LM4871
1.1W Audio Power Amplifier with Shutdown Mode

General Description

The LM4871 is a bridge-connected audio power amplifier ca-
pable of delivering typically 1.1W of continuous average
power to an 8

load with 0.5% (THD) from a 5V power sup-

ply.

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 sys-
tems.

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

THD at 1 kHz at 1W continuous

average output power into 8

0.5% (max)

Output power at 10% THD+N

at 1 kHz into 8

1.5W (typ)

Shutdown Current

0.6 µA (typ)

Features

No output coupling capacitors, bootstrap capacitors, or
snubber circuits are necessary

Small Outline or DIP packaging

Unity-gain stable

External gain configuration capability

Pin compatible with LM4861

Applications

Portable Computers

Desktop Computers

Low Voltage Audio Systems

Typical Application

Connection Diagram

Boomer

is a registered trademark of National Semiconductor Corporation.

DS100008-1

FIGURE 1. Typical Audio Amplifier Application Circuit

Small Outline and DIP Package

DS100008-2

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

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

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.

(typ) -- M08A

35°C/W

(typ) -- M08A

140°C/W

(typ) -- N08E

37°C/W

(typ) -- N08E

107°C/W

Operating Ratings

Temperature Range

MIN

MAX

-40°C

85°C

Supply Voltage

2.0V

5.5V

Electrical Characteristics

(Notes 1, 2)

The following specifications apply for V

= 5V unless otherwise specified. Limits apply for T

= 25°C.

Symbol

Parameter

Conditions

LM4871

Units

(Limits)

Typical

Limit

(Note 6)

(Note 7)

Supply Voltage

2.0

V (min)

5.5

V (max)

Quiescent Power Supply Current

= 0V, I

= 0A

6.5

10.0

mA (max)

Shutdown Current

PIN1

= V

0.6

µA (max)

Output Offset Voltage

= 0V

mV (max)

Output Power

THD = 0.5% (max); f = 1 kHz

1.10

1.0

W (min)

THD+N = 10%; f = 1 kHz

1.5

THD+N

Total Harmonic Distortion+Noise

= 1 Wrms; A

= 2; 20 Hz

20 kHz

0.25

PSRR

Power Supply Rejection Ratio

= 4.9V to 5.1V

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 and AC electrical specifications under particular test conditions which guar-

antee 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

, and the ambient temperature T

. The maximum

allowable power dissipation is P

DMAX

= (T

JMAX

or 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 k

resistor.

Note 5: Machine Model, 220 pF240 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

www.national.com

External Components Description

(

Figure 1)

Components

Functional Description

Inverting input resistance which sets the closed-loop gain in conjunction with R

. This resistor also forms a

high pass filter with C

at f

= 1/(2

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

at f

= 1/(2

). Refer to the section, Proper Selection of External Components,

for an explanation of how to determine the value of C

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

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.

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

Typical Performance Characteristics

THD+N vs Frequency

DS100008-3

THD+N vs Frequency

DS100008-4

THD+N vs Frequency

DS100008-5

THD+N vs Output Power

DS100008-6

THD+N vs Output Power

DS100008-7

THD+N vs Output Power

DS100008-8

Output Power vs
Supply Voltage

DS100008-9

Output Power vs
Supply Voltage

DS100008-10

Output Power vs
Supply Voltage

DS100008-11

LM4871

www.national.com

Application Information

BRIDGE CONFIGURATION EXPLANATION

As shown in

Figure 1, the LM4871 has two operational am-

plifiers internally, allowing for a few different amplifier con-
figurations. The first amplifier's gain is externally config-
urable, 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

to R

while

the second amplifier's gain is fixed by the two internal 40 k

resistors.

Figure 1 shows that the output of amplifier one

serves as the input to amplifier two which results in both am-
plifiers producing signals identical in magnitude, but out of
phase 180°. Consequently, the differential gain for the IC is

= 2 *(R

)

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 configura-
tion 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 condi-
tions. 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 ex-
cessive 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 amplifi-
ers. 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 configura-
tion. 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 suc-
cessful amplifier, whether the amplifier is bridged or single-
ended. A direct consequence of the increased power deliv-
ered 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.

DMAX

= 4*(V

)

/(2

)

(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 in-
crease 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 8

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 dissipa-
tion that results from Equation 2:

DMAX

= (T

JMAX

(2)

For package M08A,

= 140°C/W, and for package N08E,

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

JMAX

= 150°C

for the LM4871. The

can be decreased by using some

form of heat sinking. The resultant

will be the summation

of the

, and

is the junction to case of the

package,

is the case to heat sink thermal resistance and

is the heat sink to ambient thermal resistance. By adding

additional copper area around the LM4871, the

can be

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

, and the

, Equation 2 can be used to find the maximum internal

power dissipation supported by the IC packaging. If the re-
sult of Equation 1 is greater than that of Equation 2, then ei-
ther the supply voltage must be decreased, the load imped-
ance increased, the

decreased, or the ambient

temperature reduced. For the typical application of a 5V
power supply, with an 8

load, and no additional heatsink-

ing, the maximum ambient temperature possible without vio-
lating the maximum junction temperature is approximately
61°C provided that device operation is around the maximum
power dissipation point and assuming surface mount pack-
aging. 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. Typical applica-
tions employ a 5V regulator with 10 µF and a 0.1 µF bypass
capacitors which aid in supply stability. This does not elimi-
nate the need for bypassing the supply nodes of the
LM4871. The selection of bypass capacitors, especially C

is dependent upon PSRR requirements, click and pop per-
formance 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 am-
plifier 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 switch-
ing the shutdown pin to V

, the LM4871 supply current

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

, the idle

current may be greater than the typical value of 0.6 µA. In ei-
ther case, the shutdown pin should be tied to a definite volt-
age to avoid unwanted state changes.

In many applications, a microcontroller or microprocessor
output is used to control the shutdown circuitry which pro-
vides a quick, smooth transition into shutdown. Another solu-
tion 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 re-
sistor 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 us-
ing integrated power amplifiers is critical to optimize device
and system performance. While the LM4871 is tolerant of

LM4871

Ð­Ð»ÐµÐºÑ‚Ñ€Ð¾Ð½Ð½Ñ‹Ð¹ ÐºÐ¾Ð¼Ð¿Ð¾Ð½ÐµÐ½Ñ‚: LM4871

ÐÐ»ÐµÐºÑ‚Ñ€Ð¾Ð½Ð½Ñ‹Ð¹ ÐºÐ¾Ð¼Ð¿Ð¾Ð½ÐµÐ½Ñ‚: LM4871