major concern when designing a suc­cessful amplifier, whether the amplifier is bridged or single­ended. A direct consequence of the increased power deliv­ered to the load by a bridge amplifier is an increase in
internal power dissipation. Equation 1 states the maximum
power dissipation point for a bridge amplifier operating at a
given supply voltage and driving a specified output load.

P

DMAX

= 4*(VDD)2/(2π2RL) (1)

Since the LM4871 has two operational amplifiers in one
package, the maximum internal power dissipation is 4 times
that of a single-ended ampifier. Even with this substantial in­crease in power dissipation, the LM4871 does not require
heatsinking under most operating conditions and output
loading. From Equation 1, assuming a 5V power supply and
an 8Ω load, the maximum power dissipation point is
625 mW. The maximum power dissipation point obtained
from Equation 1 must not be greater than the power dissipa­tion that results from Equation 2:

P

DMAX

=(T

JMAX–TA

)/θ

JA

(2)

For package M08A, θ

JA

= 140˚C/W, and for package N08E,

θ

JA

= 107˚C/W assuming free air operation. T

JMAX

= 150˚C

for the LM4871. The θ

JA

can be decreased by using some

form of heat sinking. The resultant θ

JA

will be the summation

of the θ

JC

, θCS, and θSA. θJCis the junction to case of the

package, θ

CS

is the case to heat sink thermal resistance and

θ

SA

is the heat sink to ambient thermal resistance. By adding

additional copper area around the LM4871, the θ

JA

can be
reduced from its free air value of 140˚C/W for package
M08A. Depending on the ambient temperature, T

A

, and the

θ

JA

, Equation 2 can be used to find the maximum internal
power dissipation supported by the IC packaging. If the re­sult of Equation 1 is greater than that of Equation 2, then ei­ther the supply voltage must be decreased, the load imped­ance increased, the θ

JA

decreased, or the ambient
temperature reduced. For the typical application of a 5V
power supply, with an 8Ω load, and no additional heatsink­ing, the maximum ambient temperature possible without vio­lating the maximum junction temperature is approximately
61˚C provided that device operation is around the maximum
power dissipation point and assuming surface mount pack­aging. Internal power dissipation is a function of output
power. If typical operation is not around the maximum power
dissipation point, the ambient temperature can be increased.
Refer to the Typical Performance Characteristics curves
for power dissipation information for different output powers
and output loading.

POWER SUPPLY BYPASSING

As with any amplifier, proper supply bypassing is critical for
low noise performance and high power supply rejection. The
capacitor location on both the bypass and power supply pins
should be as close to the device as possible. Typicalapplica­tions employ a 5V regulator with 10 µF and a 0.1 µF bypass
capacitors which aid in supply stability. This does not elimi­nate the need for bypassing the supply nodes of the
LM4871. The selection of bypass capacitors, especially C

B

,
is dependent upon PSRR requirements, click and pop per­formance as explained in the section, Proper Selection of

External Components, system cost, and size constraints.

SHUTDOWN FUNCTION

In order to reduce power consumption while not in use, the
LM4871 contains a shutdown pin to externally turn off the
amplifier’s bias circuitry.This shutdown feature turns the am­plifier off when a logic high is placed on the shutdown pin.
The trigger point between a logic low and logic high level is
typically half- supply. It is best to switch between ground and
supply to provide maximum device performance. By switch­ing the shutdown pin to V

DD

, the LM4871 supply current
draw will be minimized in idle mode. While the device will be
disabled with shutdown pin voltages less then V

DD

, the idle
current may be greater than the typical value of 0.6 µA. In ei­ther case, the shutdown pin should be tied to a definite volt­age to avoid unwanted state changes.

In many applications, a microcontroller or microprocessor
output is used to control the shutdown circuitry which pro­vides a quick, smooth transition into shutdown. Another solu­tion is to use a single-pole, single-throw switch in conjunction
with an external pull-up resistor. When the switch is closed,
the shutdown pin is connected to ground and enables the
amplifier. If the switch is open, then the external pull-up re­sistor will disable the LM4871. This scheme guarantees that
the shutdown pin will not float thus preventing unwanted
state changes.

PROPER SELECTION OF EXTERNAL COMPONENTS

Proper selection of external components in applications us­ing integrated power amplifiers is critical to optimize device
and system performance. While the LM4871 is tolerant of

LM4871

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Application Information (Continued)

external component combinations, consideration to compo­nent values must be used to maximize overall system qual­ity.

The LM4871 is unity-gain stable which gives a designer
maximum system flexibility. The LM4871 should be used in
low gain configurations to minimize THD+N values, and
maximize the signal to noise ratio. Low gain configurations
require large input signals to obtain a given output power. In­put signals equal to or greater than 1 Vrms are available
from sources such as audio codecs. Please refer to the sec­tion, Audio Power Amplifier Design, for a more complete
explanation of proper gain selection.

Besides gain, one of the major considerations is the closed­loop bandwidth of the amplifier. To a large extent, the band­width is dictated by the choice of external components
shown in

Figure 1

. The input coupling capacitor, Ci, forms a
first order high pass filter which limits low frequency re­sponse. This value should be chosen based on needed fre­quency response for a few distinct reasons.

Selection Of Input Capacitor Size

Large input capacitors are both expensive and space hungry
for portable designs. Clearly, a certain sized capacitor is
needed to couple in low frequencies without severe attenua­tion. But in many cases the speakers used in portable sys­tems, whether internal or external, have little ability to repro­duce signals below 100 Hz to 150 Hz. Thus, using a large
input capacitor may not increase actual system perfor­mance.

In addition to system cost and size, click and pop perfor­mance is effected by the size of the input coupling capacitor,
C

i.

A larger input coupling capacitor requires more charge to

reach its quiescent DC voltage (nominally 1/2 V

DD

). This
charge comes from the output via the feedback and is apt to
create pops upon device enable. Thus, by minimizing the ca­pacitor size based on necessary low frequency response,
turn-on pops can be minimized.

Besides minimizing the input capacitor size, careful consid­eration should be paid to the bypass capacitor value. Bypass
capacitor, C

B

, is the most critical component to minimize
turn-on pops since it determines how fast the LM4871 turns
on. The slower the LM4871’s outputs ramp to their quiescent
DC voltage (nominally 1/2 V

DD

), the smaller the turn-on pop.

Choosing C

B

equal to 1.0 µF along with a small value of C

i

(in the range of 0.1 µF to 0.39 µF), should produce a virtually
clickless and popless shutdown function. While the device
will function properly, (no oscillations or motorboating), with
C

B

equal to 0.1 µF,the device will be much more susceptible

to turn-on clicks and pops. Thus, a value of C

B

equal to

1.0 µF is recommended in all but the most cost sensitive de­signs.

AUDIO POWER AMPLIFIER DESIGN
Design a 1W/8Ω Audio Amplifier

Given:

Power Output 1 Wrms
Load Impedance 8Ω
Input Level 1 Vrms
Input Impedance 20 kΩ
Bandwidth 100 Hz–20 kHz

±

0.25 dB

A designer must first determine the minimum supply rail to
obtain the specified output power. By extrapolating from the
Output Power vs Supply Voltage graphs in the Typical Per-
formance Characteristics section, the supply rail can be
easily found. A second way to determine the minimum sup­ply rail is to calculate the required V

opeak

using Equation 3
and add the output voltage. Using this method, the minimum
supply voltage would be (V

opeak

+(V

OD

TOP

+V

OD

BOT

)), where

V

OD

BOT

and V

OD

TOP

are extrapolated from the Dropout Volt­age vs Supply Voltage curve in the Typical Performance
Characteristics section.

(3)

Using the Output Power vs Supply Voltage graph for an 8Ω
load, the minimum supply rail is 4.6V. But since 5V is a stan­dard voltage in most applications, it is chosen for the supply
rail. Extra supply voltage creates headroom that allows the
LM4871 to reproduce peaks in excess of 1W without produc­ing audible distortion. At this time, the designer must make
sure that the power supply choice along with the output im­pedance does not violate the conditions explained in the
Power Dissipation section.

Once the power dissipation equations have been addressed,
the required differential gain can be determined from Equa­tion 4.

(4)

R

f/Ri

=

A

VD

/2 (5)

From Equation 4, the minimum A

VD

is 2.83; use AVD=3.

Since the desired input impedance was 20 kΩ, and with a
A

VD

impedance of 2, a ratio of 1.5:1 of Rfto Riresults in an

allocation of R

i

=20kΩand Rf=30kΩ. The final design step
is to address the bandwidth requirements which must be
stated as a pair of −3 dB frequency points. Five times away
from a −3 dB point is 0.17 dB down from passband response
which is better than the required

±

0.25 dB specified.

f

L

= 100 Hz/5 = 20 Hz

f

H

=20kHz*5=100kHz

As stated in the External Components section, R

i

in con-

junction with C

i

create a highpass filter.

C

i

≥ 1/(2π*20 kΩ*20 Hz) = 0.397 µF; use 0.39 µF

The high frequency pole is determined by the product of the
desired frequency pole, f

H

, and the differential gain, AVD.

With a A

VD

= 3 and fH= 100 kHz, the resulting GBWP =
150 kHz which is much smaller than the LM4871 GBWP of
4 MHz. This figure displays that if a designer has a need to
design an amplifier with a higher differential gain, the
LM4871 can still be used without running into bandwidth limi­tations.

LM4871

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Physical Dimensions inches (millimeters) unless otherwise noted

Order Number LM4871M

NS Package Number M08A

Order Number LM4871N

NS Package Number N08E

LM4871

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Notes

LIFE SUPPORT POLICY

NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT
DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL
COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:

1. Life support devices or systems are devices or
systems which, (a) are intended for surgical implant
into the body, or (b) support or sustain life, and
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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LM4871 1.1W Audio Power Amplifier with Shutdown Mode

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.