LM4860
Series 1W Audio Power Amplifier with Shutdown Mode

General Description

The LM4860 is a bridge-connected audio power amplifier ca-
pable of delivering 1W of continuous average power to an
8

load with less than 1% (THD+N) over the audio spectrum

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 using surface mount packaging. Since
the LM4860 does not require output coupling capacitors,
bootstrap capacitors or snubber networks, it is optimally
suited for low-power portable systems.

The LM4860 features an externally controlled, low-power
consumption shutdown mode, as well as an internal thermal
shutdown protection mechanism. It also includes two head-
phone control inputs and a headphone sense output for ex-
ternal monitoring.

The unity-gain stable LM4860 can be configured by external
gain setting resistors for differential gains of 1 to 10 without
the use of external compensation components.

Key Specifications

THD+N at 1W continuous average

output power into 8

1% (max)

Instantaneous peak output power:

Shutdown current:

0.6 µA (typ)

Features

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

Small Outline (SO) power packaging

Compatible with PC power supplies

Thermal shutdown protection circuitry

Unity-gain stable

External gain configuration capability

Two headphone control inputs and headphone sensing
output

Applications

Personal computers

Portable consumer products

Cellular phones

Self-powered speakers

Toys and games

Typical Application

Connection Diagram

The Boomer

registered trademark is licensed to National Semiconductor for audio integrated circuits by Rockford Corporation.

Patents pending.

DS011988-1

FIGURE 1. Typical Audio Amplifier Application Circuit

Small Outline Package

DS011988-2

Top View

Order Number LM4860M

See NS Package Number M16A

August 1994

LM4860

Audio

Power

Amplifier

with

Shutdown

Mode

DS011988

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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

Storage Temperature

-65°C to +150°C

Input Voltage

-0.3V to V

+ 0.3V

Power Dissipation

Internally limited

ESD Susceptibility (Note 4)

3000V

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 de-
vices.

Operating Ratings

Temperature Range

MIN

MAX

-20°C

+85°C

Supply Voltage

2.7V

5.5V

Electrical Characteristics

(Notes 1, 2) The following specifications apply for V

= 5V, R

= 8

unless otherwise specified. Limits apply for T

= 25°C.

Symbol

Parameter

Conditions

LM4860

Units

(Limits)

Typical

Limit

(Note 6)

(Note 7)

Supply Voltage

2.7

V (min)

5.5

V (max)

Quiescent Power Supply Current

= 0V, I

= 0A (Note 8)

7.0

15.0

mA (max)

Shutdown Current

pin2

= V

(Note 9)

0.6

µA

Output Offset Voltage

= 0V

5.0

50.0

mV (max)

Output Power

THD+N = 1% (max); f = 1 kHz

1.15

1.0

W (min)

THD+N

Total Harmonic Distortion + Noise

= 1 Wrms; 20 Hz

20 kHz

0.72

PSRR

Power Supply Rejection Ratio

= 4.9V to 5.1V

Output Dropout Voltage

= 0V to 5V, V

= (V

- V

)

0.6

1.0

V (max)

HP-IN High Input Voltage

HP-SENSE = 0V to 4V

2.5

HP-IN Low Input Voltage

HP-SENSE = 4V to 0V

2.5

HP-SENSE High Output Voltage

= 500 µA

2.8

2.5

V (min)

HP-SENSE Low Output Voltage

= -500 µA

0.2

0.8

V (max)

Note 1: All voltages are measured with respect to the ground pins, 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

- T

or the number given in the Absolute Maximum Ratings, whichever is lower. For the LM4860, T

JMAX

+150°C, and the typical junction-to-ambient thermal resistance, when board mounted, is 100°C/W.

Note 4: Human body model, 100 pF discharged through a 1.5 k

resistor.

Note 5: Machine Model, 200 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).

Note 8: The quiescent power supply current depends on the offset voltage when a practical load is connected to the amplifier.

Note 9: Shutdown current has a wide distribution. For Power Management sensitive designs, contact your local National Semiconductor Sales Office.

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Typical Performance Characteristics

(Continued)

Application Information

BRIDGE CONFIGURATION EXPLANATION

As shown in

Figure 1, the LM4860 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

)

By driving the load differentially through outputs V

and

, 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

LM4860 Noise Floor
vs Frequency

DS011988-18

Supply Current Distribution
vs Temperature

DS011988-19

Power Dissipation
vs Output Power

DS011988-20

Output Power vs
Load Resistance

DS011988-21

Output Power vs
Supply Voltage

DS011988-22

Open Loop
Frequency Response

DS011988-23

Power Supply
Rejection Ratio

DS011988-24

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Application Information

(Continued)

supply voltage. Consequently, 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 caus-
ing excessive clipping which will damage high frequency
transducers used in loudspeaker systems, please refer to
the Audio Power Amplifier Deslgn section.

A bridge configuration, such as the one used in Boomer Au-
dio Power Amplifiers, also creates a second advantage over
single-ended amplifiers. Since the differential outputs, V

and V

, are biased at half-supply, no net DC voltage exists

across the load. This eliminates the need for an output cou-
pling capacitor which is required in a single supply,
single-ended amplifier configuration. Without an output cou-
pling capacitor in a single supply single-ended amplifier, the
half-supply bias across the load would result in both in-
creased internal IC power dissipation and also permanent
loudspeaker damage. An output coupling capacitor forms a
high pass filter with the load requiring that a large value such
as 470 µF be used with an 8

load to preserve low fre-

quency response. This combination does not produce a flat
response down to 20 Hz, but does offer a compromise be-
tween printed circuit board size and system cost, versus low
frequency response.

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
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.

DMAX

= 4

)

/(2

(1)

Since the LM4860 has two operational amplifiers in one
package, the maximum internal power dissipation is 4 times
that of a single-ended amplifier. Even with this substantial in-
crease in power dissipation, the LM4860 does not require
heatsinking. 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 dissipation
that results from Equation 2:

DMAX

= (T

JMAX

- T

(2)

For the LM4860 surface mount package,

= 100°C/W and

JMAX

= 150°C. Depending on the ambient temperature, T

of the system surroundings, 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 de-
creased or the load impedance increased. For the typical ap-
plication of a 5V power supply, with an 8

load, the maxi-

mum ambient temperature possible without violating the
maximum junction temperature is approximately 88°C, pro-
vided that device operation is around the maximum power
dissipation point. Power dissipation is a function of output
power and thus, if typical operation is not around the maxi-
mum power dissipation point, the ambient temperature can
be increased. Refer to the Typical Performance Character-
istics curves for power dissipation information for lower out-
put powers.

POWER SUPPLY BYPASSING

As with any power amplifier, proper supply bypassing is criti-
cal for low noise performance and high power supply rejec-
tion. The capacitor location on both the bypass and power
supply pins should be as close to the device as possible. As
displayed in the Typical Performance CharacterIstIcs sec-
tion, the effect of a larger half-supply bypass capacitor is im-
proved low frequency THD+N due to increased half-supply
stability. Typical applications employ a 5V regulator with
10 µF and a 0.1 µF bypass capacitors which aid in supply
stability, but do not eliminate the need for bypassing the sup-
ply nodes of the LM4860. The selection of bypass capaci-
tors, especially C

, is thus dependant upon desired low fre-

quency THD+N, system cost, and size constraints.

SHUTDOWN FUNCTION

In order to reduce power consumption while not in use, the
LM4860 contains a shutdown pin to externally turn off the
amplifier's bias circuitry. The shutdown feature turns the am-
plifier off when a logic high is placed on the shutdown pin.
Upon going into shutdown, the output is immediately discon-
nected from the speaker. There is a built-in threshold which
produces a drop in quiescent current to 500 µA typically. For
a 5V power supply, this threshold occurs when 2V3V is ap-
plied to the shutdown pin. A typical quiescent current of
0.6 µA results when the supply voltage is applied to the shut-
down pin. In many applications, a microcontroller or micro-
processor output is used to control the shutdown circuitry
which provides a quick, smooth transition into shutdown. An-
other solution is to use a single-pole, single-throw switch that
when closed, is connected to ground and enables the ampli-
fier. If the switch is open, then a soft pull-up resistor of 47 k

will disable the LM4860. There are no soft pull-down resis-
tors inside the LM4860, so a definite shutdown pin voltage
must be appliied externally, or the internal logic gate will be
left floating which could disable the amplifier unexpectedly.

HEADPHONE CONTROL INPUTS

The LM4860 possesses two headphone control inputs that
disable the amplifier and reduce I

to less than 1 mA when

either one or both of these inputs have a logic-high voltage
placed on their pins.

Unlike the shutdown function, the headphone control func-
tion does not provide the level of current conservation that is
required for battery powered systems. Since the quiescent
current resulting from the headphone control function is
1000 times more than the shutdown function, the residual
currents in the device may create a pop at the output when
coming out of the headphone control mode. The pop effect
may be eliminated by connecting the headphone sensing
output to the shutdown pin input as shown in

Figure 4. This

solution will not only eliminate the output pop, but will also
utilize the full current conservation of the shutdown function
by reducing I

to 0.6 µA. The amplifier will then be fully

shutdown. This configuration also allows the designer to use
the control inputs as either two headphone control pins or a
headphone control pin and a shutdown pin where the lowest
level of current consumption is obtained from either function.

Figure 5 shows the implementation of the LM4860's head-
phone control function using a single-supply headphone am-
plifier. The voltage divider of R1 and R2 sets the voltage at
the HP-IN1 pin to be approximately 50 mV when there are
no headphones plugged into the system. This logic-low volt-
age at the HP-IN1 pin enables the LM4860 to amplify AC sig-
nals. Resistor R3 limits the amount of current flowing out of
the HP-IN1 pin when the voltage at that pin goes below

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Application Information

(Continued)

ground resulting from the music coming from the headphone
amplifier. The output coupling cap protects the headphones
by blocking the amplifier's half-supply DC voltage. The ca-
pacitor also protects the headphone amplifier from the low
voltage set up by resistors R1 and R2 when there aren't any
headphones plugged into the system. The tricky point to this
setup is that the AC output voltage of the headphone ampli-
fier cannot exceed the 2.0V HP-IN1 voltage threshold when
there aren't any headphones plugged into the system, as-
suming that R1 and R2 are 100k and 1k, respectively. The
LM4860 may not be fully shutdown when this level is ex-
ceeded momentarily, due to the discharging time constant of
the bias-pin voltage. This time constant is established by the
two 50k resistors (in parallel) with the series bypass capaci-
tor value.

When a set of headphones are plugged into the system, the
contact pin of the headphone jack is disconnected from the
signal pin, interrupting the voltage divider set up by resistors
R1 and R2. Resistor R1 then pulls up the HP-IN1 pin, en-
abling the headphone function and disabling the LM4860
amplifier. The headphone amplifier then drives the head-
phones, whose impedance is in parallel with resistor R2.
Since the typical impedance of headphones are 32

, resis-

tor R2 has negligible effect on the output drive capability.
Also shown in

Figure 5 are the electrical connections for the

headphone jack and plug. A 3-wire plug consists of a Tip,
Ring, and Sleave, where the Tip and Ring are signal carrying
conductors and the Sleave is the common ground return.
One control pin contact for each headphone jack is sufficient

to indicate to control inputs that the user has inserted a plug
into a jack and that another mode of operation is desired.

For a system implementation where the headphone amplifier
is designed using a split supply, the output coupling cap, C

and resistor R2 of

Figure 5, can be eliminated. The function-

ality described earlier remains the same, however.

In addition, the HP-SENSE pin, although it may be con-
nected to the SHUTDOWN pin as shown in

Figure 4, may

still be used as a control flag. It is capable of driving the input
to another logic gate or approximately 2 mA without serious
loading.

DS011988-7

FIGURE 4. HP-SENSE Pin to
SHUTDOWN Pin Connection

DS011988-8

FIGURE 5. Typical Headphone Control Input Circuitry

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Application Information

(Continued)

HIGHER GAIN AUDIO AMPLIFIER

The LM4860 is unity-gain stable and requires no external
components besides gain-setting resistors, an input coupling
capacitor, and proper supply bypassing in the typical appli-
cation. However if a closed-loop differential gain of greater
than 10 is required, then a feedback capacitor is needed, as
shown in

Figure 2, to bandwidth limit the amplifier. The feed-

back capacitor creates a low pass filter that eliminates un-
wanted high frequency oscillations. Care should be taken
when calculating the -3 dB frequency in that an incorrect
combination of R

and C

will cause rolloff before 20 kHz. A

typical combination of feedback resistor and capacitor that
will not produce audio band high frequency rolloff is R

100 k

and C

= 5 pF. These components result in a -3 dB

point of approximately 320 kHz. Once the differential gain of
the amplifier has been calculated, a choice of R

will result,

and C

can then be calculated from the formula stated in the

External Components Description section.

VOICE-BAND AUDIO AMPLIFIER

Many applications, such as telephony, only require a
voice-band frequency response. Such an application usually
requires a flat frequency response from 300 Hz to 3.5 kHz.
By adjusting the component values of

Figure 2, this common

application requirement can be implemented. The combina-
tion of R

and C

form a highpass filter while R

and C

form a

lowpass filter. Using the typical voice-band frequency range,
with a passband differential gain of approximately 100, the
following values of R

, C

, R

, and C

follow from the equa-

tions stated in the External Components Description sec-
tion.

= 10 k

, R

= 510k, C

= 0.22 µF, and C

= 15 pF

Five times away from a -3 dB point is 0.17 dB down from the
flatband response. With this selection of components, the re-
sulting -3 dB points, f

and f

, are 72 Hz and 20 kHz, re-

spectively, resulting in a flatband frequency response of bet-
ter than

0.25 dB with a rolloff of 6 dB/octave outside of the

passband. If a steeper rolloff is required, other common
bandpass filtering techniques can be used to achieve higher
order filters.

SINGLE-ENDED AUDIO AMPLIFIER

Although the typical application for the LM4860 is a bridged
monoaural amp, it can also be used to drive a load
single-endedly in applications, such as PC cards, which re-
quire that one side of the load is tied to ground.

Figure 3

shows a common single-ended application, where V

used to drive the speaker. This output is coupled through a
470 µF capacitor, which blocks the half-supply DC bias that
exists in all single-supply amplifier configurations. This ca-
pacitor, designated C

Figure 3, in conjunction with R

forms a highpass filter. The -3 dB point of this highpass filter
is 1/(2

), so care should be taken to make sure that the

product of R

and C

is large enough to pass low frequen-

cies to the load. When driving an 8

load, and if a full audio

spectrum reproduction is required, C

should be at least

470 µF. V

, the output that is not used, is connected

through a 0.1 µF capacitor to a 2 k

load to prevent instabil-

ity. While such an instability will not affect the waveform of
V

, it is good design practice to load the second output.

AUDIO POWER AMPLIFIER DESIGN

Design a 500 mW/8

Audio Amplifier

Given:

Power Output:

500 mWrms

Load Impedance:

Input Level:

1 Vrms(max)

Input Impedance:

20 k

Bandwidth:

20 Hz-20 kHz

0.25 dB

A designer must first determine the needed supply rail to ob-
tain the specified output power. Calculating the required sup-
ply rail involves knowing two parameters, V

opeak

and also the

dropout voltage. The latter is typically 0.7V. V

opeak

can be

determined from equation 3.

For 500 mW of output power into an 8

load, the required

opeak

is 2.83V. A minimum supply rail of 3.53V results from

adding V

opeak

and V

. But 3.53V is not a standard voltage

that exists in many applications and for this reason, a supply
rail of 5V is designated. Extra supply voltage creates dy-
namic headroom that allows the LM4860 to reproduce peaks
in excess of 500 mW without clipping the signal. At this time,
the designer must make sure that the power supply choice
along with the output impedance does not violate the condi-
tions explained in the Power Dissipation section.

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

From equation 4, the minimum A

is:

= 2

Since the desired input impedance was 20 k

, and with an

of 2, a ratio of 1:1 of R

to R

results in an allocation of

= R

= 20 k

. Since the A

was less than 10, a feedback

capacitor is not needed. 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 bet-
ter than the required

0.25 dB specified. This fact results in

a low and high frequency pole of 4 Hz and 100 kHz respec-
tively. As stated in the External Components section, R

conjunction with C

create a highpass filter.

1/(2

20 k

4 Hz) = 1.98 µF;

use 2.2 µF.

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

, and the differential gain, A

With a A

= 2 and f

= 100 kHz, the resulting GBWP =

100 kHz which is much smaller than the LM4860 GBWP of
7 MHz. This figure displays that if a designer has a need to
design an amplifier with a higher differential gain, the
LM4860 can still be used without running into bandwidth
problems.

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Physical Dimensions

inches (millimeters) unless otherwise noted

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.

National Semiconductor
Corporation
Americas
Tel: 1-800-272-9959
Fax: 1-800-737-7018
Email: support@nsc.com

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Fax: 81-3-5639-7507

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Small Outline Package (M)

Order Number LM4860M

NS Package Number M16A

LM4860

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.

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