Datasheet LM4881 Datasheet (National Semiconductor)

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LM4881

Dual 200 mW Headphone Amplifier with Shutdown Mode

LM4881 Dual 200 mW Headphone Amplifier with Shutdown Mode

September 1997

General Description

The LM4881 is a dual audio power amplifier capable of deliv­ering 200 mW of continuous average power into an 8Ω load
with 0.1%(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 using surface mount packaging. Since
the LM4881 does not require bootstrap capacitors or snub­ber networks, it is optimally suited for low-power portable
systems.

The LM4881 features an externally controlled, low power
consumption shutdown mode which is virtually clickless and
popless, as well as an internal thermal shutdown protection
mechanism.

The unity-gain stable LM4881 can be configured by external
gain-setting resistors.

Key Specifications

n THD at 1 kHz at 125 mW

continuous average output
power into 8Ω 0.1%(max)

n THD at 1 kHz at 75 mW continuous

average output power into 32Ω 0.02%(typ)

n Output power at 10%THD+N

at 1 kHz into 8Ω 300 mW (typ)

n Shutdown Current 0.7 µA (typ)

Features

n MSOP surface mount packaging
n Unity-gain stable
n External gain configuration capability
n Thermal shutdown protection circuitry
n No bootstrap capacitors, or snubber circuits are

necessary

Applications

n Headphone Amplifier
n Personal Computers
n Microphone Preamplifier

Typical Application Connection Diagrams

MSOP Package

DS100005-2

SOP and DIP Package

*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

Boomer®is a registered trademarkof National Semiconductor Corporation.

© 1999 National Semiconductor Corporation DS100005 www.national.com

DS100005-1

DS100005-38

Top View

Order Number LM4881MM, LM4881M, or LM4881N

See NS Package Number MUA08A, M08A, or N08E

Absolute Maximum Ratings (Note 3)

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
Power Dissipation (Note 4) Internally limited
ESD Susceptibility (Note 5) 3500V
ESD Susceptibility (Note 6) 250V
Junction Temperature 150˚C
Soldering Information (Note 1)

Small Outline Package

Vapor Phase (60 seconds) 215˚C

DD

+ 0.3V

Thermal Resistance

(MSOP) 56˚C/W

θ

JC

(MSOP) 210˚C/W

θ

JA

(SOP) 35˚C/W

θ

JC

(SOP) 170˚C/W

θ

JA

(DIP) 37˚C/W

θ

JC

(DIP) 107˚C/W

θ

JA

Operating Ratings

Temperature Range

≤ TA≤ T

T

MIN

MAX

Supply Voltage 2.7V ≤ V

Note 1: See AN-450 “Surface Mounting and their Effects on Product Reli­ability” for other methods of soldering surface mount devices.

−40˚C ≤ TA≤ 85˚C
≤ 5.5V

DD

Infrared (15 seconds) 220˚C

Electrical Characteristics (Notes 2, 3)

The following specifications apply for VDD= 5V unless otherwise specified. Limits apply for TA= 25C.

Symbol Parameter Conditions LM4881 Units (Limits)

Typ (Note 7) Limit (Note 8)

V

DD

I

DD

I

SD

V

OS

P

O

THD+N Total Harmonic Distortion +

PSRR C

Power Supply Voltage 2.7 V (min)

5.5 V (max)
Quiescent Current VIN= 0V, IO= 0A 3.6 6.0 mA (max)
Shutdown Current V

PIN1=VDD

0.7 5 µA (max)
Offset Voltage VIN= 0V 5 50 mV (max)
Output Power THD = 0.1%(max);f=1kHz;

R

=8Ω 200 125 mW (min)

L

R

=16Ω 150 mW

L

R

=32Ω 85 mW

L

THD+N=10%;f=1kHz;
R

=8Ω 300 mW

L

R

=16Ω 200 mW

L

R

=32Ω 110 mW

Noise

L

R

=16Ω,PO= 120 mWrms; 0.025

L

R

=32Ω,PO= 75 mWrms;

L

f=1kHz

= 1.0 µF, V

B

mVrms, f = 120Hz

RIPPLE

= 200

0.02

50 dB

%
%

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Electrical Characteristics (Notes 2, 3)

The following specifications apply for VDD= 3V unless otherwise specified. Limits apply for TA= 25C.

Symbol Parameter Conditions LM4881 Units (Limits)

Typ (Note 7) Limit (Note 8)

I

DD

I

SD

V
P

THD+N Total Harmonic Distortion +

PSRR Power Supply Rejection Ratio C

Quiescent Current VIN= 0V, IO= 0A 1.1 mA
Shutdown Current V
Offset Voltage VIN=0V 5 mV

OS

Output Power THD = 1%(max);

O

PIN1=VDD

f = 1 kHz;

=8Ω 70 mW

R

L

R

=16Ω 65 mW

L

R

=32Ω 30 mW

L

0.7 µA

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

R

=8Ω 95 mW

L

R

=16Ω 65 mW

L

R

=32Ω 35 mW

L

R

=16Ω,PO= 60 mWrms; 0.2

Noise

Note 2: All voltages are measured with respect to the ground pin, unless otherwise specified.
Note 3:

Absolute Maximum Ratings

tional, but do not guarantee specific performance limits.
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 4: The maximum power dissipation must be derated at elevated temperatures and is dictated by T
allowable power dissipation is P
mounted, is 210˚C/W for the MSOP Package and 107˚C/W for package N08E.

Note 5: Human body model, 100 pF discharged through a 1.5 kΩ resistor.
Note 6: Machine Model, 220 pF–240 pF discharged through all pins.
Note 7: Typicals are measured at 25˚C and represent the parametric norm.
Note 8: Limits are guaranteed to National’s AOQL (Average Outgoing Quality Level).

indicate limits beyond which damage to the device may occur.

=(T

DMAX

L

R

=32Ω,PO=

L

25 mWrms;f=1kHz

= 1.0 µF, V

B

200 mVrms, f = 100 Hz

Electrical Characteristics

)/θJA. For the LM4881, T

JMAX−TA

0.03

=

RIPPLE

state DC andAC electrical specifications under particular test conditions which guar-

= 150˚C, and the typical junction-to-ambient thermal resistance, when board

JMAX

50 dB

Operating Ratings

indicate conditions for which the device is func-

, θJA, and the ambient temperature TA. The maximum

JMAX

%
%

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External Components Description (

Compo-

nents

1. R

2. C

3. R

4. C

5. C

6. C

i

i

f
S

B

O

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

at fc=1/(2πRiCi).

i

Input coupling capacitor which blocks the DC voltage at the amplifier’s input terminals. Also creates a
highpass filter with R
Components, for and explanation of how to determine the value of C

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

i

Feedback resistance which sets closed-loop gain in conjuction with Ri.
Supply bypass capacitor which provides power supply filtering. Refer to the Application Information

section for 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
Output coupling capacitor which blocks the DC voltage at the amplifier’s output. Forms a high pass

filter with R

at fO= 1/(2πRLCO)

L

Typical Performance Characteristics

Figure 1

)

Functional Description

.

i

.

B

THD+N vs Frequency

THD+N vs Frequency

DS100005-3

DS100005-6

THD+N vs Frequency

THD+N vs Frequency

DS100005-4

DS100005-7

THD+N vs Frequency

DS100005-5

THD+N vs Frequency

DS100005-8

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Typical Performance Characteristics (Continued)

THD+N vs Output Power

THD+N vs Output Power

Output Power vs
Supply Voltage

DS100005-9

DS100005-12

THD+N vs Output Power

THD+N vs Output Power

Output Power vs
Supply Voltage

DS100005-10

DS100005-13

THD+N vs Output Power

DS100005-11

THD+N vs Output Power

DS100005-14

Output Power vs
Supply Voltage

DS100005-15

DS100005-16

DS100005-17

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Typical Performance Characteristics (Continued)

Power Dissipation vs
Output Power

Power Dissipation vs
Output Power

Channel Separation

DS100005-18

DS100005-21

Output Power vs
Load Resistance

Clipping Voltage vs
Supply Voltage

Output Attenuation in
Shutdown Mode

DS100005-19

DS100005-22

Output Power vs
Load Resistance

DS100005-20

Clipping Voltage vs
Supply Voltage

DS100005-23

Supply Current vs
Supply Voltage

DS100005-24

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DS100005-25

DS100005-26

Typical Performance Characteristics (Continued)

Power Supply
Rejection Ratio

Frequency Response vs
Output Capacitor Size

Typical Application
Frequency Response

DS100005-27

DS100005-30

Open Loop
Frequency Response

Frequency Response vs
Output Capacitor Size

Typical Application
Frequency Response

DS100005-28

DS100005-31

Noise Floor

DS100005-29

Frequency Response vs
Output Capacitor Size

DS100005-32

Power Derating Curve

DS100005-33

DS100005-34

DS100005-35

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Application Information

SHUTDOWN FUNCTION

In order to reduce power consumption while not in use, the
LM4881 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 the V
draw will be minimized in idle mode. While the device will be
disabled with shutdown pin voltages less than V
current may be greater than the typical value of 0.7 µA. In ei­ther case, the shutdown pin should be tied to a definite 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 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
LM4881. 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.

P

=(VDD)2/(2π2RL) (1)

DMAX

Since the LM4881 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 LM4881 does
not require heat sinking over a large range of ambient tem­perature. From Equation 1, assuming a 5V power supply and
an 8Ω load, the maximum power dissipation point is 158 mW
per amplifier. Thus the maximum package dissipation point
is 317 mW. The maximum power dissipation point obtained
must not be greater than the power dissipation that results
from Equation 2:

P

=(T

DMAX

For package MUA08A, θ
M08A, θ
107˚C/W. T
ambient temperature, T

= 170˚C/W, and for package N08E, θJA=

JA

= 150˚C for the LM4881. Depending on the

JMAX

tion 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 T
power supply, with an 8Ω load, the maximum ambient tem-

reduced. For the typical application of a 5V

A

perature possible without violating the maximum junction
temperature is approximately 96˚C provided 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 maximum power dissipa­tion point, the ambient temperature may be increased ac­cordingly. Refer to the Typical Performance Characteris-
tics curves for power dissipation information for lower output
powers.

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, the LM4881 supply current

DD

, the idle

DD

JMAX−TA

A

)/θJA(2)

= 230˚C/W, and for package

JA

, of the system surroundings, Equa-

POWER SUPPLY BYPASSING

As with any power amplifer, 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. As
displayed in the Typical Performance Characteristics sec­tion, the effect of a larger half supply bypass capacitor is im­proved 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 sup­ply nodes of the LM4881. The selection of bypass capaci­tors, especially C
quency PSRR, click and pop performance as explained in

, is thus dependent upon desired low fre-

B

the section, Proper Selection of External Components
section, system cost, and size constraints.

PROPER SELECTION OF EXTERNAL COMPONENTS

Selection of external components when using integrated
power amplifiers is critical to optimize device and system
performance. While the LM4881 is tolerant of external com­ponent combinations, consideration to component values
must be used to maximize overall system quality.

The LM4881 is unity gain stable and this gives a designer
maximum system flexibility. The LM4881 should be used in
low gain configurations to minimize THD+N values, and
maximum 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 closed
loop bandwidth of the amplifier. To a large extent, the band­width is dicated by the choice of external components shown
in

Figure 1

put coupling capacitor, C
which limit low frequency response. These values should be

. Both the input coupling capacitor, Ci, and the out-

, form first order high pass filters

o

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 se­vere attenuation. But in many cases the speakers used in
portable systems, whether internal or external, have little
ability to reproduce signals below 150 Hz. Thus using large
input and output capacitors may not increase system perfor­mance.

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

. A larger input coupling capacitor requires more charge to

i

reach its quiescent DC voltage (nominally 1/2 V
charge comes from the output via the feedback and is apt to

DD

). This

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 and output capacitor sizes,
careful consideration should be paid to the bypass capacitor
value. Bypass capacitor C
minimize turn on pops since it determines how fast the

is the most critical component to

B

LM4881 turns on. The slower the LM4881’s outputs ramp to
their quiescent DC voltage (nominally 1/2 V
the turn on pop. Thus choosing C
a small value of C

(in the range of 0.1 µF to 0.39 µF), the

i

equal to 1.0 µF along with

B

), the smaller

DD

Application Information (Continued)

shutdown function should be virtually clickless and popless.
While the device will function properly,(no oscillations or mo­torboating), with C
more susceptible to turn on clicks and pops. Thus, a value of
C

equal to 0.1 µF or larger is recommended in all but the

B

most cost sensitive designs.

AUDIO POWER AMPLIFIER DESIGN
Design a Dual 200mW/8Ω Audio Amplifier

Given:
Power Output 200 mWrms
Load Impedance 8Ω
Input Level 1 Vrms (max)
Input Impedance 20 kΩ
Bandwidth 100 Hz–20 kHz

A designer must first determine the needed supply rail to ob­tain the specified output power.Calculating the required sup­ply rail involves knowing two parameters, V
the dropout voltage. The latter is typically 530 mV and can
be found from the graphs in the Typical Performance Char-
acteristics. V

For 200 mW of output power into an 8Ω load, the required
V

is 1.79 volts. A minimum supply rail of 2.32V results

OPEAK

from adding V
voltage in most applications, it is chosen for the supply rail.
Extra supply voltage creates headroom that allows the
LM4881 to reproduce peaks in excess 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

equal to 0.1 µF, the device will be much

B

can be determined from Equation 3.

OPEAK

and VOD. Since 5V is a standard supply

OPEAK

OPEAK

±

0.50 dB

and also

(3)

dissipation point 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.

(4)

A

V=Rf/Ri

From Equation 4, the minimum gain is: A

(5)

= 1.26

V

Since the desired input impedance was 20 kΩ, and with a
gain of 1.26, a value of 27 kΩ is designated for R
5%tolerance resistors. This combination results in a nominal

, assuming

f

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 Ci, and Cowith RL, create first or-

i

der highpass 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 fre­quency of 20 Hz is used in the following equations to ensure
that the response is better than 0.5 dB down at 100 Hz.

C

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

i

≥ 1/(2π*8Ω* 20 Hz) = 995 µF; use 1000 µF.

C

o

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

V

. With a closed-loop gain of 1.35 and fH= 100 kHz, the re-

, and the closed-loop gain, A

H

sulting GBWP = 135 kHz which is much smaller than the
LM4881 GBWP of 18 MHz. This figure displays that if a de­signer has a need to design an amplifier with a higher gain,
the LM4881 can still be used without running into bandwidth
limitations.

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Physical Dimensions inches (millimeters) unless otherwise noted

Order Number LM4881MM

NS Package Number MUA08A

Order Number LM4881M

NS Package Number M08A

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Physical Dimensions inches (millimeters) unless otherwise noted (Continued)

Order Number LM4881N

NS Package Number N08E

LM4881 Dual 200 mW Headphone Amplifier with Shutdown Mode

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

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.

labeling, can be reasonably expected to result in a
significant injury to the user.

National Semiconductor
Corporation

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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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