LM3875

LM3875 Overture Audio Power Amplifier Series High-Performance 56W Audio

Power Amplifier

Literature Number: SNAS083C

LM3875 Overture™ Audio Power Amplifier SeriesHigh-Performance 56W Audio Power AmplifierGeneral DescriptionThe LM3875 is a high-performance audio power amplifiercapable of delivering 56W of continuous average power toan 8Ω load with 0.1% THD+N from 20Hz to 20kHz.The performance of the LM3875, utilizing its Self Peak In-stantaneous Temperature (˚Ke) (SPiKe™) protection cir-cuitry, puts it in a class above discrete and hybrid amplifiersby providing an inherently, dynamically protected Safe Op-erating Area (SOA). SPiKe protection means that theseparts are completely safeguarded at the output against ov-ervoltage, undervoltage, overloads, caused by shorts to thesupplies, thermal runaway, and instantaneous temperaturepeaks.

The LM3875 maintains an excellent signal-to-noise ratio ofgreater than 95dB(min) with a typical low noise floor of2.0µV. It exhibits extremely low THD+N values of 0.06% atthe rated output into the rated load over the audio spectrum,and provides excellent linearity with an IMD (SMPTE) typicalrating of 0.004%.

Featuresn 56W continuous average output power into 8Ωn 100W instantaneous peak output power capabilityn Signal-to-Noise Ratio >95dB (min)n Output protection from a short to ground or to the

supplies via internal current limiting circuitryn Output over-voltage protection against transients from

inductive loadsn Supply under-voltage protection, not allowing internal

biasing to occur when |V+| + |V−| ≤ 12V, thus eliminatingturn-on and turn-off transients

n 11 lead TO-220 packagen Wide supply voltage range: |V+| + |V−| = 20V to 84V

Applicationsn Component or compact stereosn Self-powered speakersn Surround-sound amplifiersn High-end stereo TVs

Typical Application

Overture™ and SPiKe™ are trademarks of National Semiconductor Corporation.

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*Optional components dependent upon specific design requirements. Refer to the External Components Description section for a component functiondescription.

FIGURE 1. Typical Audio Amplifier Application Circuit

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Connection DiagramPlastic Package (Note 8)

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Top ViewOrder Number LM3875T or LM3875TFSee NS Package Number TA11B for

Staggered Lead Non-Isolated Packageor TF11B for Staggered Lead Isolated Package

Equivalent Schematic(Excluding active protection circuitry)

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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 |V+| + |V−| (No Signal) 94V

Supply Voltage |V+| + |V−| (InputSignal) 84V

Common Mode Input Voltage (V+ or V−) and|V+| + |V−| ≤ 80V

Differential Input Voltage 60V

Output Current Internally Limited

Power Dissipation (Note 3) 125W

ESD Susceptibility (Note 4) 2500V

Junction Temperature (Note 5) 150˚C

Soldering InformationT package (10 seconds) 260˚C

Storage Temperature −40˚C to +150˚C

Thermal Resistance

θJC 1˚C/WθJA 43˚C/W

Operating Ratings (Notes 1, 2)Temperature Range

TMIN ≤ TA ≤ TMAX −20˚C ≤ TA ≤+85˚C

Supply Voltage |V+| + |V−| 20V to 84VNote: Operation is guaranteed up to 84V, however, distortion may be intro-duced from the SPiKe Protection Circuitry when operating above 70V ifproper thermal considerations are not taken into account. Refer to theThermal Considerations section for more information. (See SPiKe ProtectionResponse)

Electrical Characteristics (Notes 1, 2)The following specifications apply for V+ = +35V, V− = −35V with RL = 8Ω unless otherwise specified. Limits apply for TA =25˚C.

Symbol Parameter Conditions

LM3875Units

(Limits)Typical(Note 6)

Limit(Note 7)

|V+| + |V−| Power Supply Voltage 2084

V (Min)V (Max)

**PO Output Power (Continuous Average) THD + N = 0.1% (Max)f = 1 kHz, f = 20 kHz

56 40 W (Min)

Peak PO Instantaneous Peak Output Power 100 W

THD + N Total Harmonic Distortion Plus Noise 40W, 20 Hz ≤ f ≤ 20 kHzAV = 26 dB

0.06 %

**SR Slew Rate (Note 9) VIN = 1.414 Vrms, f = 10 kHzSquare-wave, RL = 2 kΩ

11 5V/µs(Min)

*I+ Total Quiescent Power SupplyCurrent

VCM = 0V, VO = 0V, Io = 0 mA 30 70 mA (Max)

*VOS Input Offset Voltage VCM = 0V, Io = 0 mA 1 10 mV (Max)

IB Input Bias Current VCM = 0V, Io = 0 mA 0.2 1 µA (Max)

IOS Input Offset Current VCM = 0V, Io = 0 mA 0.01 0.2 µA (Max)

Io Output Current Limit |V+| = |V−| = 10V, ton = 10 ms, VO = 0V 6 4 A(Min)

*Vod Output Dropout Voltage |V+−Vo

−|, V+ = 20V, Io = +100 mA|Vo−V

−|, V− = −20V, Io = −100 mA1.62.7

55

V (Max)V (Max)

*PSRR Power Supply Rejection Ratio V+ = 40V to 20V, V− = −40V,Vcm = 0V, Io = 0 mAV+ = 40V, V = −40V to −20V,Vcm = 0V, Io = 0 mA

120120

8585

dB (Min)

*CMRR Common Mode Rejection Ratio V+ = 60V to 20V, V− = −20V to −60V,Vcm = 20V to −20V, Io = 0 mA

120 80 dB (Min)

*AVOL Open Loop Voltage Gain |V+| = |V−| = 40V, RL = 2 kΩ, ∆VO = 60V 120 90 dB (Min)

GBWP Gain-Bandwidth Product |V+| = |V−| = 40VfO = 100 kHz, VIN = 50 mVrms

8 2MHz(Min)

**eIN Input Noise IHF − A Weighting FilterRIN = 600Ω (Input Referred)

2.0 8.0 µV (Max)

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Electrical Characteristics (Notes 1, 2) (Continued)The following specifications apply for V+ = +35V, V− = −35V with RL = 8Ω unless otherwise specified. Limits apply for TA =25˚C.

Symbol Parameter Conditions

LM3875Units

(Limits)Typical(Note 6)

Limit(Note 7)

SNR Signal-to-Noise Ratio PO = 1W, A-Weighted,Measured at 1 kHz, RS =25Ω

98 dB dB

PO = 40W, A-Weighted,Measured at 1 kHz, RS =25Ω

114 dB dB

Ppk = 100W, A-Weighted,Measured at 1 kHz, RS =25Ω

122 dB dB

IMD Intermodulation Distortion Test 60 Hz, 7 kHz, 4:1 (SMPTE)60 Hz, 7 kHz, 1:1 (SMPTE)

0.0040.006

%

*DC Electrical Test; refer to Test Circuit #1.**AC Electrical Test; refer to Test Circuit #2.

Note 1: 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 2: All voltages are measured with respect to supply GND, unless otherwise specified.

Note 3: For operating at case temperatures above 25˚C, the device must be derated based on a 150˚C maximum junction temperature and a thermal resistanceof θJC = 1.0˚C/W (junction to case). Refer to the Thermal Resistance figure in the Application Information section under Thermal Considerations.

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

Note 5: The operating junction temperature maximum is 150˚C, however, the instantaneous Safe Operating Area temperature is 250˚C.

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: The LM3875T package TA11B is a non-isolated package, setting the tab of the device and the heat sink at V− potential when the LM3875 is directly mountedto the heat sink using only thermal compound. If a mica washer is used in addition to thermal compound, θCS (case to sink) is increased, but the heat sink will beisolated from V−.

Note 9: The feedback compensation network limits the bandwidth of the closed-loop response and so the slew rate will be reduced due to the high frequency roll-off.Without feedback compensation, the slew rate is typically 16V/µs.

Note 10: The output dropout voltage is the supply voltage minus the clipping voltage. Refer to the Clipping Voltage vs. Supply Voltage graph in the TypicalPerformance Characteristics section.

Test Circuit #1 (DC Electrical Test Circuit)

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Test Circuit #2 (AC Electrical Test Circuit)

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Single Supply Application Circuit

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*Optional components dependent upon specific design requirements. Refer to the External Components Description section for a component functiondescription.

FIGURE 2. Typical Single Supply Audio Amplifier Application Circuit

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External Components Description(Figure 1 and Figure 2)

Components Functional Description

1. RIN Acts as a volume control by setting the voltage level allowed to the amplifier’s input terminals.

2. RA Provides DC voltage biasing for the single supply operation and bias current for the positive input terminal.

3. CA Provides bias filtering.

4. C Provides AC coupling at the input and output of the amplifier for single supply operation.

5. RB Prevents currents from entering the amplifier’s non-inverting input which may be passed through to the loadupon power-down of the system due to the low input impedance of the circuitry when the under-voltagecircuitry is off. This phenomenon occurs when the supply voltages are below 1.5V.

6. *CC Reduces the gain (bandwidth of the amplifier) at high frequencies to avoid quasi-saturation oscillations of theoutput transistor. The capacitor also suppresses external electromagnetic switching noise created fromfluorescent lamps.

7. Ri Inverting input resistance to provide AC Gain in conjunction with Rf1.

8. *Ci Feedback capacitor. Ensures unity gain at DC. Also a low frequency pole (highpass roll-off) at:

fc = 1/(2π Ri Ci).9. Rf1 Feedback resistance to provide AC Gain in conjunction with Ri.

10. *Rf2 At higher frequencies feedback resistance works with Cf to provide lower AC Gain in conjunction with Rf1 andRi. A high frequency pole (lowpass roll-off) exists at:

fc = [Rf1 Rf2] (s + 1/Rf2 Cf]/[(Rf1 + Rf2) (s + 1/Cf (Rf1 +Rf2))].

11. *Cf Compensation capacitor that works with Rf1 and Rf2 to reduce the AC Gain at higher frequencies.

12. *RSN Works with CSN to stabilize the output stage by creating a pole that eliminates high frequency oscillations.

13. *CSN Works with RSN to stabilize the output stage by creating a pole that eliminates high frequency oscillations. fc= 1/(2πRSN CSN).

14. *L Provides high impedance at high frequencies so that R may decouple a highly capacitive load and reduce theQ of the series resonant circuit due to capacitive load. Also provides a low impedance at low frequencies toshort out R and pass audio signals to the load.

15. *R

16. CS Provides power supply filtering and bypassing.*Optional components dependent upon specific design requirements. Refer to the Application Information section for more information.

OPTIONAL EXTERNAL COMPONENT INTERACTION

Although the optional external components have specificdesired functions that are designed to reduce the bandwidthand eliminate unwanted high frequency oscillations they maycause certain undesirable effects when they interact. Inter-action may occur for components whose reactances are inclose proximity to one another. One example would be thecoupling capacitor, CC, and the compensation capacitor, Cf.

These two components act as low impedances to certainfrequencies which will couple signals from the input to theoutput. Please take careful note of basic amplifier compo-nent functionality when designing in these components.

The optional external components shown in Figure 2 anddescribed above are applicable in both single and split volt-age supply configurations.

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Typical Performance Characteristics

Safe AreaSPiKe

Protection Response

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Supply Current vsSupply Voltage Pulse Thermal Resistance

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Pulse Thermal ResistanceSupply Current vs

Output Voltage

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Typical Performance Characteristics (Continued)

Pulse Power Limit Pulse Power Limit

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Supply Current vsCase Temperature

Clipping Voltagevs Supply Voltage

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Input Bias Current vsCase Temperature Peak Output Current

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Typical Performance Characteristics (Continued)

THD + N vs FrequencyTHD + N vs

Output Power

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THD + N vsOutput Power THD Distribution

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THD DistributionOutput Power vsLoad Resistance

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Typical Performance Characteristics (Continued)

Max Heatsink Thermal Resistance (˚C/W) at the Specified Ambient Temperature (˚C)and Maximum Power Dissipation vs Supply Voltage

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Note: The maximum heat sink thermal resistance values, ØSA, in the table above were calculated using a ØCS = 0.2˚C/W due to thermal compound.

Power Dissipation vsOutput Power

Power Dissipation vsOutput Power

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Output Power vsSupply Voltage IMD 60 Hz, 4:1

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Typical Performance Characteristics (Continued)

IMD 60 Hz, 7 kHz, 4:1 IMD 60 Hz, 7 kHz, 4:1

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IMD 60 Hz, 1:1 IMD 60 Hz, 7 kHz, 1:1

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IMD 60 Hz, 7 kHz, 1:1Power Supply Rejection

Ratio

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Typical Performance Characteristics (Continued)

Common-Mode RejectionRatio Large Signal Response

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Pulse ResponseOpen Loop

Frequency Response

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Application Information

GENERAL FEATURES

Under-Voltage Protection: Upon system power-up theunder-voltage Protection Circuitry allows the power suppliesand their corresponding caps to come up close to their fullvalues before turning on the LM3875 such that no DC outputspikes occur. Upon turn-off, the output of the LM3875 isbrought to ground before the power supplies such that notransients occur at power-down.

Over-Voltage Protection: The LM3875 contains overvolt-age protection circuitry that limits the output current to ap-proximately 4Apeak while also providing voltage clamping,though not through internal clamping diodes. The clampingeffect is quite the same, however, the output transistors aredesigned to work alternately by sinking large current spikes.

SPiKe Protection: The LM3875 is protected from instanta-neous peak-temperature stressing by the power transistorarray. The Safe Operating Area graph in the Typical Perfor-mance Characteristics section shows the area of deviceoperation where the SPiKe Protection Circuitry is not en-abled. The waveform to the right of the SOA graph exempli-fies how the dynamic protection will cause waveform distor-tion when enabled.

Thermal Protection: The LM3875 has a sophisticated ther-mal protection scheme to prevent long-term thermal stressto the device. When the temperature on the die reaches165˚C, the LM3875 shuts down. It starts operating againwhen the die temperature drops to about 155˚C, but if thetemperature again begins to rise, shutdown will occur againat 165˚C. Therefore the device is allowed to heat up to arelatively high temperature if the fault condition is temporary,but a sustained fault will cause the device to cycle in aSchmitt Trigger fashion between the thermal shutdown tem-perature limits of 165˚C and 155˚C. This greatly reduces thestress imposed on the IC by thermal cycling, which in turnimproves its reliability under sustained fault conditions.

Since the die temperature is directly dependent upon theheat sink, the heat sink should be chosen as discussed inthe Thermal Considerations section, such that thermalshutdown will not be reached during normal operation. Usingthe best heat sink possible within the cost and space con-straints of the system will improve the long-term reliability ofany power semiconductor device.

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Application Information (Continued)THERMAL CONSIDERATIONS

Heat Sinking

The choice of a heat sink for a high-power audio amplifier ismade entirely to keep the die temperature at a level suchthat the thermal protection circuitry does not operate undernormal circumstances. The heat sink should be chosen todissipate the maximum IC power for a given supply voltageand rated load.

With high-power pulses of longer duration than 100 ms, thecase temperature will heat up drastically without the use of aheat sink. Therefore the case temperature, as measured atthe center of the package bottom, is entirely dependent onheat sink design and the mounting of the IC to the heat sink.For the design of a heat sink for your audio amplifier appli-cation refer to the Determining the Correct Heat Sinksection.

Since a semiconductor manufacturer has no control overwhich heat sink is used in a particular amplifier design, wecan only inform the system designer of the parameters andthe method needed in the determination of a heat sink. Withthis in mind, the system designer must choose his supplyvoltages, a rated load, a desired output power level, andknow the ambient temperature surrounding the device.These parameters are in addition to knowing the maximumjunction temperature and the thermal resistance of the IC,both of which are provided by National Semiconductor.

As a benefit to the system designer we have provided Maxi-mum Power Dissipation vs Supply Voltages curves for vari-ous loads in the Typical Performance Characteristics sec-tion, giving an accurate figure for the maximum thermalresistance required for a particular amplifier design. Thisdata was based on θJC = 1˚C/W and θCS = 0.2˚C/W. We alsoprovide a section regarding heat sink determination for anyaudio amplifier design where θCS may be a different value. Itshould be noted that the idea behind dissipating the maxi-mum power within the IC is to provide the device with a lowresistance to convection heat transfer such as a heat sink.Therefore, it is necessary for the system designer to beconservative in his heat sink calculations. As a rule, thelower the thermal resistance of the heat sink the higher theamount of power that may be dissipated. This is, of course,guided by the cost and size requirements of the system.Convection cooling heat sinks are available commercially,and their manufacturers should be consulted for ratings.

Proper mounting of the IC is required to minimize the thermaldrop between the package and the heat sink. The heat sinkmust also have enough metal under the package to conductheat from the center of the package bottom to the finswithout excessive temperature drop.

A thermal grease such as Wakefield type 120 or ThermalloyThermacote should be used when mounting the package tothe heat sink. Without this compound, the thermal resistancewill be no better than 0.5˚C/W, and probably much worse.With the compound, thermal resistance will be 0.2˚C/W orless, assuming under 0.005 inch combined flatness runoutfor the package and heat sink. Proper torquing of the mount-ing bolts is important and can be determined from heat sinkmanufacturer’s specification sheets.

Should it be necessary to isolate V− from the heat sink, aninsulating washer is required. Hard washers like berylumoxide, anodized aluminum and mica require the use of ther-

mal compound on both faces. Two-mil mica washers aremost common, giving about 0.4˚C/W interface resistancewith the compound.

Silicone-rubber washers are also available. A 0.5˚C/W ther-mal resistance is claimed without thermal compound. Expe-rience has shown that these rubber washers deteriorate andmust be replaced should the IC be dismounted.

Determining Maximum Power Dissipation

Power dissipation within the integrated circuit package is avery important parameter requiring a thorough understand-ing if optimum power output is to be obtained. An incorrectmaximum power dissipation (PD) calculation may result ininadequate heatsinking, causing thermal shutdown circuitryto operate and limit the output power.

The following equations can be used to accurately calculatethe maximum and average integrated circuit power dissipa-tion for your amplifier design, given the supply voltage, ratedload, and output power. These equations can be directlyapplied to the Power Dissipation vs Output Power curves inthe Typical Performance Characteristics section.

Equation (1) exemplifies the maximum power dissipation ofthe IC and Equations (2), (3) exemplify the average IC powerdissipation expressed in different forms.

PDMAX = VCC2/2π2 RL (1)

where VCC is the total supply voltage

PDAVE = (VOpk/RL) [VCC/π − VOpk/2] (2)where VCC is the total supply voltage and VOpk = VCC/π

PDAVE = VCC VOpk/π RL − VOpk2/2 RL (3)where VCC is the total supply voltage.

Determining the Correct Heat Sink

Once the maximum IC power dissipation is known for agiven supply voltage, rated load, and the desired rated out-put power the maximum thermal resistance (in ˚C/W) of aheat sink can be calculated. This calculation is made usingEquation (4) and is based on the fact that thermal heat flowparameters are analogous to electrical current flow proper-ties.

It is also known that typically the thermal resistance, θJC(junction to case), of the LM3875 is 1˚C/W and that usingThermalloy Thermacote thermal compound provides a ther-mal resistance, θCS (case to heat sink), of about 0.2˚C/W asexplained in the Heat Sinking section.

Referring to the figure below, it is seen that the thermalresistance from the die (junction) to the outside air (ambient)is a combination of three thermal resistances, two of whichare known, θJC and θCS. Since convection heat flow (powerdissipation) is analogous to current flow, thermal resistanceis analogous to electrical resistance, and temperature dropsare analogous to voltage drops, the power dissipation out ofthe LM3875 is equal to the following:

PDMAX = (TJmax − TAmb)/θJAwhere θJA = θJC + θCS + θSA

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Application Information (Continued)But since we know PDMAX, θJC, and θSC for the applicationand we are looking for θSA, we have the following:

θSA = [(TJmax − TAmb) − PDMAX (θJC + θCS)]/PDMAX (4)Again it must be noted that the value of θSA is dependentupon the system designer’s amplifier application and itscorresponding parameters as described previously. If theambient temperature that the audio amplifier is to be workingunder is higher than the normal 25˚C, then the thermalresistance for the heat sink, given all other things are equal,will need to be smaller.

Equations (1), (4) are the only equations needed in thedetermination of the maximum heat sink thermal resistance.This is, of course, given that the system designer knows therequired supply voltages to drive his rated load at a particularpower output level and the parameters provided by thesemiconductor manufacturer. These parameters are thejunction to case thermal resistance, θJC, TJmax = 150˚C, andthe recommended Thermalloy Thermacote thermal com-pound resistance, θCS.

SIGNAL-TO-NOISE RATIO

In the measurement of the signal-to-noise ratio, misinterpre-tations of the numbers actually measured are common. Oneamplifier may sound much quieter than another, but due toimproper testing techniques, they appear equal in measure-ments. This is often the case when comparing integratedcircuit designs to discrete amplifier designs. Discrete transis-tor amps often “run out of gain” at high frequencies andtherefore have small bandwidths to noise as indicated below.

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Integrated circuits have additional open loop gain allowingadditional feedback loop gain in order to lower harmonicdistortion and improve frequency response. It is this addi-tional bandwidth that can lead to erroneous signal-to-noisemeasurements if not considered during the measurementprocess. In the typical example above, the difference inbandwidth appears small on a log scale but the factor of 10in bandwidth, (200 kHz to 2 MHz) can result in a 10 dBtheoretical difference in the signal-to-noise ratio (white noiseis proportional to the square root of the bandwidth in asystem).

In comparing audio amplifiers it is necessary to measure themagnitude of noise in the audible bandwidth by using a“weighting” filter (Note 11). A “weighting” filter alters thefrequency response in order to compensate for the averagehuman ear’s sensitivity to the frequency spectra. The weight-ing filters at the same time provide the bandwidth limiting asdiscussed in the previous paragraph.Note 11: CCIR/ARM: A Practical Noise Measurement Method; by RayDolby, David Robinson and Kenneth Gundry, AES Preprint No. 1353 (F-3).

In addition to noise filtering, differing meter types give differ-ent noise readings. Meter responses include:

1. RMS reading,

2. average responding,

3. peak reading, and

4. quasi peak reading.

Although theoretical noise analysis is derived using trueRMS based calculations, most actual measurements aretaken with ARM (Average Responding Meter) test equip-ment.

Typical signal-to-noise figures are listed for an A-weightedfilter which is commonly used in the measurement of noise.The shape of all weighting filters is similar, with the peak ofthe curve usually occurring in the 3 kHz–7 kHz region asshown below.

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SUPPLY BYPASSING

The LM3875 has excellent power supply rejection and doesnot require a regulated supply. However, to eliminate pos-sible oscillations all op amps and power op amps shouldhave their supply leads bypassed with low-inductance ca-pacitors having short leads and located close to the packageterminals. Inadequate power supply bypassing will manifestitself by a low frequency oscillation known as “motorboating”or by high frequency instabilities. These instabilities can beeliminated through multiple bypassing utilizing a large tanta-lum or electrolytic capacitor (10 µF or larger) which is used toabsorb low frequency variations and a small ceramic capaci-tor (0.1 µF) to prevent any high frequency feedback throughthe power supply lines.

If adequate bypassing is not provided the current in thesupply leads which is a rectified component of the loadcurrent may be fed back into internal circuitry. This signalcauses low distortion at high frequencies requiring that thesupplies be bypassed at the package terminals with anelectrolytic capacitor of 470 µF or more.

LEAD INDUCTANCE

Power op amps are sensitive to inductance in the outputlead, particularly with heavy capacitive loading. Feedback tothe input should be taken directly from the output terminal,minimizing common inductance with the load.

Lead inductance can also cause voltage surges on the sup-plies. With long leads to the power supply, energy is stored inthe lead inductance when the output is shorted. This energycan be dumped back into the supply bypass capacitors whenthe short is removed. The magnitude of this transient isreduced by increasing the size of the bypass capacitor nearthe IC. With at least a 20 µF local bypass, these voltagesurges are important only if the lead length exceeds a couplefeet (>1 µH lead inductance). Twisting together the supplyand ground leads minimizes the effect.

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Application Information (Continued)LAYOUT, GROUND LOOPS AND STABILITY

The LM3875 is designed to be stable when operated at aclosed-loop gain of 10 or greater, but as with any otherhigh-current amplifier, the LM3875 can be made to oscillateunder certain conditions. These usually involve printed cir-cuit board layout or output/input coupling.

When designing a layout, it is important to return the loadground, the output compensation ground, and the low level(feedback and input) grounds to the circuit board commonground point through separate paths. Otherwise, large cur-rents flowing along a ground conductor will generate volt-ages on the conductor which can effectively act as signals atthe input, resulting in high frequency oscillation or excessivedistortion. It is advisable to keep the output compensationcomponents and the 0.1 µF supply decoupling capacitors asclose as possible to the LM3875 to reduce the effects of PCBtrace resistance and inductance. For the same reason, theground return paths should be as short as possible.

In general, with fast, high-current circuitry, all sorts of prob-lems can arise from improper grounding which again can beavoided by returning all grounds separately to a commonpoint. Without isolating the ground signals and returning thegrounds to a common point, ground loops may occur.

“Ground Loop” is the term used to describe situations occur-ring in ground systems where a difference in potential existsbetween two ground points. Ideally a ground is a ground, butunfortunately, in order for this to be true, ground conductorswith zero resistance are necessary. Since real world groundleads possess finite resistance, currents running throughthem will cause finite voltage drops to exist. If two groundreturn lines tie into the same path at different points there willbe a voltage drop between them. The first figure belowshows a common ground example where the positive inputground and the load ground are returned to the supplyground point via the same wire. The addition of the finite wireresistance, R2, results in a voltage difference between thetwo points as shown below.

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The load current IL will be much larger than input bias currentI1, thus V1 will follow the output voltage directly, i.e., inphase. Therefore the voltage appearing at the non-invertinginput is effectively positive feedback and the circuit mayoscillate. If there were only one device to worry about thenthe values of R1 and R2 would probably be small enough tobe ignored; however, several devices normally comprise atotal system. Any ground return of a separate device, whoseoutput is in phase, can feedback in a similar manner andcause instabilities. Out of phase ground loops also aretroublesome, causing unexpected gain and phase errors.

The solution to most ground loop problems is to always usea single-point ground system, although this is sometimesimpractical. The third figure above is an example of a single-point ground system.

The single-point ground concept should be applied rigor-ously to all components and all circuits when possible. Vio-lations of single-point grounding are most common amongprinted circuit board designs, since the circuit is surroundedby large ground areas which invite the temptation to run adevice to the closest ground spot. As a final rule, make allground returns low resistance and low inductance by usinglarge wire and wide traces.

Occasionally, current in the output leads (which function asantennas) can be coupled through the air to the amplifierinput, resulting in high-frequency oscillation. This normallyhappens when the source impedance is high or the inputleads are long. The problem can be eliminated by placing asmall capacitor, CC, (on the order of 50 pF–500 pF) acrossthe LM3875 input terminals. Refer to the External Compo-nents Description section relating to component interactionwith Cf.

REACTIVE LOADING

It is hard for most power amplifiers to drive highly capacitiveloads very effectively and normally results in oscillations orringing on the square wave response. If the output of theLM3875 is connected directly to a capacitor with no series

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Application Information (Continued)resistance, the square wave response will exhibit ringing ifthe capacitance is greater than about 0.2 µF. If highly ca-pacitive loads are expected due to long speaker cables, amethod commonly employed to protect amplifiers from lowimpedances at high frequencies is to couple to the loadthrough a 10Ω resistor in parallel with a 0.7 µH inductor. Theinductor-resistor combination as shown in the Typical Ap-plication Circuit isolates the feedback amplifier from theload by providing high output impedance at high frequenciesthus allowing the 10Ω resistor to decouple the capacitiveload and reduce the Q of the series resonant circuit. The LRcombination also provides low output impedance at lowfrequencies thus shorting out the 10Ω resistor and allowingthe amplifier to drive the series RC load (large capacitiveload due to long speaker cables) directly.

GENERALIZED AUDIO POWER AMPLIFIER DESIGN

The system designer usually knows some of the followingparameters when starting an audio amplifier design:

Desired Power Output Input Level

Input Impedance Load Impedance

Maximum Supply Voltage Bandwidth

The power output and load impedance determine the powersupply requirements, however, depending upon the applica-tion some system designers may be limited to certain maxi-mum supply voltages. If the designer does have a powersupply limitation, he should choose a practical load imped-ance which would allow the amplifier to provide the desiredoutput power, keeping in mind the current limiting capabili-ties of the device. In any case, the output signal swing andcurrent are found from (where PO is the average outputpower):

(5)

(6)

To determine the maximum supply voltage the followingparameters must be considered. Add the dropout voltage(5 volts for LM3875) to the peak output swing, Vopeak, to getthe supply rail value, (i.e. + Vopeak + Vod) at a current ofIopeak). The regulation of the supply determines the unloadedvoltage, usually about 15% higher. Supply voltage will alsorise 10% during high line conditions. Therefore, the maxi-mum supply voltage is obtained from the following equation:

max. supplies ≈ ± (Vopeak + Vod(1 + regulation)(1.1) (7)The input sensitivity and the output power specs determinethe minimum required gain as depicted below:

(8)

Normally the gain is set between 20 and 200; for a 40W, 8Ωaudio amplifier this results in a sensitivity of 894 mV and89 mV, respectively. Although higher gain amplifiers providegreater output power and dynamic headroom capabilities,there are certain shortcomings that go along with the socalled “gain”. The input referred noise floor is increased andhence the SNR is worse. With the increase in gain, there isalso a reduction of the power bandwidth which results in adecrease in feedback thus not allowing the amplifier to re-

spond as quickly to nonlinearities. This decreased ability torespond to nonlinearities increases the THD + N specifica-tion.

The desired input impedance is set by RIN. Very high valuescan cause board layout problems and DC offsets at theoutput. The value for the feedback resistance, Rf1, should bechosen to be a relatively large value (10 kΩ–100 kΩ), andthe other feedback resistance, Ri, is calculated using stan-dard op amp configuration gain equations. Most audio am-plifiers are designed from the non-inverting amplifier configu-ration.

DESIGN A 40W/8Ω AUDIO AMPLIFIERGiven:

Power Output 40W

Load Impedance 8ΩInput Level 1V(max)Input Impedance 100 kΩBandwidth 20 Hz–20 kHz ±0.25 dB

Equations (5), (6) give:

40W/8Ω Vopeak = 25.3V Iopeak = 3.16ATherefore the supply required is: ±30.3V @3.16AWith 15% regulation and high line the final supply voltage is±38.3V using Equation (7). At this point it is a good idea tocheck the Power Output vs Supply Voltage to ensure that therequired output power is obtainable from the device whilemaintaining low THD + N. It is also good to check the PowerDissipation vs Supply Voltage to ensure that the device canhandle the internal power dissipation. At the same timedesigning in a relatively practical sized heat sink with a lowthermal resistance is also important. Refer to Typical Per-formance Characteristics graphs and the Thermal Con-siderations section for more information.

The minimum gain from Equation (8) is: AV ≥ 18We select a gain of 21 (Non-Inverting Amplifier); resulting ina sensitivity of 894 mV.

Letting RIN equal 100 kΩ gives the required input imped-ance, however, this would eliminate the “volume control”unless an additional input impedance was placed in serieswith the 10 kΩ potentiometer that is depicted in Figure 1.Adding the additional 100 kΩ resistor would ensure theminimum required input impedance.

For low DC offsets at the output we let Rf1 = 100 kΩ. Solvingfor Ri (Non-Inverting Amplifier) gives the following:

Ri = Rf1/(AV − 1) = 100k/(21 − 1) = 5 kΩ; use 5.1 kΩThe bandwidth requirement must be stated as a pole, i.e.,the 3 dB frequency. Five times away from a pole give0.17 dB down, which is better than the required 0.25 dB.Therefore:

fL = 20 Hz/5 = 4 Hz

fH = 20 kHz x 5 = 100 kHz

At this point, it is a good idea to ensure that the GainBandwidth Product for the part will provide the designed gainout to the upper 3 dB point of 100 kHz. This is why theminimum GBWP of the LM3875 is important.

GBWP = AV x f3 dB = 21 x 100 kHz = 2.1 MHz

GBWP = 2.0 MHz (min) for LM3875

Solving for the low frequency roll-off capacitor, Ci, we have:

Ci > 1/(2π Ri fL) = 7.8 µF; use 10 µF.

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Definition of TermsInput Offset Voltage: The absolute value of the voltagewhich must be applied between the input terminals throughtwo equal resistances to obtain zero output voltage andcurrent.

Input Bias Current: The absolute value of the average ofthe two input currents with the output voltage and current atzero.

Input Offset Current: The absolute value of the differencein the two input currents with the output voltage and currentat zero.

Input Common-Mode Voltage Range (or Input VoltageRange): The range of voltages on the input terminals forwhich the amplifier is operational. Note that the specifica-tions are not guaranteed over the full common-mode voltagerange unless specifically stated.

Common-Mode Rejection: The ratio of the input common-mode voltage range to the peak-to-peak change in inputoffset voltage over this range.

Power Supply Rejection: The ratio of the change in inputoffset voltage to the change in power supply voltages pro-ducing it.

Quiescent Supply Current: The current required from thepower supply to operate the amplifier with no load and theoutput voltage and current at zero.

Slew Rate: The internally limited rate of change in outputvoltage with a large amplitude step function applied to theinput.

Class B Amplifier: The most common type of audio poweramplifier that consists of two output devices each of whichconducts for 180˚ of the input cycle. The LM3875 is aQuasi-AB type amplifier.

Crossover Distortion: Distortion caused in the output stageof a class B amplifier. It can result from inadequate biascurrent providing a dead zone where the output does notrespond to the input as the input cycle goes through its zerocrossing point. Also for ICs an inadequate frequency re-sponse of the output PNP device can cause a turn-on delaygiving crossover distortion on the negative going transistionthrough zero crossing at the higher audio frequencies.

THD + N: Total Harmonic Distortion plus Noise refers to themeasurement technique in which the fundamental compo-nent is removed by a bandreject (notch) filter and all remain-ing energy is measured including harmonics and noise.

Signal-to-Noise Ratio: The ratio of a system’s output signallevel to the system’s output noise level obtained in theabsence of a signal. The output reference signal is eitherspecified or measured at a specified distortion level.

Continuous Average Output Power: The minimum sinewave continuous average power output in watts (or dBW)that can be delivered into the rated load, over the ratedbandwidth, at the rated maximum total harmonic distortion.

Music Power: A measurement of the peak output powercapability of an amplifier with either a signal duration suffi-ciently short that the amplifier power supply does not sagduring the measurement, or when high quality externalpower supplies are used. This measurement (an IHF stan-dard) assumes that with normal music program material theamplifier power supplies will sag insignificantly.

Peak Power: Most commonly referred to as the power out-put capability of an amplifier that can be delivered to theload; specified by the part’s maximum voltage swing.

Headroom: The margin between an actual signal operatinglevel (usually the power rating of the amplifier with particularsupply voltages, a rated load value, and a rated THD + Nfigure) and the level just before clipping distortion occurs,expressed in decibels.

Large Signal Voltage Gain: The ratio of the output voltageswing to the differential input voltage required to drive theoutput from zero to either swing limit. The output swing limitis the supply voltage less a specified quasi-saturation volt-age. A pulse of short enough duration to minimize thermaleffects is used as a measurement signal.

Output-Current Limit: The output current with a fixed out-put voltage and a large input overdrive. The limiting currentdrops with time once SPiKe protection circuitry is activated.

Output Saturation Threshold (Clipping Point): The outputswing limit for a specified input drive beyond that required forzero output. It is measured with respect to the supply towhich the output is swinging.

Output Resistance: The ratio of the change in output volt-age to the change in output current with the output aroundzero.

Power Dissipation Rating: The power that can be dissi-pated for a specified time interval without activating theprotection circuitry. For time intervals in excess of 100 ms,dissipation capability is determined by heat sinking of the ICpackage rather than by the IC itself.

Thermal Resistance: The peak, junction-temperature rise,per unit of internal power dissipation (units in ˚C/W), abovethe case temperature as measured at the center of thepackage bottom.

The DC thermal resistance applies when one output transis-tor is operating continuously. The AC thermal resistanceapplies with the output transistors conducting alternately at ahigh enough frequency that the peak capability of neithertransistor is exceeded.

Power Bandwidth: The power bandwidth of an audio am-plifier is the frequency range over which the amplifier voltagegain does not fall below 0.707 of the flat band voltage gainspecified for a given load and output power.

Power bandwidth also can be measured by the frequenciesat which a specified level of distortion is obtained while theamplifier delivers a power output 3 dB below the rated out-put. For example, an amplifier rated at 60W with ≤0.25%THD + N, would make its power bandwidth measured as thedifference between the upper and lower frequencies at which0.25% distortion was obtained while the amplifier was deliv-ering 30W.

Gain-Bandwidth Product: The Gain-Bandwidth Product isa way of predicting the high-frequency usefulness of an opamp. The Gain-Bandwidth Product is sometimes called theunity-gain frequency or unity-gain cross frequency becausethe open-loop gain characteristic passes through or crossesunity gain at this frequency. Simply, we have the followingrelationship:

ACL1 x f1 = ACL2 x f2Assuming that at unity-gain

(ACL1 = 1 or 0 dB) fu = f1 = GBWP,

then we have the following:

GBWP = ACL2 x f2This says that once fu (GBWP) is known for an amplifier,then the open-loop gain can be found at any frequency. This

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Definition of Terms (Continued)is also an excellent equation to determine the 3 dB point of aclosed-loop gain, assuming that you know the GBWP of thedevice. Refer to the diagram below.

Bi-amplification: The technique of splitting the audio fre-quency spectrum into two sections and using individualpower amplifiers to drive a separate woofer and tweeter.Crossover frequencies for the amplifiers usually vary be-tween 500 Hz and 1600 Hz. “Biamping” has the advantagesof allowing smaller power amps to produce a given soundpressure level and reducing distortion effects produced byoverdrive in one part of the frequency spectrum affecting theother part.

C.C.I.R./A.R.M.:Literally: International Radio Consultative Committee

Average Responding Meter

This refers to a weighted noise measurement for a Dolby Btype noise reduction system. A filter characteristic is usedthat gives a closer correlation of the measurement with thesubjective annoyance of noise to the ear. Measurementsmade with this filter cannot necessarily be related to un-weighted noise measurements by some fixed conversionfactor since the answers obtained will depend on the spec-trum of the noise source.

S.P.L.: Sound Pressure Level — usually measured with amicrophone/meter combination calibrated to a pressure levelof 0.0002 µBars (approximately the threshold hearing level).

S.P.L. = 20 Log 10P/0.0002 dB

Where P is the R.M.S sound pressure in microbars. (1 Bar =1 atmosphere = 14.5 lb./in2 = 194 dB S.P.L.).

01144914

FIGURE 3.

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Physical Dimensions inches (millimeters) unless otherwise noted

Order Number LM3875TNS Package Number TA11B

Order Number LM3875TFNS Package Number TF11B

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Notes

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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LM3875 Overture(TM) Audio Power Amplifier SeriesGeneral DescriptionFeaturesApplicationsTypical ApplicationFIGURE 1. Typical Audio Amplifier Application Circuit

Connection DiagramEquivalent Schematic Absolute Maximum Ratings Operating Ratings (Notes , )Electrical Characteristics (Notes , )Test Circuit #1 Test Circuit #2 Single Supply Application CircuitFIGURE 2. Typical Single Supply Audio Amplifier Application Circuit

External Components Description OPTIONAL EXTERNAL COMPONENT INTERACTION

Typical Performance CharacteristicsApplication InformationGENERAL FEATURESTHERMAL CONSIDERATIONSHeat SinkingDetermining Maximum Power DissipationDetermining the Correct Heat Sink

SIGNAL-TO-NOISE RATIOSUPPLY BYPASSINGLEAD INDUCTANCELAYOUT, GROUND LOOPS AND STABILITYREACTIVE LOADINGGENERALIZED AUDIO POWER AMPLIFIER DESIGNDESIGN A 40W/8Ohm AUDIO AMPLIFIER

Definition of TermsC.C.I.R./A.R.M.:FIGURE 3.

Physical Dimensions