LM4880
Dual 250 mW Audio Power Amplifier with Shutdown
Mode

General Description

The LM4880 is a dual audio power amplifier capable of deliv-
ering typically 250 mW per channel of continuous average
power to an 8

load with 0.1% (THD) using 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 using surface mount packaging.

Since the LM4880 does not require bootstrap capacitors or
snubber networks, it is optimally suited for low-power por-
table systems.

The LM4880 features an externally controlled, low-power
consumption shutdown mode, as well as an internal thermal
shutdown protection mechanism.

The unity-gain stable LM4880 can be configured by external
gain-setting resistors.

Key Specifications

THD at 1 kHz at 200 mW continuous average output
power into 8

0.1% (max)

THD at 1 kHz at 85 mW continuous average output
power into 32

0.1% (typ)

Output power at 10% THD + N at 1 kHz into 8

325 mW (typ)

Shutdown Current:

0.7 µA (typ)

Features

No bootstrap capacitors or snubber circuits are
necessary

Small Outline (SO) and DIP packaging

Unity-gain stable

External gain configuration capability

Applications

Headphone Amplifier

Personal Computers

CD-ROM Players

Typical Application

Boomer

is a registered trademark of National Semiconductor Corporation.

DS012343-1

Refer to the Application Information section for information concerning proper selection of the input and output coupling capacitors.

FIGURE 1. Typical Audio Amplifier Application Circuit

November 1995

LM4880

Boomer

Audio

Power

Amplifier

Series

Dual

250

Audio

Power

Amplifier

with

Shutdown

Mode

DS012343

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

Storage Temperature

-65°C to +150°C

Input Voltage

-0.3V to V

+ 0.3V

Power Dissipation (Note 3)

Internally limited

ESD Susceptibility (Note 4)

3500V

ESD Susceptibility (Note 5)

250V

Junction Temperature

150°C

Soldering Information

Small Outline Package

Vapor Phase (60 sec.)
Infrared (15 sec.)

215°C
220°C

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

Thermal Resistance

(DIP)

37°C/W

(DIP)

107°C/W

(SO)

35°C/W

(SO)

170°C/W

Operating Ratings

Temperature Range

MIN

MAX

-40°C

+85°C

Supply Voltage

2.7V

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

LM4880

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

3.6

6.0

(max)

Shutdown Current

PIN5

0.7

µA

(max)

Output Offset Voltage

=0V

(max)

Output Power

THD=0.1% (max); f=1 kHz;

250

200

(min)

=32

THD+N=10%; f=1 kHz

325

=32

110

THD+N

Total Harmonic Distortion+Noise

, P

=200 mW;

0.03

=32

, P

=75 mW;

0.02

f=1 kHz

PSRR

Power Supply Rejection Ratio

= 1.0 µF,

RIPPLE

=200 mVrms, f = 100 Hz

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 may occur. Operating Ratings indicate conditions for which the device is functional, but do
not guarantee specific performance limits. Electrical Characteristics state DC and AC 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

, 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 LM4880, T

JMAX

= 150°C,

and the typical junction-to-ambient thermal resistance is 170°C/W for package M08A and 107°C/W for package 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).

www.national.com

Typical Performance Characteristics

(Continued)

Application Information

SHUTDOWN FUNCTION

In order to reduce power consumption while not in use, the
LM4880 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
the supply to provide maximum device performance. By
switching the shutdown pin to V

, the LM4880 supply cur-

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

the idle current may be greater than the typical value of 0.7
µA. In either case, the shutdown pin should be tied to a defi-
nite voltage because leaving the pin floating may result in an
unwanted shutdown condition.

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 LM4880. This scheme guarantees that
the shutdown pin will not float which will prevent unwanted
state changes.

POWER DISSIPATION

Power dissipation is a major concern when using any power
amplifier and must be thoroughly understood to ensure a
successful design.

Equation (1) states the maximum power

dissipation point for a single-ended amplifier operating at a
given supply voltage and driving a specified output load.

DMAX

= (V

)

/(2

)

(1)

Since the LM4880 has two operational amplifiers in one
package, the maximum internal power dissipation point is
twice that of the number which results from

Equation (1).

Even with the large internal power dissipation, the LM4880
does not require heat sinking over a large range of ambient
temperatures. From

Equation (1), assuming a 5V power sup-

ply and an 8

load, the maximum power dissipation point is

158 mW per amplifier. Thus the maximum package dissipa-
tion point is 317 mW. The maximum power dissipation point
obtained must not be greater than the power dissipation that
results from

Equation (2):

DMAX

= (T

JMAX

-T

(2)

For the LM4880 surface mount package,

= 170° C/W

and T

JMAX

= 150°C. Depending on the ambient temperature,

, 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

decreased, the load impedance increased, or the ambient
temperature reduced. For the typical application of a 5V
power supply, with an 8

load, the maximum ambient tem-

perature possible without violating the maximum junction
temperature is approximately 96°C provided that device op-
eration is around the maximum power dissipation point.
Power dissipation is a function of output power and thus, if
typical operation is not around the maximum power dissipa-
tion point, the ambient temperature may be increased ac-
cordingly. Refer to the Typical Performance Characteris-
tics curves for power dissipation information for lower output
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 PSRR 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 LM4880. The selection of bypass capaci-
tors, especially C

, is thus dependant upon desired low fre-

quency PSRR, click and pop performance as explained in
the section, Proper Selection of External Components
section, system cost, and size constraints.

AUTOMATIC SHUTDOWN CIRCUIT

As shown in

Figure 2, the LM4880 can be set up to automati-

cally shutdown when a load is not connected. This circuit is
based upon a single control pin common in many head-
phone jacks. This control pin forms a normally closed switch
with one of the output pins. The output of this circuit (the volt-
age on pin 5 of the LM4880) has two states based on the
state of the switch. When the switch is open, signifying that
headphones are inserted, the LM4880 should be enabled.
When the switch is closed, the LM4880 should be off to mini-
mize power consumption.

Typical Application
Frequency Response

DS012343-32

Typical Application
Frequency Response

DS012343-33

Power Derating Curve

DS012343-34

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Application Information

(Continued)

The operation of this circuit is rather simple. With the switch
closed, R

and R

form a resistor divider which produces a

gate voltage of less than 5 mV. This gate voltage keeps the
NMOS inverter off and R

pulls the shutdown pin of the

LM4880 to the supply voltage. This places the LM4880 in
shutdown mode which reduces the supply current to 0.7 µA
typically. When the switch is open, the opposite condition is
produced. Resistor R

pulls the gate of the NMOS high

which turns on the inverter and produces a logic low signal
on the shutdown pin of the LM4880. This state enables the
LM4880 and places the amplifier in its normal mode of op-
eration.

This type of circuit is clearly valuable in portable products
where battery life is critical, but is also benefical for power
conscious designs such as "Green PC's".

AUTOMATIC SWITCHING CIRCUIT

A circuit closely related to the Automatic Shutdown Circuit
is the Automatic Switching Circuit of

Figure 3. The Auto-

matic Switching Circuit utilizes both the input and output of
the NMOS inverter to toggle the states of two different audio
power amplifiers. The LM4880 is used to drive stereo single
ended loads, while the LM4861 drives bridged internal
speakers.

In this application, the LM4880 and LM4861 are never on at
the same time. When the switch inside the headphone jack
is open, the LM4880 is enabled and the LM4861 is disabled
since the NMOS inverter is on. If a headphone jack is not
present, it is assumed that the internal speakers should be
on and thus the voltage on the LM4861 shutdown pin is low
and the voltage at the LM4880 pin is high. This results in the
LM4880 being shutdown and the LM4861 being enabled.

Only one channel of this circuit is shown in

Figure 3 to keep

the drawing simple but the typical application would a
LM4880 driving a stereo external headphone jack and two
LM4861's driving the internal stereo speakers. If only one in-
ternal speaker is required, a single LM4861 can be used as
a summer to mix the left and right inputs into a single mono
channel.

PROPER SELECTION OF EXTERNAL COMPONENTS

Selection of external components when using integrated
power amplifiers is critical to optimize device and system
performance. While the LM4880 is tolerant of external com-
ponent combinations, care must be exercised when choos-
ing component values.

The LM4880 is unity-gain stable which gives a designer
maximum system flexibility. The LM4880 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. In-
put signals equal to or greater than 1 Vrms are available
from sources such as audio codecs. Please refer to the sec-
tion, Audio Power Amplifier Design, for a more complete
explanation of proper gain selection.

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

Figure 1. Both the input coupling capacitor, C

, and

the output coupling capacitor, C

, form first order high pass

filters which limit low frequency response. These values
should be chosen based on needed frequency response for
a few distinct reasons.

Selection of Input and Output Capacitor Size

Large input and output capacitors are both expensive and
space hungry for portable designs. Clearly a certain sized
capacitor is needed to couple in low frequencies without se-
vere attenuation. But in many cases the transducers used in
portable systems, whether internal or external, have little
ability to reproduce signals below 100 Hz150 Hz. Thus us-
ing large input and output capacitors may not increase sys-
tem performance.

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

. A larger input coupling capacitor requires more charge to

reach its quiescent DC voltage (normally 1/2 V

.) This

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

Besides minimizing the input and output capacitor sizes,
careful consideration should be paid to the bypass capacitor
size. The bypass capacitor, C

, is the most critical compo-

nent to minimize turn-on pops since it determines how fast
the LM4880 turns on. The slower the LM4880's outputs
ramp to their quiescent DC voltage (nominally 1/2 V

), the

smaller the turn-on pop. Choosing C

equal to 1.0 µF along

with a small value of C

(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 oscilla-
tions or motorboating), with C

equal to 0.1 µF, the device

will be much more susceptible to turn-on clicks and pops.
Thus, a value of C

equal to 1.0 µF or larger is recom-

mended in all but the most cost sensitive designs.

AUDIO POWER AMPLIFIER DESIGN

Design a Dual 200 mW/8

Audio Amplifier

Given:

Power Output:

200 mWrms

Load Impedance:

Input Level:

1 Vrms (max)

Input Impedance:

20 k

Bandwidth:

100 Hz20 kHz

0.50 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. As shown in the Typical Performance
Curves, the dropout voltage is typically 0.5V. V

opeak

can be

determined from

Equation (3).

(3)

For 200 mW of output power into an 8

load, the required

opeak

is 1.79V. Since this is a single supply application, the

minimum supply voltage is twice the sum of V

opeak

and V

Since 5V is a standard supply voltage in most applications, it
is chosen for the supply rail. Extra supply voltage creates
headroom that allows the LM4880 to reproduce peaks in ex-
cess of 200 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 conditions
explained in the Power Dissipation section. Remember that
the maximum power dissipation value from

Equation (1)

must be multiplied by two since there are two independent
amplifiers inside the package.

Once the power dissipation equations have been addressed,
the required gain can be determined from

Equation (4).

www.national.com

Application Information

(Continued)

(4)

= -R

(5)

From

Equation (4), the minimum gain is::

= -1.26

Since the desired input impedance was 20 k

, and with a

gain of -1.26, a value of 27 k

is designated for R

, assum-

ing 5% tolerance resistors. This combination results in a
nominal gain of -1.35. 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 assuming a single
pole roll-off. As stated in the External Components section,
both R

in conjunction with C

, and C

with R

, create first or-

der high pass filters. Thus to obtain the desired frequency
low response of 100 Hz within

0.5 dB, both poles must be

taken into consideration. The combination of two single order
filters at the same frequency forms a second order response.
This results in a signal which is down 0.34 dB at five times
away from the single order filter -3 dB point. Thus, a fre-
quency of 20 Hz is used in the following equations to ensure
that the response if better than 0.5 dB down at 100 Hz.

1/(2

20k

20Hz) = 0.397 µF; use 0.39 µF

1/(2

20Hz) = 995 µF; use 1000 µF

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

, and the closed-loop gain,

. With a closed-loop gain magnitude of 1.35 and f

= 100

kHz, the resulting GBWP = 135 kHz which is much smaller
than the LM4880 GBWP of 12.5 MHz. This figure displays
that if a designer has a need top design an amplifier with a
higher gain, the LM4880 can still be used without running
into bandwidth limitations.

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Notes

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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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Corporation
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www.national.com

LM4880

Boomer

Audio

Power

Amplifier

Series

Dual

250

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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