LM4872 1 Watt Audio Power Amplifier in micro SMD package

LM4872

1 Watt Audio Power Amplifier in micro SMD package

General Description

The LM4872 is a bridge-connected audio power amplifier ca-

pable of delivering 1 W of continuous average power to an

Ω

load with less than .2% (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 LM4872 does not require

output coupling capacitors or bootstrap capacitors. It is opti-

mally suited for low-power portable applications.

The LM4872 features an externally controlled, low-power

consumption shutdown mode, as well as an internal thermal

shutdown protection mechanism.

The unity-gain stable LM4872 can be configured by external

gain-setting resistors.

Key Specifications

Power Output at 0.2% THD

1 W (typ)

Shutdown Current

0.01 µA (typ)

Features

micro SMD package (see App. note AN-1112)

5V - 2V operation

No output coupling capacitors or bootstrap capacitors.

Unity-gain stable

External gain configuration capability

Applications

Cellular Phones

Portable Computers

Low Voltage Audio Systems

Typical Application

Connection Diagram

Boomer

is a registered trademark of National Semiconductor Corporation.

DS101230-1

FIGURE 1. Typical Audio Amplifier Application Circuit

8 Bump micro SMD

DS101230-23

Top View

Order Number LM4872IBP, LM4872IBPX

See NS Package Number BPA08B6B

February 2000

LM4872

att

Audio

Power

Amplifier

micro

SMD

package

DS101230

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)

2500V

ESD Susceptibility (Note 5)

250V

Junction Temperature

150˚C

Soldering Information

See AN-1112

″

Micro-SMD Wafers Level Chip Scale

Package

″

Operating Ratings

Temperature Range

MIN

≤

MAX

−40˚C

≤

85˚C

Supply Voltage

2.0V

≤

5.5V

Electrical Characteristics V

= 5V

(Notes 1, 2, 9)

The following specifications apply for V

= 5V and 8

Ω

Load unless otherwise specified. Limits apply for T

= 25˚C.

Symbol

Parameter

Conditions

LM4872

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

5.3

mA (max)

Shutdown Current

PIN1

= V

0.01

µA (max)

Output Offset Voltage

= 0V

mV (max)

Output Power

THD = 0.2% (max); f = 1 kHz

THD+N

Total Harmonic Distortion+Noise

= 0.25 Wrms; A

= 2; 20 Hz

≤

20 kHz

0.1

PSRR

Power Supply Rejection Ratio

= 4.9V to 5.1V

Electrical Characteristics V

= 3.3V

(Notes 1, 2, 9)

The following specifications apply for V

= 3.3V and 8

Ω

Load unless otherwise specified. Limits apply for T

= 25˚C.

Symbol

Parameter

Conditions

LM4872

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

mA (max)

Shutdown Current

PIN1

= V

0.01

µA (max)

Output Offset Voltage

= 0V

mV (max)

Output Power

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

.45

THD+N

Total Harmonic Distortion+Noise

= 0.25 Wrms; A

= 2; 20 Hz

≤

20 kHz

0.15

PSRR

Power Supply Rejection Ratio

= 3.2V to 3.4V

Electrical Characteristics V

= 2.6V

(Notes 1, 2, 8, 9)

The following specifications apply for V

= 2.6V and 8

Ω

Load unless otherwise specified. Limits apply for T

= 25˚C.

Symbol

Parameter

Conditions

LM4872

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

3.4

mA (max)

LM4872

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Electrical Characteristics V

= 2.6V

(Notes 1, 2, 8, 9)

The following specifications apply for V

= 2.6V and 8

Ω

Load unless otherwise specified. Limits apply for T

25˚C. (Continued)

Symbol

Parameter

Conditions

LM4872

Units

(Limits)

Typical

Limit

(Note 6)

(Note 7)

Shutdown Current

PIN1

= V

0.01

µA (max)

Output Offset Voltage

= 0V

mV (max)

Output Power ( 8

Ω

)

Output Power ( 4

Ω

)

THD = 0.3% (max); f = 1 kHz

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

0.25

0.5

THD+N

Total Harmonic Distortion+Noise

= 0.25 Wrms; A

= 2; 20 Hz

≤

20 kHz

0.25

PSRR

Power Supply Rejection Ratio

= 2.5V to 2.7V

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

–T

or the number given in Absolute Maximum Ratings, whichever is lower. For the LM4872, T

JMAX

= 150˚C. The

typical junction-to-ambient thermal resistance is 150˚C/W.

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

Ω

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).

Note 8: Low Voltage Circuit - See Fig. 4

Note 9: Shutdown current is measured in a Normal Room Environment. Exposure to direct sunlight will increase I

by a maximum of 2µA.

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

LM4872

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Application Information

BRIDGE CONFIGURATION EXPLANATION

As shown in

Figure 1, the LM4872 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 10 k

Ω

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 by 180˚. Consequently, the differential gain for the IC

= 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 LM4872,

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. Since the LM4872 has two opera-

tional amplifiers in one package, the maximum internal

power dissipation is 4 times that of a single-ended amplifier.

The maximum power dissipation for a given application can

be derived from the power dissipation graphs or from Equa-

tion 1.

DMAX

= 4*(V

)

/(2

)

(1)

It is critical that the maximum junction temperature T

JMAX

150˚C is not exceeded. T

JMAX

can be determined from the

power derating curves by using P

DMAX

and the PC board foil

area. By adding additional copper foil, the thermal resistance

of the application can be reduced from a free air value of

150˚C/W, resulting in higher P

DMAX

. Additional copper foil

can be added to any of the leads connected to the LM4872.

It is especially effective when connected to V

, G

, and

the output pins. Refer to the application information on the

LM4872 reference design board for an example of good heat

sinking. If T

JMAX

still exceeds 150˚C, then additional

changes must be made. These changes can include re-

duced supply voltage, higher load impedance, or reduced

ambient temperature. The National Reference Design board

using a 5V supply and an 8 ohm load will run in a 110˚C am-

bient environement without exceeding T

JMAX

. Internal power

dissipation is a function of output power. 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 Tantalum or electro-

lytic capacitor and a 0.1 µF bypass capacitor which aid in

supply stability. This does not eliminate the need for bypass-

ing the supply nodes of the LM4872. The selection of bypass

capacitor, especially C

, is dependent upon PSRR require-

ments, click and pop performance as explained in the sec-

tion, Proper Selection of External Components, system

cost, and size constraints.

SHUTDOWN FUNCTION

In order to reduce power consumption while not in use, the

LM4872 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 LM4872 supply current

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 0.01 µA. In

either case, the shutdown pin should be tied to a stable 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 LM4872. 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 LM4872 is tolerant of

external component combinations, consideration to compo-

nent values must be used to maximize overall system qual-

ity.

The LM4872 is unity-gain stable which gives a designer

maximum system flexibility. The LM4872 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 considerations is the closed-

loop bandwidth of the amplifier. To a large extent, the band-

width is dictated by the choice of external components

LM4872

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Application Information

(Continued)

shown in

Figure 1. The input coupling capacitor, C

, forms a

first order high pass filter which limits low frequency re-

sponse. This value should be chosen based on needed fre-

quency 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 attenua-

tion. But in many cases the speakers used in portable sys-

tems, whether internal or external, have little ability to repro-

duce signals below 100 Hz to 150 Hz. Thus, using a large

input capacitor may not increase actual system perfor-

mance.

In addition to system cost and size, click and pop perfor-

mance is effected by the size of the input coupling capacitor,

A larger input coupling capacitor requires more charge to

reach its quiescent DC voltage (nominally 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 capacitor size, careful consid-

eration should be paid to the bypass capacitor value. Bypass

capacitor, C

, is the most critical component to minimize

turn-on pops since it determines how fast the LM4872 turns

on. The slower the LM4872’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 oscillations or motorboating), with

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 is recommended in all but the most cost sensitive de-

signs.

LOW VOLTAGE APPLICATIONS ( BELOW 3.0 V

)

The Lm4872 will function at voltages below 3 volts but this

mode of operation requires the addition of a 1k

Ω

resistor

from each of the differential output pins ( pins 8 and 4 ) di-

rectly to ground. The addition of the pair of 1k

Ω

resistors ( R4

& R5 ) assures stable operation below 3 Volt Vdd operation.

The addition of the two resistors will however increase the

idle current by as much as 5mA. This is because at 0v input

both of the outputs of the LM4872’s 2 internal opamps go to

1/2 V

( 2.5 volts for a 5v power supply ), causing current to

flow through the 1K resistors from output to ground. See fig

Jumper options have been included on the reference design,

Fig. 4, to accommodate the low voltage application. J2 & J3

connect R4 and R5 to the outputs. J1 operates the shutdown

function. J1 must be installed to operate the part. A switch

may be installed in place of J1 for easier evaluation of the

shutdown function.

AUDIO POWER AMPLIFIER DESIGN

A 1W/8

Ω

AUDIO AMPLIFIER

Given:

Power Output

1 Wrms

Load Impedance

Ω

Input Level

1 Vrms

Input Impedance

20 k

Ω

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 sup-

ply rail is to calculate the required V

opeak

using Equation 2

and add the output voltage. Using this method, the minimum

supply voltage would be (V

opeak

+ (V

ODTOP

+ V

ODBOT

)), where

ODBOT

and V

ODTOP

are extrapolated from the Dropout Volt-

age vs Supply Voltage curve in the Typical Performance

Characteristics section.

(2)

Using the Output Power vs Supply Voltage graph for an 8

Ω

load, the minimum supply rail is 4.6V. But since 5V is a stan-

dard voltage in most applications, it is chosen for the supply

rail. Extra supply voltage creates headroom that allows the

LM4872 to reproduce peaks in excess of 1W without produc-

ing audible distortion. At this time, the designer must make

sure that the power supply choice along with the output im-

pedance 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 Equa-

tion 3.

(3)

= A

From Equation 3, the minimum A

is 2.83; use A

= 3.

Since the desired input impedance was 20 k

Ω

, and with a

impedance of 2, a ratio of 1.5:1 of R

to R

results in an

allocation of R

= 20 k

Ω

and R

= 30 k

Ω

. 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.

= 100 Hz/5 = 20 Hz

= 20 kHz * 5 = 100 kHz

As stated in the External Components section, R

in con-

junction with C

create a highpass filter.

≥

1/(2

*20 k

Ω

*20 Hz) = 0.397 µF; use 0.39 µF

The high frequency pole is determined by the product of the

desired frequency pole, f

, and the differential gain, A

With a A

= 3 and f

= 100 kHz, the resulting GBWP =

150 kHz which is much smaller than the LM4872 GBWP of

4 MHz. This figure displays that if a designer has a need to

design an amplifier with a higher differential gain, the

LM4872 can still be used without running into bandwidth limi-

tations.

LM4872