National Semiconductor LM4756 Dual 7W Audio Power Amplifier w/Mute Standby Volume Control handbook
Contents
1. 188 010 20 020 251 177 005 34 4 550 121 2X 45 X 1204 010 1 110 2 19 N 1005 010 17 1850 25 10 69 0 12 8 860 860 020 21 841 121 84 0 51 866 4 020 2240 5 866 221 Jas yo 4 B j ih M 028 003 ME 15X 016 5 002 JL 0 7150 07 050 0 4150 051 010 0 251 80 1 21 024 10 69 200 2700 17 181 5 08 CONTROLLING DIMENSION IS INCH VALUES IN 1 ARE MILLIMETERS Staggered 15 Lead Non Isolated TO 220 Package Order Number LM4756TA See NS Package Number TA15A TA15A Rev B www national com 16 Physical Dimensions inches millimeters unless otherwise noted Continued 788 010 20 0240 25 151 002 3 84 0 05 421 005 10 69 0 12 0271 003 700 012 0 69 0 07 17 78 0 3 J o 0 TYP 20 0 56 010 039 009 0 25 99 0 33 177 005 14 5 0 12 060 003 1 5200 689 005 17 5 0 12 150 012 3 81 0 3 010 t0 251 A E to 251 4 014 018 36 0 46 089 103 2 26 2 62 CONTROLLING DIMENSION IS INCH VALUES IN ARE MILLIMETERS TA15D Rev Staggered 15 Lead Non Isolated 220 Package with 90 degree lead bend Order Number LM4756TA Spec LB10 See NS Package Number TA15D LIFE SUPPORT POLICY NATIONAL S P2. VCC GND Out Right Volume IN Left RSNI T MUTE stey 19 x CSNI poy il n IN Right C vai 51 su 82 OFF PLAY PLAY 20064442 Bottom Layer e e A vey 100 8 110182 o ureu 20064443 www national com 14 Application Information continued BILL OF MATERIALS FOR REFERENCE PCB Symbol Value Tolerance Type Description Comment Bou Pius 1 50 1 4 Watt Ry 10kQ 1 4 Watt Cina Cine 47 Metallized Polyester Film Crus Cus 0 01uF Monolithic Ceramic Cano 0 1uF 20 Monolithic Ceramic Qr 29 Bones 20 Wap Couri Courz 1 000 Electrolytic 25V Cai 0 1uF Monolithic Ceramic Cs2 10uF Electrolytic 35V 1 000 Electrolytic 35V 5 Ja J4 Non Switched PC Mount RCA Jack Jo Je PCB Banana Jack BLACK Ja Js PCB Banana Jack RED 2N3904 Small Signal 1 CNN U1 15 lead TO 220 Power Socket LM4756 IC 15 www national com 9S vIWN 1l LM4756 Physical Dimensions inches millimeters unless otherwise noted
3. Standby Mute Pin Function Table Standby Pin 9 Mute Pin 10 l orOpes Open Standby H H or Open Standby www national com Typical Performance Characteristics THD N vs Frequency Vec 18V Pour 1W Channel 40 80kHz BW 2 FREQUENCY Hz 20064449 THD N vs Output Power Vec 18V f 1kHz 40 80kHz BW 20 10 D I H 9 2 z H 0 1 0 1 1 10 20 OUTPUT POWER W 20064450 Output Power vs Supply Voltage f 1kHz 49 80kHz BW 10 9 8 10 THD N gt 7 lt 5 6 1 THD N 5 5 4 a 3 2 amp E 2 2 1 0 14 16 18 20 22 SUPPLY VOLTAGE V 20064439 THD N vs Frequency 80 kHz filter HHH x 20 100 tk 10k 20k Frequency Hz 20064412 THD N vs Output Power 22V R 80 f 1kHz 80 kHz filter E fa n H Output Power W 20064418 Output Power vs Supply Voltage f 1kHz 80 80kHz BW 14 7 2 10 10 THD N 8 6 a E 4 i a 5 2 2 s 0 14 16 18 20 22 24 26 28 30 SUPPLY V
4. and 0s is the sink to ambient thermal resistance Once the maximum power dissipation is calculated from Equation 2 above the minimum heat sink thermal resis tance can be calculated from Equation 4 below Osa Tumax m Pomax 4 Example Vcc 22 8Q 1 C W 0 5 C W 1 Pomax 2 22V 2 21 80 6W 2 150 25 6W 1 C W 0 5 C W eW 19 C W Therefore the minimum heat sink thermal resistance re quired is 19 C W for both channels being driven simulta neously at maximum power dissipation into an 8Q load using 22V voltage supply Again remember to take into account the unregulated supply voltage and reactive load impedance dips Should it be necessary to isolate the tab of the IC from the heat sink an insulating washer can be used There are many different types of insulating washers with varying thermal resistances Good washers can be obtained from Thermal loy or Berquist Refer to the References list for contact information for these manufacturers Supply Bypassing The LM4756 has good power supply rejection however for all power amplifiers proper power supply bypassing is re quired To prevent oscillations and instability all op amps www national com 12 Application Information continued and power op amps should have their supply leads by passed with low inductance capacitors having short leads All high fr
5. 781 4756 hy fg National Semiconductor LM4756 September 2003 Dual 7W Audio Power Amplifier w Mute Standby and Volume Control General Description The LM4756 is a stereo audio amplifier capable of delivering 7W channel at 10 distortion into 4Q or 80 load The power amp has an internally set gain of 30dB A 0 5 DC controlled volume block provides 80dB of attenuation from input to line out Line outputs are available after the volume control for signal routing The amplifier has a smooth transition fade in out mute and a power conserving standby function which are controlled through TTL or CMOS logic Both functions provide over 75dB of attenuation The LM4756 maintains an excellent Signal to Noise ratio of greater than 70dB with a low noise floor less than 2mV The IC also maintains above 50dB of channel separation The LM4756 is available in a 15 lead non isolated plastic package and is designed for use in TV applications requiring single supply operation Connection Diagrams Plastic Package PWRGNDR VOUTR Vcc VOUTL PWRGNDL MUTE STBY GND BIAS NC VINL VAROUTL VOLUME VAROUTR VINR 20064433 Top View Order Number LM4756TA See NS Package Number TA15A 90 Degree Lead Bend Order Number LM4756TA Spec LB10 See NS Package Number TA15D Key Specifications m Output power into 40 or 8Q at 10 THD 7W typ m Maximum operating voltage 28V max m Power output stage Noise floor 2 typ m Li
6. on AC waveforms only DC shorts from the output to ground are not protected Generally this is not a concern as there is a DC blocking capacitor on the output to protect the speaker from single supply DC bias Thermal Shutdown Protection The LM4756 has a thermal shutdown protection scheme that limits the drive capability of each amplifier output when the internal die temperature reaches the temperature trip point of 150 C The limiting of the output current drive capability is proportional to increasing die temperature When the IC is in thermal shutdown mode all of the DC biases of the IC remain unchanged It is only the current drive capability of the output power transistors that is limited This thermal shutdown mechanism provides for smooth au dio attenuation rather than abruptly pulling the outputs to ground When the outputs are being limited the maximum www national com 9SZVINT LM4756 Application Information continued voltage swing will be reduced creating a clipping effect as shown in Figure 7 With further increases in die temperature the maximum voltage swing will be further reduced The thermal sensing mechanism monitors the global die temperature and is not intended to operate quickly enough to shutdown the IC for extremely high power dissipation pulses created by driving very low impedance loads In Figure 7 a 50kHz input signal is used to show the clipping and attenuating effect of the LM4756 when comi
7. OLTAGE V 20064438 www national com 9S vIN 1l LM4756 Typical Performance Characteristics continued Power Dissipation vs Output Power R 40 f 1kHz THD lt 1 0 Power Dissipation W 0 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 Output Power W 20064421 Channel Separation vs Frequency T 5 Kk a E c o 20 100 1k 10k 100k Frequency Hz 20064423 Volume Attenuation vs DC Voltage gt 10 5 OJR 10ka a 5 10 f 1kHz n o 2V o gt 30 o Z 40 50 50 c 2 70 S 80 90 lt 100 0 0 0 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 4 5 5 0 DC Volts Pin 3 Volume 20064425 Attenuation dBr SUPPLY CURRENT mA Power Dissipation vs Output Power 14 13 R 80 12 f 1kHz 11 THD lt 1 0 10 28V e Power Dissipation W QC O4 UD CO lt lt 0 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 Output Power W 20064422 Attenuation vs Frequency Frequency Hz 20064424 Supply Current vs Supply Voltage 14 16 18 20 22 24 26 28 30 SUPPLY VOLTAGE V 20064437 www national com Typical Performance Characteristics Continued Supply Curren
8. RODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION As used herein 1 Life support devices or systems are devices or systems which a are intended for surgical implant into the body or b support or sustain life and whose failure to perform when properly used in accordance with instructions for use provided in the labeling can be reasonably expected to result in a significant injury to the user www national com National Semiconductor National Semiconductor Americas Customer Europe Customer Support Center Support Center Fax 49 0 180 530 85 86 Email new feedback 9 nsc com Email europe support nsc com Tel 1 800 272 9959 Deutsch Tel 49 0 69 9508 6208 English Tel 44 0 870 24 0 2171 Fran ais Tel 33 0 1 41 91 8790 2 A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system or to affect its safety or effectiveness National Semiconductor National Semiconductor Asia Pacific Customer Support Center Email nsc com Fax 81 3 5639 7507 Email jpn feedback 9 nsc com Tel 81 3 5639 7560 Japan Customer Support Center National does not assume any responsibility for use of any circuitry described no circuit pa
9. alue the single supply biasing of the output stage will then begin to charge up to 2 The pop performance under this condition is quite good however it is highly recommended that the Mute and Standby pin voltages are high at 5V while the main power supply voltage Vcc is ramping up Once the main supply voltage is up to its full value the standby function can then be brought low to OV The biasing of the amplifier and the output stage will then begin to charge up to 2 Notice that the supply current draw is approxi mately 7 until the standby function is disabled at which point the supply current increases to approximately 13mA while in mute mode Once the single supply biasing is established the mute pin voltage can be brought down to OV allowing the IC to amplify the input signal As shown in Figure 4 the input signal that is applied to the IC all throughout the power up process is not passed to the speaker until the mute function is disabled The typical quiescent power supply current while in play mode is approximately 80mA The same sequence should be applied when powering down the device First the IC should be placed into mute mode muting the output then placed into standby mode where the bias and output coupling caps are gradually discharged to ground Once the biasing of the IC is brought to ground the main power supplies can be powered down This power up and power down sequence is highly recommended Abrup
10. ause a flyback voltage that may damage the IC As shown in Figure 6 first notice that music is playing at the output When the mechanical standby switch is toggled from ground play mode to 5V standby mode transistor Q1 is quickly turned on discharging capacitor C7 bringing the voltage at the volume pin pin 3 to ground This quickly attenuates the audio signal at the output as shown in Figure 6 While the input signal is being attenuated the diode D1 becomes reverse biased and the voltage at the standby pin starts to charge through R4 C8 and C9 There is also a finite amount of current flowing through R5 as well but because of its high resistance we can neglect it in the charge up timing of pin 9 Note that when the standby switch was grounded the diode D1 was clamping the standby pin low setting the initial voltage condition of C8 at a low voltage Once C8 starts charging up diode D2 becomes forward biased and C9 also starts charging up This brings the standby and mute pin voltages up simultaneously By the time the standby pin www national com 9SZVINT LM4756 Application Information continued voltage enables the standby function the voltage at the volume pin will already have been ramped down to OV and the output signal will be close to OV When the IC is in standby mode the biasing of the IC is brought down to ground and the quiescent supply current is around 7 mA When the standby switch in Figure 5 is tog
11. equency bypass capacitors should be located as close to the package terminals as possible and have a clear unobstructed current return path to ground It is typical to use capacitor values that are a factor of 100 different from each other to minimize interaction with each other The LM4756 should be bypassed with 0 1 ceramic and 100pF tantalum capacitors for optimum performance The 100uF tantalum can be replaced with an electrolytic but the bypassing per formance of the tantalum will be better There should also be large supply reservoir capacitors of about 4700yF on each supply rail A larger reservoir capacitor will reduce the supply ripple and will supply larger current burst requirements in stead of requiring those large currents to come from the main power supply transformer If adequate bypassing is not provided the current in the supply leads which is a rectified component of the load current may be fed back into internal circuitry This signal may cause signal distortion to increase Layout and Ground Loops When designing a printed circuit board layout it is important to return the load ground any output compensation ground and the low level feedback and input grounds to the circuit board common ground point through separate paths Large currents flowing along a ground conductor will generate voltages which effectively act as signals to the input ground reference This can result in high frequency oscillation or excess
12. es but then attenuate fast to 80dB at lower DC voltages This means that when the volume control is turned down the amplification is quickly attenuated while at normal listening levels attenuation changes are more gradual Please refer to the Volume Attenuation vs DC Volt age curve in the Typical Performance Characteristics sec tion The DC voltage to pin 3 can be controlled with a potentiom eter as shown in Figures 1 3 100kQO resistor and a capacitor form an RC lowpass filter that keeps any unnec essary noise from coupling into the device Any noise that is coupled into the device is gained up by 40dB 51kQ0 Volume vr Pin 3 T 20064427 FIGURE 3 Volume Pin Lowpass Filter Turn On Off Characteristics In order to minimize turn on and off pops the LM4756 should be powered up by using the sequence described below Figure 4 shows the sequence for turn on and off Since the power supply voltage of the power amplifier is about 4 times more than a 5V power supply it is assumed that the logic voltage supply for the standby and mute func tions is up before the large power supply reservoir capacitors are charged The LM4756 should be placed into standby mode before the undervoltage protection circuitry is dis abled The undervoltage protection circuitry will keep the outputs of the LM4756 at OV until the voltage from Vec to GND is about 9 5V If the standby function is disabled when the supply voltage exceeds this v
13. gled to ground for play mode transistor Q1 is quickly cut off and diode D1 is forward biased When D1 is forward biased capacitor C8 is quickly discharged to ground bringing the standby pin voltage to OV When C8 is discharged diode D2 becomes reverse biased allowing capacitor C9 to discharge to ground through R5 Diode D2 was clamping the voltage on C9 to the same voltage as C8 Because R5 is 10 times R4 it takes longer for the mute function to be disabled While the mute voltage is decreasing the biasing of the amplifier is charging up since the standby function has already been 5V VOLUME 51k R14 10k Q1 c7 2N3904 22 10V VOLUME CONTROL POT 22V 5V 1N4152 disabled While the mute pin voltage is decreasing the vol ume pin voltage is slowly increasing through the charge up capacitor C7 Charging of the volume pin is from the 5V through the volume control pot R6 and 7 Notice that the time constant of the volume pin charging is greater than the mute pin discharging As shown in Figure 6 the volume control function finally ramps up the input signal allowing music to be amplified at the output Please notice that with this circuit the standby switch will override the mute switch as required in the IC s functional truth table in Table 1 Also note once again that most systems typically utilize a microprocessor or COP microcontroller to interface with the logic control functions of t
14. he LM4756 When a clean logic signal is used as from a microcontroller RC lowpass filtering is not required for the mute and standby functions 5V MUTE PIN 10 5V 20064429 FIGURE 5 Turn On Off External Circuitry www national com 10 Application Information continued OUTPUT OV STANDBY SWITCH STANDBY MUTE SWITCH MUTE VOLUME t 20064430 FIGURE 6 Turn On Off External Circuitry Sequence Line Out The line out function for the LM4756 is intended to provide preamp output control for signal routing to an external power amplifier An example of this would be in a TV where the TV s remote control provides volume control on the audio signals that may be sent to a home theater receiver The line out amplifier is only able to drive high impedance loads like 2kQ and 10kQ Since the LM4756 utilizes a single 22V power supply the output of the line out amplifier is biased at 1 2 of Voc or 11V Because of this its output should be capacitor coupled to any other processing IC The value of the capaci tor is chosen by using Equation 1 f 1 2nRC 1 where R is the processing IC input impedance and f is the lowest audio frequency to be passed like 20Hz The value of capacitance is then calculated For a 10kO impedance C 1pF AC Short Circuit Protection The LM4756 is AC short circuit protected with a current limiting setting minimum of 2 0A Current limiting protection works
15. ion Plus Noise 1W f 1 kHz 8Q 0 4 1 max Note 2 Xtalk Channel Separation f21kHz P 25W R 80 50 dB Note 2 Power Amp Closed Loop Gain Error Internal Gain 30 dB 0 5 dB max SR Slew Rate Vin 100 mVp p trise 2 ns 80 3 V us Note 2 Rin Input Impedance 32 1 lo Output Current Limit Vin 100 mV DC ton 1 ms 19 2 5 2 0 A min Note 1 Ei PSRR Power Supply Rejection Ratio Vpin 13 AC 1 Vrms f 100 Hz 50 dB Note 2 Vom OV lo 0 mA EB GBWP Gain Bandwidth Product f 100 kHz Vy 50 mvrms 2 ME Power Bandwidth 3 dB Bandwidth at 5W k VCA Output Noise IHF Weighting Filter 55 uV Rin 250 Gut Power Amp Output Noise IHF A Weighting Filter 1 8 mV Rin 250 www national com 9SZVINT LM4756 Electrical Characteristics Notes 3 4 Continued The following specifications apply for Vec 22V and Volume at OdB unless otherwise specified Limits apply for T4 25 LM4756 Symbol Parameter Typical Limit ipis Note 8 Note 9 SNR Signal to Noise Ratio Measured at 1 kHz 250 P 4 8W A Weighted dB Standby Vit Standby Low Input Voltage 0 8 V max Vin Standby High Input Voltage 2 0 V min Mute Vit Mute Low Input Voltage 0 8 V max Vin Mute High Input Voltage 2 0 V min Note 1 DC Electrical Test Note 2 AC Electrical Test Note 3 Absolute Maximum Ratings indicate limits beyond which damage t
16. ive distortion Output compensation components and the high frequency supply bypass capacitors should be placed as close as possible to the IC to reduce the effects of PCB trace resistance and inductance For cases where long traces must exist widen the traces to minimize their induc tance References International Electronic Research Corporation P O Box 7704 Burbank California 91510 7704 818 842 7277 Thermalloy Inc P O Box 810839 Dallas Tx 75381 0839 214 243 4321 www thermalloy com 13 www national com 9S vIWN 1 LM4756 Application Information continued LM4756 REFERENCE BOARD ARTWORK Composite Layer National Semiconductor LM4756 Stereo 7 Watt Amplifier With DC Volume Control MUTE and STBY modes Evaluation Board J4 Line Line IN Left e IN Right 51 OFF PLAY 20064441 Top Layer 20064440 Silk Layer National Semiconductor LM4756 Stereo 7 Watt Amplifier With DC Volume Control MUTE and STBY modes Evaluation Board COut2 1 15 8 ago Line OutL n ai CB a O 8 De z Gut Left cng C02 Tg 8 a CN3 Cot GR 33 Line OutR R8 2 ae CN2 COuti R2
17. ne output Noise floor 55yV typ m OV 5V DC controlled volume attenuation 80dB typ m Mute attenuation 75dB typ m Standby mode supply current 7mA typ Features m Quiet fade in out mute function m Stereo variable line out pins m AC output short circuit protection m Thermal shutdown protection Applications m Audio amplifier for stereo TVs m Audio amplifier component stereo m Audio amplifier compact stereo Plastic Package 20064444 Top View U Wafer Fab Code Z Assembly Plant Code XY Date Code TT Die Traceability LM4756TA LM4756TA 2003 National Semiconductor Corporation DS200644 www national com euinjoA pue qpuejs einjy M AZ eng 96 77 LM4756 Typical Application L 10 pF 100 pF 0 1 pF 4700 uF 10 pF Line OUT 10k L T 7 13 7 VAROUTL internal circuit VourL LM4753 Mute amp Standby volume Circuitr Control y VAROUTR R 10 uF Line OUT 10k FIGURE 1 Typical Audio Amplifier Application Circuit PWRGNDL VOUTR PWRGNDR 20064401 www national com 2 Absolute Maximum Ratings Notes 3 4 If Military Aerospace specified devices are T Package 10 sec Storage Temperature required please contact the National Semiconductor Sales Office Distributors for availability and specifications Supply Voltage Output Current Power Dissipation N
18. ng out of thermal shutdown Tek SIME 20 5 5 20064431 FIGURE 7 Thermal Shutdown Response THERMAL CONSIDERATIONS Determining Maximum Power Dissipation It is important to determine the maximum amount of package power dissipation in order to choose an adequate heat sink Improper heat sinking can lead to premature thermal shut down operation causing music to cut out Equation 2 can be used to calculate the approximate maximum integrated circuit power dissipation for your amplifier design given the supply voltage and rated load with both channels being driven simultaneously Pomax 2 2 2 To ensure that a proper heat sink is chosen be sure to take into account the effects of the unregulated power supply voltage variation and the highly reactive load impedance variation over frequency A poorly regulated power supply can have a supply voltage variation of more than 10V Be sure to take into account the no load power supply voltage A nominally rated 8Q load can have an impedance dip down to 5Q at low frequencies As well the load is not purely resistive and this causes the amplifier output current to be out of phase with the output voltage When the current and voltage are out of phase the internal power dissipation actually increases Equation 2 can be directly applied to the Power Dissipation vs Output Power curves in the Typical Performance Charac teristics section However the curves take in
19. o the device may occur Operating Ratings indicate conditions for which the device is functional but do not guarantee specific performance limits Electrical Characteristics state DC and AC electrical specifications under particular test conditions which guarantee specific performance limits This assumes that the device is within the Operating Ratings Specifications are not guaranteed for parameters where no limit is given however the typical value is a good indication of device performance Note 4 All voltages are measured with respect to the ground pin 8 unless otherwise specified Note 5 The maximum power dissipation must be derated at elevated temperatures and is dictated by T jyAx 8ya and the ambient temperature Ta The maximum allowable power dissipation is PDMAX TA 8jA or the number given in the Absolute Maximum Ratings whichever is lower For operating at case temperatures above 25 C the device must be derated based on a 150 C maximum junction temperature and a thermal resistance of 0jc 1 5 C W junction to case Note 6 Human body model 100pF discharged through a 1 5kQ resistor Note 7 Machine model 200pF 240pF discharge through all pins Note 8 Typicals are measured at 25 C and represent the parametric norm Note 9 Tested limits are guaranteed to National s AOQL Average Outgoing Quality Level Note 10 Datasheet min max specification limits are guaranteed by design test or statistical analysis
20. ot affected by the mute function Please refer to Table 1 for each input condition To prevent mechanical switch bouncing from adversely af fecting the functionality of the IC an RC lowpass filter should be used as shown in Figure 2 This circuit replaces the need for a debounce circuit when using a mechanical switch to control the IC logic functions However most systems typi cally utilize a microprocessor or COP microcontroller to in terface with the logic control functions of the LM4756 When a clean logic signal is used as from a microcontroller the RC lowpass filter is not required Supply Current vs Mute Voltage 100 SUPPLY CURRENT mA a 0 0 1 0 2 0 3 0 4 0 5 0 MUTE VOLTAGE V 20064447 51k0 5k0 Mute Standby Pin 10 Pin 9 20064426 FIGURE 2 Mute and Standby Pin Lowpass Filters Standby Function The standby function allows the user to place the LM4756 into a power conserving mode that draws less than 10 mA of quiescent power supply current With the IC in this mode while using 22 for the supply voltage the IC draws about 150mW of power The standby function is enabled by placing a logic 1 or 5V onto the standby pin pin 9 To disable the function allowing music to be passed to the output a logic 0 or OV should be placed on the standby pin When the standby function is enabled it overrides the mute function and places the IC in its powe
21. ote 5 ESD Susceptibility Note 6 ESD Susceptibility Note 7 Junction Temperature Soldering Information Electrical Characteristics Notes Internally Limited Input Signal Voltage Range 260 C 40 C to 150 C 3V Operating Ratings Notes 3 4 30V Temperature Range Twin 5 lt Tmax d Supply Volt push MS i Ambient 250V dnd i ient 150 C Jc Junction to Case 3 4 40 C lt T lt 85 15V to 28V 35 C W 1 5 C W The following specifications apply for Vcc 22V and Volume at OdB unless otherwise specified Limits apply for Ta 25 C LM4756 Symbol Parameter Conditions Typical pas Note 8 Note 9 lca Total Quiescent Power Supply Current Vem OV V OV 1 0 mA 20 mA min Note 1 80 140 mA max Standby Current 5V Standby on 7 10 mA max Note 1 luurE Mute Current VmurTe 5V Mute on 13 20 mA Mute Attenuation Vuute 5V OV Mute on 75 dB min Note 2 Signal Input OV OV Mute off 5 dB 2 Vrms il Volume Attenuation Range 80 dB min Volume Absolute Attenuation Pin 3 OV 80 dB 2V 14 dB 3 5 dB max Line out 3V 8 GB 4V dB 5V 0 Line out Offset Voltage 20 55 mV max Po Output Power Continuous Average THD N 10 max Note 1 f 1 kHz 40 18V 7 W f 1 kHz 80 Voc 22V 7 6 0 W min THD N Total Harmonic Distort
22. r conserving mode If the mute function is enabled while in standby mode the IC will continue to remain in standby mode After the standby function is disabled the IC will be placed into mute mode Please refer to the Table 1 for each input condition TABLE 1 Mute and Standby Functional Conditions Standby Pin 9 Mute Pin 10 Operating Conditions OV or open OV Music Plays OV or open 5V or open oV 5V or open Mute Mode Standby Mode Standby Mode To prevent mechanical switch bouncing from adversely af fecting the functionality of an IC an RC lowpass filter should be used as shown in Figure 2 This circuit replaces the need www national com 9S vIN 1l LM4756 Application Information continued for a debounce circuit when using a mechanical switch to control the IC logic functions However most systems typi cally utilize a microprocessor or COP microcontroller to in terface with the logic control functions of the LM4756 When a clean logic signal is used as from a microcontroller the RC lowpass filter is not required DC Volume Control The DC volume control for the LM4756 works between OV and 5V When the volume pin pin 3 is OV the IC s preamp stage is fully attenuated to 80dB When the volume pin is at 5V the preamp stage passes audio at OdB The DC volume attenuation curve for the LM4756 is in tended to provide smooth accurate attenuation changes at higher DC voltag
23. t changes in output current from enabling standby while the output is driving an inductive load like a speaker may cause the IC to handle extreme levels of power due to inductive kickback The IC may not be able to handle this and should be avoided www national com Application Information continued STBY V MUTE V BIAS V INPUT mV PLAY OUTPUT V Vcc 2 t Icco me 80 mA NO INPUT 13 mA 7 gt t 20064428 FIGURE 4 Turn On Off Sequence If the sequence described above and shown in Figure 4 is not used then the external circuitry shown in Figure 5 should be used to minimize turn on off pops and protect the output stage against SOA violations In Figure 5 there are only a few components that are differ ent than the ones described earlier for lowpass filtering the pin voltages The new components are Q1 R2 R3 D1 and D2 All of the other components will perform the same func tions that were previously described The explanation of how the circuit in Figure 5 works will be related to the timing waveforms in Figure 6 The circuit in Figure 5 protects the LM4756 from SOA violations by ensur ing that the enabling of the standby function when music is playing will not quickly bring the biasing to ground before the input signal is smoothly attenuated through the volume func tion Again this is important because any quick changes in output current when driving an inductive load will c
24. t vs Standby Voltage 100 SUPPLY CURRENT mA a 0 0 1 0 2 0 3 0 4 0 5 0 STANDBY VOLTAGE V 20064448 Application Information GENERAL FEATURES The LM4756 has a number of valuable functions that make this audio amplifier IC an all in one solution The IC has a stereo audio path from input to output with a DC voltage controlled volume attenuator in the preamp section After the volume attenuator is a line out connection for preamp out control The attenuation curve versus DC voltage can be found by referring to the Volume Attenuation vs DC Voltage graph in the Typical Performance Characteristics section The IC also possesses a mute function to provide audio attenuation as used on a remote control for a TV as well as a standby function for power conservation when not being used The IC is well protected with thermal shutdown and output AC short circuit protection Mute Function The muting function of the LM4756 allows the user to mute the music going into the amplifier providing over 60 dB of attenuation from input to output The function is enabled by placing a logic 1 or 5V onto the mute pin pin 10 To disable the function allowing music to be passed to the output a logic 0 or OV should be placed on the mute pin By placing the device into mute mode each of the power amplifier outputs are simultaneously muted The DC volume control and line out amplifiers are n
25. tent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications euinjoA pue qpuejs einjy M oipny AZ eng 9
26. to account qui escent power dissipation which Equation 2 does not The curves are to be used as a guideline in determining the required heat sink and are not intended to provide exact power dissipation values Heat Sinking Choosing a heat sink for a high power audio amplifier is made entirely to keep the die temperature below its maxi mum junction temperature so that the thermal protection circuitry does not operate under normal circumstances The heat sink should be chosen to dissipate the maximum IC power for the maximum no load supply voltage and the minimum load impedance Referring to Figure 8 the thermal resistance from the die junction to the outside air ambient is a combination of three thermal resistances and 54 Two of these thermal resistances are provided by National and O AAN O NNN O NNN O acs Osa TIME TAMB 9 NWN o OJA 20064432 FIGURE 8 Thermal Model Since convection heat flow power dissipation is analogous to current flow thermal resistance is analogous to electrical resistance and temperature drops are analogous to voltage drops the power dissipation out of the LM4756 is equal to the following Pomax Timax 3 The thermal resistance 0 4 is equal to 65 where is the junction to case thermal resistance Ocs is the case to sink thermal resistance thermal compound
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National Semiconductor LM4756 Dual 7W Audio Power Amplifier w/Mute Standby Volume Control handbook