LM4923LQBD - National Semiconductor

LM4923 February 19, 2009

1.1 Watt Fully Differential Audio Power Amplifier With

Shutdown Select

General Description

The LM4923 is a fully differential audio power amplifier pri-

marily designed for demanding applications in mobile phones

and other portable communication device applications. It is

capable of delivering 1.1 watt of continuous average power to

an 8Ω BTL load with less than 1% distortion (THD+N) from a

5VDC power supply.

Boomer audio power amplifiers were designed specifically to

provide high quality output power with a minimal amount of

external components. The LM4923 does not require output

coupling capacitors or bootstrap capacitors, and therefore is

ideally suited for mobile phone and other low voltage appli-

cations where minimal power consumption is a primary re-

quirement.

The LM4923 features a low-power consumption shutdown

mode. To facilitate this, Shutdown may be enabled by logic

low. Additionally, the LM4923 features an internal thermal

shutdown protection mechanism.

The LM4923 contains advanced pop & click circuitry which

eliminates noises which would otherwise occur during turn-on

and turn-off transitions.

Key Specifications

■ Improved PSRR at 217Hz 85dB(typ)

■ Power Output at 5.0V

@ 1% THD+N 1.1W(typ)

■ Power Output at 3.3V

@ 1% THD+N 400mW(typ)

■ Shutdown Current 0.1µA(typ)

Features

■Fully differential amplification

■Available in space-saving LLP package

■Ultra low current shutdown mode

■Can drive capacitive loads up to 100pF

■Improved pop & click circuitry eliminates noises during

turn-on and turn-off transitions

■2.4 - 5.5V operation

■No output coupling capacitors, snubber networks or

bootstrap capacitors required

Applications

■Mobile phones

■PDAs

■Portable electronic devices

Connection Diagrams

LQ Package

20071330

Top View

Order Number LM4923LQ

See NS Package Number LQB08A

8 Pin LQ Marking

20071302

X − Date Code

TT − Die Traceability

G − Boomer

B2 − LM4923LQ

8 Pin MSOP Package

200713a1

Top View

See Order Number LM4923MM

See NS Package Number MUA08A

Boomer® is a registered trademark of National Semiconductor Corporation.

LM4923 1.1 Watt Fully Differential Audio Power Amplifier With Shutdown Select

Typical Application

20071313

FIGURE 1. Typical Audio Amplifier Application Circuit

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LM4923

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 VDD +0.3V

Power Dissipation (Note 3) Internally Limited

ESD Susceptibility (Note 4) 2000V

ESD Susceptibility (Note 5) 200V

Junction Temperature 150°C

Thermal Resistance

θJA (LLP) 140°C/W

θJA (MM) 210°C/W

θJC (MM) 56°C/W

Soldering Information

See AN-1187

Operating Ratings

Temperature Range

TMIN ≤ TA ≤ TMAX −40°C ≤ TA ≤ 85°C

Supply Voltage 2.4V ≤ VDD ≤ 5.5V

Electrical Characteristics VDD = 5V (Notes 1, 2)

The following specifications apply for VDD = 5V, AV = 1, and 8Ω load unless otherwise specified. Limits apply for TA = 25°C.

Symbol Parameter Conditions

LM4923 Units

(Limits)

Typical Limit

(Note 6) (Note 7)

IDD Quiescent Power Supply Current VIN = 0V, no load

VIN = 0V, RL = 8Ω

9mA (max)

ISD Shutdown Current VSHUTDOWN = GND 0.1 1 µA (max)

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

LM4923, RL = 8Ω 1.1 1

THD+N Total Harmonic Distortion+Noise Po = 0.4 Wrms; f = 1kHz 0.02 %

PSRR Power Supply Rejection Ratio Vripple = 200mV sine p-p

f = 217Hz (Note 8) 85 73

f = 1kHz (Note 8) 85 73

CMRR Common_Mode Rejection Ratio f = 217Hz,

VCM = 200mVpp

50 dB

VOS Output Offset VIN = 0V 4 mV

VSDIH Shutdown Voltage Input High 0.9 V

VSDIL Shutdown Voltage Input Low 0.7 V

Electrical Characteristics VDD = 3V (Notes 1, 2)

The following specifications apply for VDD = 3V, AV = 1, and 8Ω load unless otherwise specified. Limits apply for TA = 25°C.

Symbol Parameter Conditions

LM4923 Units

(Limits)

Typical Limit

(Note 6) (Note 7)

IDD Quiescent Power Supply Current VIN = 0V, no load

VIN = 0V, RL = 8Ω

5.5

5.5 mA (max)

ISD Shutdown Current VSHUTDOWN = GND 0.1 1 µA (max)

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

LM4923, RL = 8Ω 0.375 W

THD+N Total Harmonic Distortion+Noise Po = 0.25Wrms; f = 1kHz 0.02 %

PSRR Power Supply Rejection Ratio Vripple = 200mV sine p-p

f = 217Hz (Note 8) 85

f = 1kHz (Note 8) 85 73

CMRR Common-Mode Rejection Ratio f = 217Hz

VCM = 200mVpp

50 dB

VOS Output Offset VIN = 0V 4 mV

3 www.national.com

LM4923

Symbol Parameter Conditions

LM4923 Units

(Limits)

Typical Limit

(Note 6) (Note 7)

VSDIH Shutdown Voltage Input High 0.8 V

VSDIL Shutdown Voltage Input Low 0.6 V

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

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 TJMAX, θJA, and the ambient temperature TA. The maximum

allowable power dissipation is PDMAX = (TJMAX – TA) / θJA or the number given in Absolute Maximum Ratings, whichever is lower. For the LM4923, see power

derating curve for additional information.

Note 4: Human body model, 100pF discharged through a 1.5kΩ resistor.

Note 5: Machine Model, 220pF – 240pF discharged through all pins.

Note 6: Typicals are measured at 25°C and represent the parametric norm.

Note 7: Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis.

Note 8: 10Ω terminated input.

External Components Description

(Figure 1)

Components Functional Description

1. RiInverting input resistance which sets the closed-loop gain in conjunction with Rf.

2. RfFeedback resistance which sets the closed-loop gain in conjunction with Ri.

3. CSSupply 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.

4. CBBypass pin capacitor which provides half-supply filtering. Refer to the section, Proper Selection of External

Components, for information concerning proper placement and selection of CB.

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LM4923

Typical Performance Characteristics

THD+N vs Frequency

VDD = 2.6V, RL = 8Ω, PO = 150mW

20071306

THD+N vs Frequency

VDD = 2.6V, RL = 4Ω, PO = 150mW

20071305

THD+N vs Frequency

VDD = 5V, RL = 8Ω, PO = 400mW

20071309

THD+N vs Frequency

VDD = 3V, RL = 8Ω, PO = 275mW

20071308

THD+N vs Frequency

VDD = 3V, RL = 4Ω, PO = 225mW

20071307

THD+N vs Output Power

VDD = 2.6V, RL = 8Ω

20071311

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LM4923

THD+N vs Output Power

VDD = 2.6V, RL = 4Ω

20071310

THD+N vs Output Power

VDD = 5V, RL = 8Ω

20071315

THD+N vs Output Power

VDD = 3V, RL = 8Ω

20071314

THD+N vs Output Power

VDD = 3V, RL = 4Ω

20071312

PSRR vs Frequency

VDD = 5V, RL = 8Ω, Input terminated

20071304

PSRR vs Frequency

VDD = 3V, RL = 8Ω, Input terminated

20071303

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LM4923

Output Power vs Supply Voltage

RL = 8Ω

20071301

CMRR vs Frequency

VDD = 5V, RL = 8Ω

20071317

CMRR vs Frequency

VDD = 3V, RL = 8Ω

20071326

PSRR vs Common Mode Voltage

VDD = 3V, RL = 8Ω, f = 217Hz

20071322

PSRR vs Common Mode Voltage

VDD = 5V, RL = 8Ω, f = 217Hz

20071316

Power Dissipation vs Output Power

VDD = 2.6V, RL = 8Ω and 4Ω

20071321

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LM4923

Power Dissipation vs Output Power

VDD = 5V, RL = 8Ω

20071319

Power Dissipation vs Output Power

VDD = 3V, RL = 8Ω

20071325

Power Derating Curve

20071318

Noise Floor

VDD = 5V

20071324

Noise Floor

VDD = 3V

20071320

Clipping Voltage vs Supply Voltage

20071323

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LM4923

Output Power vs Load Resistance

20071327

Supply Current Shutdown Voltage

20071328

Application Information

DIFFERENTIAL AMPLIFIER EXPLANATION

The LM4923 is a fully differential audio amplifier that features

differential input and output stages. Internally this is accom-

plished by two circuits: a differential amplifier and a common

mode feedback amplifier that adjusts the output voltages so

that the average value remains VDD / 2. When setting the dif-

ferential gain, the amplifier can be considered to have

"halves". Each half uses an input and feedback resistor (Ri1

and RF1) to set its respective closed-loop gain (see Figure 1).

With Ri1 = Ri2 and RF1 = RF2, the gain is set at -RF / Ri for each

half. This results in a differential gain of

AVD = -RF/Ri(1)

It is extremely important to match the input resistors to each

other, as well as the feedback resistors to each other for best

amplifier performance. See the Proper Selection of Exter-

nal Components section for more information. A differential

amplifier works in a manner where the difference between the

two input signals is amplified. In most applications, this would

require input signals that are 180° out of phase with each oth-

er. The LM4923 can be used, however, as a single ended

input amplifier while still retaining its fully differential benefits.

In fact, completely unrelated signals may be placed on the

input pins. The LM4923 simply amplifies the difference be-

tween them.

All of these applications provide what is known as a "bridged

mode" output (bridge-tied-load, BTL). This results in output

signals at Vo1 and Vo2 that are 180° out of phase with respect

to each other. Bridged mode operation is different from the

single-ended amplifier configuration that connects the load

between the amplifier output and ground. A bridged amplifier

design has distinct advantages over the single-ended config-

uration: it provides differential drive to the load, thus doubling

maximum possible output swing for a specific supply voltage.

Four times the output power is possible compared with a sin-

gle-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 causing excess clipping, please re-

fer to the Audio Power Amplifier Design section.

A bridged configuration, such as the one used in the LM4923,

also creates a second advantage over single-ended ampli-

fiers. Since the differential outputs, Vo1 and Vo2, are biased at

half-supply, no net DC voltage exists across the load. This

assumes that the input resistor pair and the feedback resistor

pair are properly matched (see Proper Selection of External

Components). BTL configuration eliminates the output cou-

pling capacitor required in single-supply, single-ended ampli-

fier configurations. If an output coupling capacitor is not used

in a single-ended output configuration, the half-supply bias

across the load would result in both increased internal IC

power dissipation as well as permanent loudspeaker dam-

age. Further advantages of bridged mode operation specific

to fully differential amplifiers like the LM4923 include in-

creased power supply rejection ratio, common-mode noise

reduction, and click and pop reduction.

EXPOSED-DAP PACKAGE PCB MOUNTING

CONSIDERATIONS

The LM4923's exposed-DAP (die attach paddle) package

(LLP) provide a low thermal resistance between the die and

the PCB to which the part is mounted and soldered. This al-

lows rapid heat transfer from the die to the surrounding PCB

copper traces, ground plane and, finally, surrounding air. Fail-

ing to optimize thermal design may compromise the LM4923's

high power performance and activate unwanted, though nec-

essary, thermal shutdown protection. The LLP package must

have its DAP soldered to a copper pad on the PCB. The DAP's

PCB copper pad is connected to a large plane of continuous

unbroken copper. This plane forms a thermal mass and heat

sink and radiation area. Place the heat sink area on either

outside plane in the case of a two-sided PCB, or on an inner

layer of a board with more than two layers. Connect the DAP

copper pad to the inner layer or backside copper heat sink

area with a thermal via. The via diameter should be 0.012in -

0.013in. Ensure efficient thermal conductivity by plating-

through and solder-filling the vias.

Best thermal performance is achieved with the largest prac-

tical copper heat sink area. In all circumstances and condi-

tions, the junction temperature must be held below 150°C to

prevent activating the LM4923's thermal shutdown protection.

The LM4923's power de-rating curve in the Typical Perfor-

mance Characteristics shows the maximum power dissipa-

tion versus temperature. Example PCB layouts are shown in

the Demonstration Board Layout section. Further detailed

and specific information concerning PCB layout, fabrication,

and mounting an LLP package is available from National

9 www.national.com

LM4923

Semiconductor's package Engineering Group under applica-

tion note AN1187.

PCB LAYOUT AND SUPPLY REGULATION

CONSIDERATIONS FOR DRIVING 4Ω LOADS

Power dissipated by a load is a function of the voltage swing

across the load and the load's impedance. As load impedance

decreases, load dissipation becomes increasingly dependent

on the interconnect (PCB trace and wire) resistance between

the amplifier output pins and the load's connections. Residual

trace resistance causes a voltage drop, which results in power

dissipated in the trace and not in the load as desired. This

problem of decreased load dissipation is exacerbated as load

impedance decreases. Therefore, to maintain the highest

load dissipation and widest output voltage swing, PCB traces

that connect the output pins to a load must be as wide as

possible.

Poor power supply regulation adversely affects maximum

output power. A poorly regulated supply's output voltage de-

creases with increasing load current. Reduced supply voltage

causes decreased headroom, output signal clipping, and re-

duced output power. Even with tightly regulated supplies,

trace resistance creates the same effects as poor supply reg-

ulation. Therefore, making the power supply traces as wide

as possible helps maintain full output voltage swing.

POWER DISSIPATION

Power dissipation is a major concern when designing a suc-

cessful amplifer, whether the amplifier is bridged or single-

ended. Equation 2 states the maximum power dissipation

point for a single-ended amplifier operating at a given supply

voltage and driving a specified output load.

PDMAX = (VDD)2 / (2π2RL) Single-Ended (2)

However, a direct consequence of the increased power de-

livered to the load by a bridge amplifier is an increase in

internal power dissipation versus a single-ended amplifier op-

erating at the same conditions.

PDMAX = 4 * (VDD)2 / (2π2RL) Bridge Mode (3)

Since the LM4923 has bridged outputs, the maximum internal

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

Even with this substantial increase in power dissipation, the

LM4923 does not require additional heatsinking under most

operating conditions and output loading. From Equation 3,

assuming a 5V power supply and an 8Ω load, the maximum

power dissipation point is 625mW. The maximum power dis-

sipation point obtained from Equation 3 must not be greater

than the power dissipation results from Equation 4:

PDMAX = (TJMAX - TA) / θJA (4)

The LM4923's θJA in an LQB08A package is 140°C/W. De-

pending on the ambient temperature, TA, of the system sur-

roundings, Equation 4 can be used to find the maximum

internal power dissipation supported by the IC packaging. If

the result of Equation 3 is greater than that of Equation 4, then

either the supply voltage must be decreased, the load

impedance increased, the ambient temperature reduced, or

the θJA reduced with heatsinking. In many cases, larger traces

near the output, VDD, and GND pins can be used to lower the

θJA. The larger areas of copper provide a form of heatsinking

allowing higher power dissipation. For the typical application

of a 5V power supply, with an 8Ω load, the maximum ambient

temperature possible without violating the maximum junction

temperature is approximately 62°C provided that device op-

eration is around the maximum power dissipation point. Re-

call that internal power dissipation is a function of output

power. If typical operation is not around the maximum power

dissipation point, the LM4923 can operate at higher ambient

temperatures. Refer to the Typical Performance Charac-

teristics curves for power dissipation information.

POWER SUPPLY BYPASSING

As with any power amplifier, proper supply bypassing is crit-

ical for low noise performance and high power supply rejec-

tion ratio (PSRR). The capacitor location on both the bypass

and power supply pins should be as close to the device as

possible. A larger half-supply bypass capacitor improves

PSRR because it increases half-supply stability. Typical ap-

plications employ a 5V regulator with 10µF and 0.1µF bypass

capacitors that increase supply stability. This, however, does

not eliminate the need for bypassing the supply nodes of the

LM4923. The LM4923 will operate without the bypass capac-

itor CB, although the PSRR may decrease. A 1µF capacitor is

recommended for CB. This value maximizes PSRR perfor-

mance. Lesser values may be used, but PSRR decreases at

frequencies below 1kHz. The issue of CB selection is thus

dependant upon desired PSRR and click and pop perfor-

mance as explained in the section Proper Selection of Ex-

ternal Components.

SHUTDOWN FUNCTION

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

LM4923 contains shutdown circuitry that is used to turn off the

amplifier's bias circuitry. The device may then be placed into

shutdown mode by toggling the Shutdown Select pin to logic

low. The trigger point for shutdown is shown as a typical value

in the Supply Current vs Shutdown Voltage graphs in the

Typical Performance Characteristics section. It is best to

switch between ground and supply for maximum perfor-

mance. While the device may be disabled with shutdown

voltages in between ground and supply, the idle current may

be greater than the typical value of 0.1µA. In either case, the

shutdown pin should be tied to a definite voltage to avoid un-

wanted state changes.

In many applications, a microcontroller or microprocessor

output is used to control the shutdown circuitry, which pro-

vides a quick, smooth transition to shutdown. Another solution

is to use a single-throw switch in conjunction with an external

pull-up resistor. 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 using

integrated power amplifiers is critical when optimizing device

and system performance. Although the LM4923 is tolerant to

a variety of external component combinations, consideration

of component values must be made when maximizing overall

system quality.

The LM4923 is unity-gain stable, giving the designer maxi-

mum system flexibility. The LM4923 should be used in low

closed-loop gain configurations to minimize THD+N values

and maximize 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 1Vrms are available from

sources such as audio codecs. Please refer to the Audio

Power Amplifier Design section for a more complete expla-

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LM4923

nation of proper gain selection. When used in its typical

application as a fully differential power amplifier the LM4923

does not require input coupling capacitors for input sources

with DC common-mode voltages of less than VDD. Exact al-

lowable input common-mode voltage levels are actually a

function of VDD, Ri, and Rf and may be determined by Equa-

tion 5:

VCMi < (VDD-1.2)*((Rf+(Ri)/(Rf)-VDD*(Ri / 2Rf) (5)

-RF / RI = AVD (6)

Special care must be taken to match the values of the feed-

back resistors (RF1 and RF2) to each other as well as matching

the input resistors (Ri1 and Ri2) to each other (see Figure 1)

more in front. Because of the balanced nature of differential

amplifiers, resistor matching differences can result in net DC

currents across the load. This DC current can increase power

consumption, internal IC power dissipation, reduce PSRR,

and possibly damaging the loudspeaker. The chart below

demonstrates this problem by showing the effects of differing

values between the feedback resistors while assuming that

the input resistors are perfectly matched. The results below

apply to the application circuit shown in Figure 1, and as-

sumes that VDD = 5V, RL = 8Ω, and the system has DC

coupled inputs tied to ground.

Tolerance RF1 RF2 V02 - V01 ILOAD

20% 0.8R 1.2R -0.500V 62.5mA

10% 0.9R 1.1R -0.250V 31.25mA

5% 0.95R 1.05R -0.125V 15.63mA

1% 0.99R 1.01R -0.025V 3.125mA

0% R R 0 0

Similar results would occur if the input resistors were not

carefully matched. Adding input coupling capacitors in be-

tween the signal source and the input resistors will eliminate

this problem, however, to achieve best performance with min-

imum component count it is highly recommended that both

the feedback and input resistors matched to 1% tolerance or

better.

AUDIO POWER AMPLIFIER DESIGN

Design a 1W/8Ω Audio Amplifier

Given:

Power Output 1Wrms

Load Impedance 8Ω

Input Level 1Vrms

Input Impedance 20kΩ

Bandwidth 100Hz–20kHz ± 0.25dB

A designer must first determine the minimum supply rail to

obtain the specified output power. The supply rail can easily

be found by extrapolating from the Output Power vs Supply

Voltage graphs in the Typical Performance Characteris-

tics section. A second way to determine the minimum supply

rail is to calculate the required VOPEAK using Equation 7 and

add the dropout voltages. Using this method, the minimum

supply voltage is (Vopeak + (VDO TOP + (VDO BOT )), where

VDO BOT and VDO TOP are extrapolated from the Dropout Volt-

age vs Supply Voltage curve in the Typical Performance

Characteristics section.

(7)

Using the Output Power vs Supply Voltage graph for an 8Ω

load, the minimum supply rail just about 5V. Extra supply volt-

age creates headroom that allows the LM4923 to reproduce

peaks in excess of 1W without producing audible distortion.

At this time, the designer must make sure that the power sup-

ply choice along with the output impedance 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 8.

(8)

Rf / Ri = AVD

From Equation 7, the minimum AVD is 2.83. Since the desired

input impedance was 20kΩ, a ratio of 2.83:1 of Rf to Ri results

in an allocation of Ri = 20kΩ for both input resistors and Rf =

60kΩ for both feedback resistors. The final design step is to

address the bandwidth requirement which must be stated as

a single -3dB frequency point. Five times away from a -3dB

point is 0.17dB down from passband response which is better

than the required ±0.25dB specified.

fH = 20kHz * 5 = 100kHz

The high frequency pole is determined by the product of the

desired frequency pole, fH , and the differential gain, AVD .

With a AVD = 2.83 and fH = 100kHz, the resulting GBWP =

150kHz which is much smaller than the LM4923 GBWP of

10MHz. This figure displays that if a designer has a need to

design an amplifier with a higher differential gain, the LM4923

can still be used without running into bandwidth limitations.

11 www.national.com

LM4923

Revision History

Rev Date Description

1.0 09/28/07 Added the MSOP package, then released.

1.01 12/17/07 Updated the mktg outline LQB08A into the rev B.

1.02 02/19/09 Fixed typo labels on the typical circuit diagram.

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LM4923

Physical Dimensions inches (millimeters) unless otherwise noted

LQ Package

Order Number LM4923LQ

NS Package Number LQB08A

MSOP Package

Order Number LM4923MM

NS Package Number MUA08A

13 www.national.com

LM4923

Notes

LM4923 1.1 Watt Fully Differential Audio Power Amplifier With Shutdown Select

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