LM3886Overture ™Audio Power Amplifier SeriesHigh-Performance68W Audio Power Amplifier w/MuteGeneral DescriptionThe LM3886is a high-performance audio power amplifier capable of delivering 68W of continuous average power to a 4Ωload and 38W into 8Ωwith 0.1%(THD +N)from 20Hz–20kHz.The performance of the LM3886,utilizing its Self Peak In-stantaneous Temperature (˚Ke)(SPiKe ™)Protection Cir-cuitry,puts it in a class above discrete and hybrid amplifiers by providing an inherently,dynamically protected Safe Oper-ating Area (SOA).SPiKe Protection means that these parts are completely safeguarded at the output against overvolt-age,undervoltage,overloads,including shorts to the sup-plies,thermal runaway,and instantaneous temperature peaks.The LM3886maintains an excellent Signal-to-Noise Ratio of greater than 92dB with a typical low noise floor of 2.0µV.It exhibits extremely low (THD +N)values of 0.03%at the rated output into the rated load over the audio spectrum,and provides excellent linearity with an IMD (SMPTE)typical rat-ing of 0.004%.Featuresn 68W cont.avg.output power into 4Ωat V CC =±28Vn 38W cont.avg.output power into 8Ωat V CC =±28V n 50W cont.avg.output power into 8Ωat V CC =±35V n 135W instantaneous peak output power capability n Signal-to-Noise Ratio ≥92dB n An input mute functionnOutput 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 EE |+|V CC |≤12V,thus eliminating turn-on and turn-off transients n 11-lead TO-220package Applicationsn Component stereo n Compact stereon Self-powered speakers n Surround-sound amplifiers nHigh-end stereo TVsTypical ApplicationOverture ™and SPiKe ™Protection are trademarks of National Semiconductor Corporation.DS011833-1*Optional components dependent upon specific design requirements.Refer to the External Components Description section for a component functional description.FIGURE 1.Typical Audio Amplifier Application CircuitMay 1999LM3886Overture Audio Power Amplifier Series High-Performance 68W Audio Power Amplifier w/Mute©1999National Semiconductor Corporation Connection DiagramPlastic Package(Note12)DS011833-2 Note1:Preliminary:call you local National Sales Rep.or distributor for availabilityTop ViewOrder Number LM3886Tor LM3886TFSee NS Package Number TA11B forStaggered Lead Non-IsolatedPackage or TF11B(Note1)forStaggered Lead Isolated Package 2Absolute Maximum Ratings(Notes6,5)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−|(Input Signal)84V Common Mode Input Voltage(V+or V−)and|V+|+|V−|≤80V Differential Input Voltage(Note16)60V Output Current Internally Limited Power Dissipation(Note7)125W ESD Susceptibility(Note8)3000V Junction Temperature(Note9)150˚C Soldering InformationT Package(10seconds)260˚C Storage Temperature−40˚C to+150˚C Thermal ResistanceθJC1˚C/W θJA43˚C/W Operating Ratings(Notes5,6)Temperature RangeT MIN≤T A≤T MAX−20˚C≤T A≤+85˚C Supply Voltage|V+|+|V−|20V to84VElectrical Characteristics(Notes5,6)The following specifications apply for V+=+28V,V−=−28V,I MUTE=−0.5mA with R L=4Ωunless otherwise specified.Limits apply for T A=25˚C.Symbol Parameter ConditionsLM3886Units(Limits) Typical(Note10)Limit(Note11)|V+|+|V−|Power Supply Voltage(Note14)V pin7−V−≥9V182084V(min)V(max)A M Mute Attenuation Pin8Open or at0V,Mute:OnCurrent out of Pin8>0.5mA,Mute:Off11580dB(min)P O (Note4)Output Power(Continuous Average)THD+N=0.1%(max)f=1kHz;f=20kHz|V+|=|V−|=28V,R L=4Ω|V+|=|V−|=28V,R L=8Ω|V+|=|V−|=35V,R L=8Ω6838506030W(min)W(min)WPeak P O Instantaneous Peak Output Power135W THD+N Total Harmonic Distortion Plus Noise60W,R L=4Ω,30W,R L=8Ω,20Hz≤f≤20kHz A V=26dB 0.030.03%%SR (Note4)Slew Rate(Note13)V IN=2.0Vp-p,t RISE=2ns198V/µs(min)I+(Note4)Total Quiescent Power SupplyCurrent V CM=0V,V o=0V,I o=0A5085mA(max)V OS (Note3)Input Offset Voltage V CM=0V,I o=0mA110mV(max)I B Input Bias Current V CM=0V,I o=0mA0.21µA(max) I OS Input Offset Current V CM=0V,I o=0mA0.010.2µA(max) I o Output Current Limit|V+|=|V−|=20V,t ON=10ms,V O=0V11.57A(min)V od (Note3)Output Dropout Voltage(Note15)|V+–V O|,V+=28V,I o=+100mA|V O–V−|,V−=−28V,I o=−100mA1.62.52.03.0V(max)V(max)PSRR (Note3)Power Supply Rejection Ratio V+=40V to20V,V−=−40V,V CM=0V,I o=0mAV+=40V,V−=−40V to−20V,V CM=0V,I o=0mA1201058585dB(min)dB(min)CMRR (Note3)Common Mode Rejection Ratio V+=60V to20V,V−=−20V to−60V,V CM=20V to−20V,I o=0mA11085dB(min)A VOL (Note3)Open Loop Voltage Gain|V+|=|V−|=28V,R L=2kΩ,∆V O=40V11590dB(min)GBWP Gain-Bandwidth Product|V+|=|V−|=30Vf O=100kHz,V IN=50mVrms 82MHz(min)3Electrical Characteristics(Notes5,6)(Continued)The following specifications apply for V+=+28V,V−=−28V,I MUTE=−0.5mA with R L=4Ωunless otherwise specified.Limits apply for T A=25˚C.Symbol Parameter ConditionsLM3886Units(Limits) Typical(Note10)Limit(Note11)e IN (Note4)Input Noise IHF—A Weighting FilterR IN=600Ω(Input Referred)2.010µV(max)SNR Signal-to-Noise Ratio P O=1W,A-Weighted,Measured at1kHz,R S=25Ω92.5dBP O=60W,A-Weighted,Measured at1kHz,R S=25Ω110dBIMD Intermodulation Distortion Test60Hz,7kHz,4:1(SMPTE)60Hz,7kHz,1:1(SMPTE)0.0040.009%Note2:Operation is guaranteed up to84V,however,distortion may be introduced from SPIKe Protection Circuitry if proper thermal considerations are not taken into account.Refer to the Thermal Considerations section for more information.(See SPIKe Protection Response)Note3:DC Electrical Test;refer to Test Circuit#1.Note4:AC Electrical Test;refer to Test Circuit#2.Note5:All voltages are measured with respect to the GND pin(pin7),unless otherwise specified.Note6: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.Note7:For operating at case temperatures above25˚C,the device must be derated based on a150˚C maximum junction temperature and a thermal resistance of θJC=1.0˚C/W(junction to case).Refer to the Thermal Resistance figure in the Application Information section under Thermal Considerations.Note8:Human body model,100pF discharged through a1.5kΩresistor.Note9:The operating junction temperature maximum is150˚C,however,the instantaneous Safe Operating Area temperature is250˚C.Note10:Typicals are measured at25˚C and represent the parametric norm.Note11:Limits are guaranteed to National’s AOQL(Average Outgoing Quality Level).Note12:The LM3886T package TA11B is a non-isolated package,setting the tab of the device and the heat sink at V−potential when the LM3886is directly mounted to 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 be isolated from V−.Note13: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 larger.Note14:V−must have at least−9V at its pin with reference to ground in order for the under-voltage protection circuitry to be disabled.Note15:The output dropout voltage is the supply voltage minus the clipping voltage.Refer to the Clipping Voltage vs Supply Voltage graph in the Typical Perfor-mance Characteristics section.Note16:The Differential Input Voltage Absolute Maximum Rating is based on supply voltages of V+=+40V and V−=−40V.Test Circuit#1(DC Electrical Test Circuit)DS011833-34Test Circuit#2(AC Electrical Test Circuit)DS011833-4Single Supply Application CircuitDS011833-5*Optional components dependent upon specific design requirements.Refer to the ExternalComponents Description section for a component functional description.FIGURE2.Typical Single Supply Audio Amplifier Application Circuit5Equivalent Schematic(excluding active protection circuitry)6External Components Description(Figure1and Figure2)Components Functional Description1.R IN Acts as a volume control by setting the voltage level allowed to the amplifier’s input terminals.2.R A Provides DC voltage biasing for the single supply operation and bias current for the positive input terminal.3.C A Provides bias filtering.4.C Provides AC coupling at the input and output of the amplifier for single supply operation.5.R B 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 below1.5V.6.C C(Note17)Reduces the gain(bandwidth of the amplifier)at high frequencies to avoid quasi-saturation oscillations of the output transistor.The capacitor also suppresses external electromagnetic switching noise created from fluorescent lamps.7.Ri Inverting input resistance to provide AC Gain in conjunction with R f1.8.Ci(Note17)Feedback capacitor.Ensures unity gain at DC.Also a low frequency pole(highpass roll-off)at: f c=1/(2πRi Ci)9.R f1Feedback resistance to provide AC Gain in conjunction with Ri.10.R f2(Note17)At higher frequencies feedback resistance works with C f to provide lower AC Gain in conjunction with R f1 and Ri.A high frequency pole(lowpass roll-off)exists at:f c=[R f1R f2(s+1/R f2C f)]/[(R f1+R f2)(s+1/C f(R f1+R f2))]11.C f(Note17)Compensation capacitor that works with R f1and R f2to reduce the AC Gain at higher frequencies.12.R M Mute resistance set up to allow0.5mA to be drawn from pin8to turn the muting function off.→RMis calculated using:R M≤(|V EE|−2.6V)/I8where I8≥0.5mA.Refer to the Mute Attenuation vs.Mute Current curves in the Typical Performance Characteristics section.13.C M Mute capacitance set up to create a large time constant for turn-on and turn-off muting.14.R SN(Note17)Works with C SN to stabilize the output stage by creating a pole that eliminates high frequency oscillations.15.C SN(Note17)Works with R SN to stabilize the output stage by creating a pole that eliminates high frequency oscillations.f c=1/(2πR SN C SN)16.L(Note17)Provides high impedance at high frequencies so that R may decouple a highly capacitive load and reduce the Q of the series resonant circuit due to capacitive load.Also provides a low impedance at low frequencies to short out R and pass audio signals to the load.17.R(Note17)18.C S Provides power supply filtering and bypassing.19.S1Mute switch that mutes the music going into the amplifier when opened.Note17:Optional components dependent upon specific design requirements.Refer to the Application Information section for more information.OPTIONAL EXTERNAL COMPONENT INTERACTIONAlthough the optional external components have specific desired functions that are designed to reduce the bandwidth and elimi-nate unwanted high frequency oscillations they may cause certain undesirable effects when they interact.Interaction may occurfor components whose reactances are in close proximity to one another.One example would be the coupling capacitor,C C,andthe compensation capacitor,C f.These two components act as low impedances to certain frequencies which will couple signals from the input to the output.Please take careful note of basic amplifier component functionality when designing in these compo-nents.The optional external components shown in Figure2and described above are applicable in both single and split voltage supply configurations.7Typical Performance CharacteristicsSafeAreaDS011833-18SPiKeProtection ResponseDS011833-19Supply Current vs Supply VoltageDS011833-20Pulse Thermal Resistance DS011833-21Pulse Thermal ResistanceDS011833-65Supply Current vs Output VoltageDS011833-22Pulse Power Limit DS011833-23Pulse Power LimitDS011833-24Supply Current vs Case TemperatureDS011833-25 8Typical Performance Characteristics(Continued)Input Bias Current vs Case TemperatureDS011833-26Clipping Voltage vs Supply VoltageDS011833-27Clipping Voltage vs Supply VoltageDS011833-28THD +N vs Frequency DS011833-29THD +N vs Frequency DS011833-30THD +N vs FrequencyDS011833-31THD +N vs Output Power DS011833-32THD +N vs Output Power DS011833-33THD +N vs Output PowerDS011833-34THD +N vs Output Power DS011833-35THD +N vs Output Power DS011833-36THD +N vs Output PowerDS011833-379Typical Performance Characteristics(Continued)THD +N vs Output PowerDS011833-38THD +N vs Output PowerDS011833-39THD +N vs Output PowerDS011833-40THD +N Distribution DS011833-41THD +N Distribution DS011833-42THD +N DistributionDS011833-43THD +N Distribution DS011833-44THD +N DistributionDS011833-45Output Power vs Load ResistanceDS011833-46 10Typical Performance Characteristics(Continued)Max Heatsink Thermal Resistance(˚C/W)at the Specified Ambient Temperature(˚C)Maximum Power Dissipation vs Supply VoltageDS011833-9 Note:The maximum heat sink thermal resistance values,øSA,in the table above were calculated using aøCS=0.2˚C/W due to thermal compound.Power Dissipationvs Output PowerDS011833-47Power Dissipationvs Output PowerDS011833-48Output Powervs Supply VoltageDS011833-49IMD60Hz,4:1DS011833-50IMD60Hz,7kHz,4:1DS011833-51IMD60Hz,7kHz,4:1DS011833-52 11Typical Performance Characteristics(Continued)Application InformationGENERAL FEATURESMute Function:The muting function of the LM3886allows the user to mute the music going into the amplifier by draw-ing less than 0.5mA out of pin 8of the device.This is accom-plished as shown in the Typical Application Circuit where the resistor R M is chosen with reference to your negative supply voltage and is used in conjuction with a switch.The switch (when opened)cuts off the current flow from pin 8to V −,thus placing the LM3886into mute mode.Refer to the Mute At-tenuation vs Mute Current curves in the Typical Perfor-mance Characteristics section for values of attenuation per current out of pin 8.The resistance R M is calculated by the following equation:R M (|V EE |−2.6V)/I8where I8≥0.5mA.Under-Voltage Protection:Upon system power-up the under-voltage protection circuitry allows the power supplies and their corresponding caps to come up close to their full values before turning on the LM3886such that no DC output spikes occur.Upon turn-off,the output of the LM3886is brought to ground before the power supplies such that no transients occur at power-down.IMD 60Hz,1:1DS011833-53IMD 60Hz,7kHz 1:1DS011833-54IMD 60Hz,7kHz,1:1DS011833-55Mute Attenuation vs Mute CurrentDS011833-56Mute Attenuation vs Mute CurrentDS011833-57Large Signal ResponseDS011833-58Power Supply Rejection RatioDS011833-59Common-Mode Rejection RatioDS011833-60Open LoopFrequency ResponseDS011833-6112Application Information(Continued)Over-Voltage Protection:The LM3886contains overvolt-age protection circuitry that limits the output current to ap-proximately 11Apeak while also providing voltage clamping,though not through internal clamping diodes.The clamping effect is quite the same,however,the output transistors are designed to work alternately by sinking large current spikes.SPiKe Protection:The LM3886is protected from instanta-neous peak-temperature stressing by the power transistor array.The Safe Operating Area graph in the Typical Perfor-mance Characteristics section shows the area of device operation 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 LM3886has a sophisticated ther-mal protection scheme to prevent long-term thermal stress to the device.When the temperature on the die reaches 165˚C,the LM3886shuts down.It starts operating again when the die temperature drops to about 155˚C,but if the temperature again begins to rise,shutdown will occur again at 165˚C.Therefore the device is allowed to heat up to a relatively high temperature if the fault condition is temporary,but a sustained fault will cause the device to cycle in a Schmitt Trigger fashion between the thermal shutdown tem-perature limits of 165˚C and 155˚C.This greatly reduces the stress imposed on the IC by thermal cycling,which in turn improves its reliability under sustained fault conditions.Since the die temperature is directly dependent upon the heat sink,the heat sink should be chosen as discussed in the Thermal Considerations section,such that thermal shutdown will not be reached during normal ing the best heat sink possible within the cost and space con-straints of the system will improve the long-term reliability of any power semiconductor device.THERMAL CONSIDERATIONSHeat SinkingThe choice of a heat sink for a high-power audio amplifier is made entirely to keep the die temperature at a level such that the thermal protection circuitry does not operate under normal circumstances.The heat sink should be chosen to dissipate the maximum IC power for a given supply voltage and rated load.With high-power pulses of longer duration than 100ms,the case temperature will heat up drastically without the use of a heat sink.Therefore the case temperature,as measured at the center of the package bottom,is entirely dependent on heat sink design and the mounting of the IC to the heat sink.For the design of a heat sink for your audio amplifier applica-tion refer to the Determining The Correct Heat Sink sec-tion.Since a semiconductor manufacturer has no control over which heat sink is used in a particular amplifier design,we can only inform the system designer of the parameters and the method needed in the determination of a heat sink.With this in mind,the system designer must choose his supply voltages,a rated load,a desired output power level,and know the ambient temperature surrounding the device.These parameters are in addition to knowing the maximum junction 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 thermal resistance required for a particular amplifier design.This data was based on θJC =1˚C/W and θCS =0.2˚C/W.We also provide a section regarding heat sink determination for any audio amplifier design where θCS may be a different value.It should be noted that the idea behind dissipating the maxi-mum power within the IC is to provide the device with a low resistance to convection heat transfer such as a heat sink.Therefore,it is necessary for the system designer to be con-servative in his heat sink calculations.As a rule,the lower the thermal resistance of the heat sink the higher the amount 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 thermal drop between the package and the heat sink.The heat sink must also have enough metal under the package to conduct heat from the center of the package bottom to the fins with-out excessive temperature drop.A thermal grease such as Wakefield type 120or Thermalloy Thermacote should be used when mounting the package to the heat sink.Without this compound,thermal resistance will be no better than 0.5˚C/W,and probably much worse.With the compound,thermal resistance will be 0.2˚C/W or less,assuming under 0.005inch combined flatness runout for the package and heat sink.Proper torquing of the mounting bolts is important and can be determined from heat sink manufacturer’s specification sheets.Should it be necessary to isolate V −from the heat sink,an in-sulating washer is required.Hard washers like beryluum ox-ide,anodized aluminum and mica require the use of thermal compound on both faces.Two-mil mica washers are most common,giving about 0.4˚C/W interface resistance with 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 and must be replaced should the IC be dismounted.Determining Maximum Power DissipationPower dissipation within the integrated circuit package is a very important parameter requiring a thorough understand-ing if optimum power output is to be obtained.An incorrect maximum power dissipation (P D )calculation may result in in-adequate heat sinking,causing thermal shutdown circuitry to operate and limit the output power.The following equations can be used to acccurately calculate the maximum and average integrated circuit power dissipa-tion for your amplifier design,given the supply voltage,rated load,and output power.These equations can be directly ap-plied to the Power Dissipation vs Output Power curves in the Typical Performance Characteristics section.Equation (1)exemplifies the maximum power dissipation of the IC and Equations (2),(3)exemplify the average IC power dissipation expressed in different forms.P DMAX =V CC 2/2π2R L (1)where V CC is the total supply voltageP DAVE =(V Opk /R L )[V CC /π−V Opk /2](2)where V CC is the total supply voltage and V Opk =V CC /π13Application Information(Continued)P DAVE=V CC V Opk/πR L−V Opk2/2R L(3)where V CC is the total supply voltage.Determining the Correct Heat SinkOnce 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 LM3886is1˚C/W and that usingThermalloy Thermacote thermal compound provides a ther-mal resistance,θCS(case to heat sink),of about0.2˚C/W asexplained in the Heat Sinking section.Referring to the figure below,it is seen that the thermal resis-tance from the die(junction)to the outside air(ambient)is acombination of three thermal resistances,two of which areknown,θJC andθCS.Since convection heat flow(power dis-sipation)is analogous to current flow,thermal resistance isanalogous to electrical resistance,and temperature dropsare analogous to voltage drops,the power dissipation out ofthe LM3886is equal to the following:P DMAX=(T Jmax−T Amb)/θJAwhereθJA=θJC+θCS+θSABut since we know P DMAX,θJC,andθSC for the applicationand we are looking forθSA,we have the following:θSA=[(T Jmax−T Amb)−P DMAX(θJC+θCS)]/P DMAX(4)Again it must be noted that the value ofθSA is dependentupon the system designer’s amplifier application and its cor-responding parameters as described previously.If the ambi-ent temperature that the audio amplifier is to be working un-der is higher than the normal25˚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 the de-termination 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 the semi-conductor manufacturer.These parameters are the junctionto case thermal resistance,θJC,T Jmax=150˚C,and the rec-ommended Thermalloy Thermacote thermal compound re-sistance,θCS.SIGNAL-TO-NOISE RATIOIn 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 integrated cir-cuit designs to discrete amplifier designs.Discrete transistoramps often“run out of gain”at high frequencies and there-fore have small bandwidths to noise as indicated below.Integrated circuits have additional open loop gain allowingadditional feedback loop gain in order to lower harmonic dis-tortion and improve frequency response.It is this additionalbandwidth that can lead to erroneous signal-to-noise mea-surements if not considered during the measurement pro-cess.In the typical example above,the difference in band-width appears small on a log scale but the factor of10inbandwidth,(200kHz to2MHz)can result in a10dB theoreti-cal difference in the signal-to-noise ratio(white noise is pro-portional to the square root of the bandwidth in a system).In comparing audio amplifiers it is necessary to measure themagnitude of noise in the audible bandwidth by using a“weighting”filter(Note18).A“weighting”filter alters the fre-quency response in order to compensate for the average hu-man ear’s sensitivity to the frequency spectra.The weightingfilters at the same time provide the bandwidth limiting as dis-cussed in the previous paragraph.Note18: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,and4.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-weighted fil-ter 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 the3kHz–7kHz region asshown below.DS011833-12DS011833-13DS011833-1414Application Information(Continued)SUPPLY BYPASSINGThe LM3886has excellent power supply rejection and does not require a regulated supply.However,to eliminate pos-sible oscillations all op amps and power op amps should have their supply leads bypassed with low-inductance ca-pacitors having short leads and located close to the package terminals.Inadequate power supply bypassing will manifest itself by a low frequency oscillation known as “motorboating”or by high frequency instabilities.These instabilities can be eliminated through multiple bypassing utilizing a large tanta-lum or electrolytic capacitor (10µF or larger)which is used to absorb low frequency variations and a small ceramic capaci-tor (0.1µF)to prevent any high frequency feedback through the power supply lines.If adequate bypassing is not provided the current in the sup-ply leads which is a rectified component of the load current may be fed back into internal circuitry.This signal causes low distortion at high frequencies requiring that the supplies be bypassed at the package terminals with an electrolytic ca-pacitor of 470µF or more.LEAD INDUCTANCEPower op amps are sensitive to inductance in the output lead,particularly with heavy capacitive loading.Feedback to the 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 in the lead inductance when the output is shorted.This energy can be dumped back into the supply bypass capacitors when the short is removed.The magnitude of this transient is re-duced by increasing the size of the bypass capacitor near the IC.With at least a 20µF local bypass,these voltage surges are important only if the lead length exceeds a couple feet (>1µH lead inductance).Twisting together the supply and ground leads minimizes the effect.LAYOUT,GROUND LOOPS AND STABILITYThe LM3886is designed to be stable when operated at a closed-loop gain of 10or greater,but as with any other high-current amplifier,the LM3886can be made to oscillate under certain conditions.These usually involve printed cir-cuit board layout or output/input coupling.When designing a layout,it is important to return the load ground,the output compensation ground,and the low level (feedback and input)grounds to the circuit board common ground 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 at the input,resulting in high frequency oscillation or excessive distortion.It is advisable to keep the output compensation components and the 0.1µF supply decoupling capacitors as close as possible to the LM3886to reduce the effects of PCB trace resistance and inductance.For the same reason,the ground 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 be avoided by returning all grounds separately to a common point.Without isolating the ground signals and returning the grounds 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 exists between two ground points.Ideally a ground is a ground,butunfortunately,in order for this to be true,ground conductors with zero resistance are necessary.Since real world ground leads possess finite resistance,currents running through them will cause finite voltage drops to exist.If two ground re-turn lines tie into the same path at different points there will be a voltage drop between them.The first figure below shows a common ground example where the positive input ground and the load ground are returned to the supply ground point via the same wire.The addition of the finite wire resistance,R 2,results in a voltage difference between the two points as shown below.The load current I L will be much larger than input bias current I I ,thus V 1will follow the output voltage directly,i.e.in phase.Therefore the voltage appearing at the non-inverting input is effectively positive feedback and the circuit may oscillate.If there were only one device to worry about then the values of R 1and R 2would probably be small enough to be ignored;however,several devices normally comprise a total system.Any ground return of a separate device,whose output is in phase,can feedback in a similar manner and cause instabili-ties.Out of phase ground loops also are troublesome,caus-ing unexpected gain and phase errors.The solution to most ground loop problems is to always use a single-point ground system,although this is sometimes im-practical.The third figure below 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.Viola-tions of single-point grounding are most common among printed circuit board designs,since the circuit is surrounded by large ground areas which invite the temptation to run a device to the closest ground spot.As a final rule,make all ground returns low resistance and low inductance by using large wire and wide traces.Occasionally,current in the output leads (which function as antennas)can be coupled through the air to the amplifier in-put,resulting in high-frequency oscillation.This normally happens when the source impedance is high or the input leads are long.The problem can be eliminated by placing aDS011833-1515。
