LM4680

LM4680 10W High-Efficiency Mono BTL Audio Power Amplifier

Literature Number: SNAS246

LM468010W High-Efficiency Mono BTL Audio Power AmplifierGeneral DescriptionThe LM4680 is a high efficiency switching audio poweramplifier primarily designed for demanding applications inflat panel monitors and TV’s. It is capable of delivering 10Wto an 8Ω mono BTL load with less than 10% distortion(THD+N) when powered from a 14VDC power supply.

Boomer audio power amplifiers were designed specifically toprovide high quality output power with a minimal amount ofexternal components. The LM4680 features a micro-power,active-low shutdown mode, an internal thermal shutdownprotection mechanism, output fault detect, and short circuitprotection.

The LM4680 contains advanced transient (“pop and click”)suppression circuitry that eliminates noises that would oth-erwise occur during turn-on and turn-off transitions.

Key Specificationsj Power Output BTL (VDD = 14V,

fIN = 1kHz, THD+N = 10%,RL = 8Ω) 10W (typ)

j Quiescent Power Supply Current 25mA (typ)

j Efficiency (VDD = 12V, fIN = 1kHz,RL = 8Ω, POUT = 6W) 81% (typ)

j Shutdown Current 0.1mA (typ)

j Fixed Gain 30dB (typ)

Featuresn Soft-start circuitry eliminates noise during turn-on

transitionn Low current shutdown moden Low quiescent currentn 6W BTL output, RL = 8Ω, THD+N = 1%n Short circuit protectionn Fixed, internally set gain of 30dBn Internal clamp diodes protect amplifier outputs

Applicationsn Flat Panel Monitorsn Flat Panel TVsn Computer Sound Cards

Connection DiagramPlastic Package

20095802

Top ViewOrder Number LM4680SD

See NS Package Number SRC14A

Boomer® is a registered trademark of National Semiconductor Corporation.

January 2005LM

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© 2005 National Semiconductor Corporation DS200958 www.national.com

Typical Application

LM4680SD Demo Board Bill of MaterialItem Part Description Package Size Qty Ref Designator Remark Supplier

1 LM4680SD Audio Amplifier LLP14 1 U1

2 Cer Cap 0.1µF 16V 10% 0805 1 C4 PCC1812CT-ND Digi - Key

3 Cer Cap 0.27µF 16V 10% 0805 3 C5, C7, C8 PCC1916CT-ND Digi - Key

4 Cer Cap 1.0µF 25V 10% 0805 2 C1 - C2 PCC2319CT-ND Digi - Key

5 Tant Cap 1.0µF 16V 10% Size = A (3216) 2 C9 399-1583-2-ND Digi - Key

6 Tant Cap 10.0µF 16V 10% Size = A (3216) 2 C10 478-1655-2-ND Digi - Key

7 Tant Cap 10.0µF 16V 10% Size = A (3216) 1 C3 478-1655-2-ND Digi - Key

8 Inductor 4922 Series 27µHSMT

2 L1, L2 DN2218CT-ND Digi - Key

20095801

FIGURE 1. Typical Audio Amplifier Application Circuit

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Absolute Maximum Ratings (Notes 1, 2)

If Military/Aerospace specified devices are required,please contact the National Semiconductor Sales Office/Distributors for availability and specifications.

Supply Voltage 16V

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

θJC (LD) 2˚C/W

θJA (LD) 40˚C/W

Operating RatingsTemperature Range

TMIN ≤ TA ≤ TMAX −40˚C ≤ TA ≤ 85˚C

Supply Voltage (Note 10) 9.0V ≤ VDD ≤ 14V

Electrical Characteristics for the LM4680 (Note 1)

The following specifications apply for the circuit shown in Figure 1 operating with VDD = 12V, RL = 8Ω, and fIN = 1kHz,unless otherwise specified. Limits apply for TA = 25˚C.

Symbol Parameter Conditions LM4680 Units(Limits)Typical Limit

(Note 6) (Notes 7,8)

IDD Quiescent Power Supply Current VIN = 0V, IO = 0A, RL = 8Ω 28 52 mA

ISD Shutdown Current VSHUTDOWN = GND (Note 9) 0.1 mA

AV Amplifier Gain BTL output voltage with respect to inputvoltage

30 dB

PO Output Power THD+N = 1% (max)THD+N = 10%, VDD = 14V

610

5W

THD+N Total Harmonic Distortion + Noise POUT = 1WRMS 0.2 %

fBW Frequency Response Bandwidth POUT = 6W, post filter,-3dB relative to 1kHz

2020000

HzHz

η Efficiency POUT = 6W 81 %

éN Output Noise A-Weighted Filter, VIN = 0V,Input Referred

10 µV

SNR Signal-to-Noise Ratio A-Weighted Filter, POUT = 6WInput Referred

116 dB

PSRR Power Supply Rejection Ratio VRIPPLE = 200mVp-p, CBYPASS_1 = 10µF,Input Referredf = 50Hzf = 60Hzf = 100Hzf = 120Hzf = 1kHz

99101102102104

dB

tWU Wake-Up time CBYPASS = 10µF 600 ms

TSD Thermal Shutdown Temperature170

˚C˚C

VSDIH Shutdown Voltage Input High 4 V (min)

VSDIL Shutdown Voltage Input Low 1.5 V (max)

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Note 1: All voltages are measured with respect to the GND 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 isfunctional, but do not guarantee specific performance limits. Electrical Characteristics state DC and AC electrical specifications under particular test conditions whichguarantee specific performance limits. This assumes that the device is within the Operating Ratings. Specifications are not guaranteed for parameters where no limitis 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 maximumallowable power dissipation is PDMAX = (TJMAX − TA)/θJA or the number given in Absolute Maximum Ratings, whichever is lower. For the LM4680 typical application(shown in Figure 1) with VDD = 12V, RL = 8Ω stereo operation, the total power dissipation is 900mW. θJA = 40˚C/W

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: Limits are guaranteed to National’s AOQL (Average Outgoing Quality Level).

Note 8: Datasheets min/max specification limits are guaranteed by design, test, or statistical analysis.

Note 9: Shutdown current is measured in a normal room environment. The SHUTDOWN pin should be driven as close as possible to GND for minimum shutdowncurrent.

Note 10: Please refer to “Under Voltage Protection” on page 8 under “General Features.”

Typical Performance CharacteristicsTHD+N vs Frequency

VDD = 9V, RL = 8Ω, PO = 1WTHD+N vs Frequency

VDD = 12V, RL = 8Ω, PO = 1W

20095806 20095807

THD+N vs FrequencyVDD = 14V, RL = 8Ω, PO = 1W

THD+N vs Output PowerRL = 8Ω, VDD = 9V, f = 1kHz

20095808 20095809

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Typical Performance Characteristics (Continued)

THD+N vs Output PowerRL = 8Ω, VDD = 12V, f = 1kHz

THD+N vs Output PowerRL = 8Ω, VDD = 14V, f = 1kHz

20095810 20095811

Amplifier Output Powervs Power Supply Voltage

RL = 8Ω, f = 1kHz

Amplifier Output Magnitudevs Frequency

RL = 8Ω, VDD = 12V

20095812 20095813

Power Rejection Ratio vs FrequencyVDD = 9V, RL = 8Ω, Input Referred

Power Rejection Ratio vs FrequencyVDD = 12V, RL = 8Ω, Input Referred

20095825 20095826

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Typical Performance Characteristics (Continued)

Power Rejection Ratio vs FrequencyVDD = 14V, RL = 8Ω, Input Referred

Amplifier Power Dissipationvs Amplifier Load DissipationVDD = 14V, RL = 8Ω, f = 1kHz

2009582720095817

Amplifier Power Dissipationvs Load Power Dissipation

VDD = 12V, RL = 8Ω, f = 1kHz

Amplifier Power Dissipationvs Total Load Power Dissipation

VDD = 9V, RL = 8Ω, f = 1kHz

20095818 20095819

Output Powervs Load ResistanceVDD = 14V, f = 1kHz

Output Powervs Load ResistanceVDD = 12V, f = 1kHz

20095820 20095821

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Typical Performance Characteristics (Continued)

Output Powervs Load ResistanceVDD = 9V, f = 1kHz

Power Supply Currentvs Power Supply Voltage

VIN = 0V, RL = 8Ω

20095822 20095823

Power Dissipationvs Ambient Temperature

20095824

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General Features

SYSTEM FUNCTIONAL INFORMATION

Modulation Technique

Unlike typical Class D amplifiers that use single-ended com-parators to generate a pulse-width modulated switchingwaveform and RC timing circuits to set the switching fre-quency, the LM4680 uses a balanced differential floatingmodulator. Oscillation is a result of injecting complimentarycurrents onto the respective plates of a floating, on-die ca-pacitor. The value of the floating capacitor and value of thecomponents in the modulator’s feedback network and setsthe nominal switching frequency at 450kHz. Modulation re-sults from imbalances in the injected currents. The amountof current imbalance is directly proportional to the appliedinput signal’s magnitude and frequency.

Using a balanced, floating modulator produces a Class Damplifier that is immune to common mode noise sourcessuch as substrate noise. This noise occurs because of thehigh frequency, high current switching in the amplifier’s out-put stage. The LM4680 is immune to this type of noisebecause the modulator, the components that set its switch-ing frequency, and even the load all float with respect toground.

The balanced modulator’s pulse width modulated outputdrives the gates of the LM4680’s H-bridge configured outputpower MOSFETs. The pulse-train present at the powerMOSFETs’ output is applied to an LC low pass filter thatremoves the 450kHz energy component. The filter’s outputsignal, which is applied to the driven load, is an amplifiedreplica of the audio input signal.

Shutdown Function

The LM4680’s active-low shutdown function allows the userto place the amplifier in a shutdown mode while the systempower supply remains active. Activating shutdown deacti-vates the output switching waveform and minimizes thequiescent current. Applying logic 0 (GND) to pin 8 enablesthe shutdown function. Applying logic 1 (4V ≤ VLOGIC ≤ VDD)to pin 8 disables the shutdown function and restores fullamplifier operation.

Under Voltage Proctection

The under voltage protection disables the output driver sec-tion of the LM4680 while the supply voltage is below 8V. Thiscondition may occur as power is first applied or during lowline conditions, changes in load resistance, or when powersupply sag occurs. The under voltage protection ensuresthat all of the LM4680’s power MOSFETs are off. This actioneliminates shoot-through current and minimizes output tran-sients during turn-on and turn-off. The under voltage protec-tion gives the digital logic time to stabilize into known states,further minimizing turn output transients.

Turn-On Time

The LM4680 has an internal timer that determines the am-plifier’s turn-on time. After power is first applied or the partreturns from shutdown, the nominal turn-on time is 600ms.This delay allows all externally applied capacitors to chargeto a final value of VDD/2. Further, during turn-on, the outputsare muted. This minimizes output transients that may occurwhile the part settles into is quiescent operating mode.

Output Stage Current Limit and Fault DetectionProtection

The output stage MOSFETs are protected against outputconditions that could otherwise compromise their operationalstatus. The first stage of protection is output current limiting.When conditions that require high currents to drive a load,the LM4680’s current limit circuitry clamps the output currentat a nominal value of 2.5A. The output waveform is present,but may be clipped or its amplitude reduced. The same 2.5Anominal current limit also occurs if the amplifier outputs areshorted together or either output is shorted to VDD or GND.

The second stage of protection is an onboard fault detectioncircuit that continuously monitors the signal on each outputMOSFET’s gate and compares it against the respectivedrain voltage. When a condition is detected that violates aMOSFET’s Safe Operating Area (SOA) and the drive signalis disconnected from the output MOSFETs’ gates. The faultdetect circuit maintains this protective condition for approxi-mately 600ms, at which time the drive signal is reconnected.If the fault condition is no longer present, normal operationresumes. If the fault condition remains, however, the drivesignal is again disconnected.

Thermal Protection

The LM4680 has thermal shutdown circuitry that monitorsthe die temperature. Once the LM4680 die temperaturereaches 170˚C, the LM4680 disables the output switchingwaveform and remains disabled until the die temperaturefalls below 140˚C (typ).

Over-Modulation Protection

The LM4680’s over-modulation protection is a result of thepreamplifier’s (AMP1 and AMP2, Figure 1) inability to pro-duce signal magnitudes that equal the power supply volt-ages. Since the preamplifier’s output magnitude will alwaysbe less than the supply voltage, the duty cycle of the ampli-fier’s switching output will never reach zero. Peak modula-tion is limited to a nominal 95%.

Application Hints

SUPPLY BYPASSING

Correct power supply bypassing has two important goals.The first is to reduce noise on the power supply lines andminimize deleterious effects that the noise may cause to theamplifier’s operation. The second is to help stabilize anunregulated power supply and to improve the supply’s tran-sient response under heavy current demands. These twogoals require different capacitor value ranges. Therefore,various types and values are recommended for supply by-passing. For noise de-coupling, generally small ceramic ca-pacitors (0.01µF to 0.1µF) are recommended. Larger value(1µF to 10µF) tantalum capacitors are needed for the tran-sient current demands. These two capacitors in parallel willdo an adequate job of removing most noise from the supplyrails and providing the necessary transient current. Thesecapacitors should be placed as close as possible to eachIC’s supply pin(s) using leads as short as possible.

The LM4680 has two VDD pins: a power VDD (PVDD) and asignal VDD (SVDD). The parallel combination of the low valueceramic (0.1µF) and high value tantalum (10µF) should beused to bypass the PVDD pin. A small value (0.1µF) ceramicor tantalum can be used to bypass the SVDD pin.

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Application Hints (Continued)

OUTPUT STAGE FILTERING

The LM4680 requires a low pass filter connected betweenthe amplifier’s bridge output and the load. Figure 1 showsthe recommended LC filter. A minimum value of 27µH isrecommended. As shown in Figure 1, using the values of thecomponents connected between the amplifier BTL outputsand the load achieves a 2nd-order lowpass filter responsewith a -3dB cutoff frequency of 25kHz.

THD+N MEASUREMENTS AND OUT OF AUDIO BANDNOISE

THD+N (Total Harmonic Distortion plus Noise) is a veryimportant parameter by which all audio amplifiers are mea-sured. Often it is shown as a graph where either the outputpower or frequency is changed over the operating range. Avery important variable in the measurement of THD+N is thebandwidth-limiting filter at the input of the test equipment.Class D amplifiers, by design, switch their output powerdevices at a much higher frequency than the accepted audiorange (20Hz - 20kHz). Alternately switching the output volt-age between VDD and GND allows the LM4680 to operate atmuch higher efficiency than that achieved by traditionalClass AB amplifiers. Switching the outputs at high frequencyalso increases the out-of-band noise. Under normal circum-

stances the output lowpass filter significantly reduces thisout-of-band noise. If the low pass filter is not optimized for agiven switching frequency, there can be significant increasein out-of-band noise. THD+N measurements can be signifi-cantly affected by out-of-band noise, resulting in a higherthan expected THD+N measurement. To achieve a moreaccurate measurement of THD, the test equipment’s inputbandwidth of the must be limited. Some common upper filterpoints are 22kHz, 30kHz, and 80kHz. The input filter limitsthe noise component of the THD+N measurement to asmaller bandwidth resulting in a more real-world THD+Nvalue.

Recommended Printed CircuitBoard LayoutFigures 2 through 4 show the recommended two-layer PCboard layout that is optimized for the 14-pin SD-packagedLM4680 and associated external components. This circuit isdesigned for use with an external 12V supply and 8W speak-ers (or load resistors). This circuit board is easy to use. Apply12V and ground to the board’s VDD and GND terminals,respectively. Connect speakers (or load resistors) betweenthe board’s -OUT and +OUT terminals. Apply the input signalto the input pin labeled -IN.

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Demonstration Board Layout

20095803

FIGURE 2. Recommended SD PCB LayoutTop Silkscreen

20095804

FIGURE 3. Recommended SD PCB LayoutTop Layer

20095805

FIGURE 4. Recommended SD PCB LayoutBottom Layer

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Physical Dimensions inches (millimeters)unless otherwise noted

LD PackageOrder Number LM4680SD

NS Package Number SRC14A

National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reservesthe right at any time without notice to change said circuitry and specifications.

For the most current product information visit us at www.national.com.

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1. Life support devices or systems are devices or systemswhich, (a) are intended for surgical implant into the body, or(b) support or sustain life, and whose failure to perform whenproperly used in accordance with instructions for useprovided in the labeling, can be reasonably expected to resultin a significant injury to the user.

2. A critical component is any component of a life supportdevice or system whose failure to perform can be reasonablyexpected to cause the failure of the life support device orsystem, or to affect its safety or effectiveness.

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LM4680

10WH

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Mono

BTL

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mplifier

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