LM4928 1.2 Watt Stereo Fully Differential Audio Amplifier with RF Suppression and Shutdown Low General Description Key Specifications The LM4928 is an stereo fully differential stereo audio power amplifier primarily designed for demanding applications in mobile phones and other portable communication devices. It is capable of delivering 1.2 watts of continuous average power to a 8Ω load with less than 1% distortion (THD+N) from a 5VDC power supply. Boomer audio power amplifiers were designed specifically to provide high quality output power with a minimal amount of external components. The LM4928 does not require output coupling capacitors or bootstrap capacitors, and therefore is ideally suited for mobile phone and other low voltage applications where minimal power consumption is a primary requirement. The LM4928 features a low-power consumption shutdown mode. To facilitate this, shutdown may be enabled by logic low. Additionally, the LM4928 features an internal thermal shutdown protection mechanism. The LM4928 contains advanced pop & click circuitry which eliminates noises which would otherwise occur during turn-on and turn-off transitions. j Improved PSRR at 217Hz 90dB (typ) j Output Power at 5.0V @ 1% THD+N (8Ω) 1.2W (typ) j Output Power at 3.0V @ 1% THD+N (8Ω)400mW (typ) j Shutdown Current 0.1µA (typ) Features RF Suppression Circuitry Fully differential amplification Available in space-saving micro SMD and LLP packages Ultra low current shutdown mode Can drive capacitive loads up to 100pF Improved pop & click circuitry eliminates noises during turn-on and turn-off transitions n 2.4 - 5.5V operation n No output coupling capacitors, snubber networks or bootstrap capacitors required n n n n n n Applications n Mobile phones n PDAs n Portable electronic devices and accessories Boomer ® is a registered trademark of National Semiconductor Corporation. © 2006 National Semiconductor Corporation DS201600 www.national.com LM4928 1.2 Watt Stereo Fully Differential Audio Power Amplifier with RF Suppression and Shutdown Low February 2006 LM4928 Typical Application 201600B7 FIGURE 1. Typical Audio Amplifier Application Circuit www.national.com 2 LM4928 Connection Diagrams LLP Package LLP Package Marking 20160004 Top View Z — Assembly Plant Code XY — 2 Digit Date Code TT — Die Traceability L4928 — LM4928 20160006 Top View Order Number LM4928SD See NS Package Number SDA14A micro SMD Package micro SMD Package Marking 20160005 Top View XY — 2 Digit Date COde TT — Die Traceability G — Boomer Family F9 — LM4928T 20160003 Top View Order Number LM4928TL See NS Package Number TLA16 LM4928TL Pin Descriptions A1 IN1+ B1 IN1– C1 IN2– D1 IN2+ A2 VDD B2 BYPASS C2 SHUTDOWN D2 VDD A3 OUT1– B3 OUT1+ C3 OUT2+ D3 OUT2– A4 GND B4 NC C4 NC D4 GND 3 www.national.com LM4928 Absolute Maximum Ratings (Note 2) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Supply Voltage θJA (SD) 50˚C/W θJA (micro SMD) 74˚C/W Soldering Information See AN-1187 6.0V Storage Temperature −65˚C to +150˚C Operating Ratings −0.3V to VDD +0.3V Input Voltage Power Dissipation (Note 3) Internally Limited ESD Susceptibility (Note 4) 2000V ESD Susceptibility (Note 5) Junction Temperature Temperature Range TMIN ≤ TA ≤ TMAX −40˚C ≤ TA ≤ 85˚C 2.4V ≤ VDD ≤ 5.5V Supply Voltage 200V 150˚C Thermal Resistance Electrical Characteristics VDD = 5V (Notes 1, 2) The following specifications apply for VDD = 5V, AV = 1, and 8Ω load unless otherwise specified. Limits apply for TA = 25˚C. LM4928 Symbol IDD Parameter Quiescent Power Supply Current Conditions Typical Limit Units (Limits) (Note 6) (Note 7) VIN = 0V, no load VIN = 0V, RL = 8Ω (Both amplifiers) 4 4 7.5 mA (max) 0.1 1.0 µA (max) 1.0 W ISD Shutdown Current VSHUTDOWN = GND (Both amplifiers) Po Output Power THD = 1% (max); f = 1 kHz LM4928SD, RL = 4Ω (Note 9) RL = 8Ω 1.8 1.2 THD = 10% (max); f = 1 kHz LM4928SD, RL = 4Ω (Note 9) RL = 8Ω 2.2 1.5 W Po = 1 Wrms; f = 1kHz 0.04 % THD+N Total Harmonic Distortion + Noise PSRR Power Supply Rejection Ratio Vripple = 200mV sine p-p f = 217Hz (Note 8) 90 f = 1kHz (Note 8) 90 dB CMRR Common-Mode Rejection Ratio f = 217Hz, VCM = 200mVpp 70 VOS Output Offset VIN = 0V 4 VSDIH Shutdown Voltage Input High VSDIL Shutdown Voltage Input Low 0.4 V 50 dB (min) 18 mV (max) 1.4 V SNR Signal-to-Noise Ratio PO = 1W, f = 1kHz 105 dB TWU Wake-up time from Shutdown Cbypass = 1µF 13 ms Electrical Characteristics VDD = 3V (Notes 1, 2) The following specifications apply for VDD = 3V, AV = 1, and 8Ω load unless otherwise specified. Limits apply for TA = 25˚C. LM4928 Symbol IDD ISD Parameter Quiescent Power Supply Current Shutdown Current www.national.com Conditions Typical Limit (Note 6) (Note 7) VIN = 0V, no load VIN = 0V, RL = 8Ω (Both amplifiers) 3.5 3.5 VSHUTDOWN = GND (Both amplifiers) 0.1 4 Units (Limits) mA 1 µA (max) LM4928 Electrical Characteristics VDD = 3V (Notes 1, 2) The following specifications apply for VDD = 3V, AV = 1, and 8Ω load unless otherwise specified. Limits apply for TA = 25˚C. (Continued) LM4928 Symbol Parameter Output Power Po THD+N Total Harmonic Distortion + Noise PSRR Power Supply Rejection Ratio Conditions Typical Limit (Note 6) (Note 7) Units (Limits) THD = 1% (max); f = 1 kHz RL = 4Ω RL = 8Ω 0.55 0.40 W THD = 10% (max); f = 1 kHz RL = 4Ω RL = 8Ω 0.68 0.50 W Po = 0.25Wrms; f = 1kHz 0.05 % Vripple = 200mV sine p-p f = 217Hz (Note 8) 90 f = 1kHz (Note 8) 90 dB CMRR Common-Mode Rejection Ratio f = 217Hz, VCM = 200mVpp 70 50 dB (min) VOS Output Offset VIN = 0V 4 18 mV (max) VSDIH Shutdown Voltage Input High 1.4 V VSDIL Shutdown Voltage Input Low 0.4 V SNR Signal-to-Noise Ratio PO = 0.4W, f = 1kHz TWU Wake-up time from Shutdown Cbypass = 1µF 105 dB 9 ms Note 1: All voltages are measured with respect to the ground pin, unless otherwise specified. Note 2: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is functional, but do not guarantee specific performance limits. Electrical Characteristics state DC and AC electrical specifications under particular test conditions which guarantee specific performance limits. This assumes that the device is within the Operating Ratings. Specifications are not guaranteed for parameters where no limit is given, however, the typical value is a good indication of device performance. Note 3: The maximum power dissipation must be derated at elevated temperatures and is dictated by TJMAX, θJA, and the ambient temperature TA. The maximum allowable power dissipation is PDMAX = (TJMAX – TA) / θJA or the number given in Absolute Maximum Ratings, whichever is lower. For the LM4928, see power derating curve for additional information. Note 4: Human body model, 100pF discharged through a 1.5kΩ resistor. Note 5: Machine Model, 220pF – 240pF discharged through all pins. Note 6: Typicals are measured at 25˚C and represent the parametric norm. Note 7: Limits are guaranteed to National’s AOQL (Average Outgoing Quality Level). Note 8: Inputs are AC terminated to GND. Note 9: When driving 4Ω loads from a 5V power supply, the LM4928SD must be mounted to a circuit board with the exposed-DAP area soldered down to at least 4in2 plane of 1oz, copper. Note 10: Data taken with BW = 80kHz and AV = 1 except where specified. Note 11: Maximum Power Dissipation (PDMAX) in the device occurs at an output power level significantly below full output power. PDMAX can be calculated using Equation 4 shown in the Application section. It may also be obtained from the Power Dissipation graphs. External Components Description (Figure 1) Components Functional Description 1. CS Supply bypass capacitor which provides power supply filtering. Refer to the Power Supply Bypassing section for information concerning proper placement and selection of the supply bypass capacitor. 2. CB Bypass pin capacitor which provides half-supply filtering. Refer to the Power Supply Bypassing section for information concerning proper placement and selection of CB. 3. Ri Inverting input resistance which sets the closed-loop gain in conjunction with Rf. 4. Rf External feedback resistance which sets the closed-loop gain in conjunction with Ri. 5 www.national.com LM4928 Typical Performance Characteristics (Note 10) THD+N vs Frequency VDD = 2.6V, RL = 8Ω, PO = 150mW THD+N vs Frequency VDD = 2.6V, RL = 4Ω, PO = 150mW 20160016 20160017 THD+N vs Frequency VDD = 3V, RL = 8Ω, PO = 250mW THD+N vs Frequency VDD = 3V, RL = 4Ω, PO = 250mW 20160018 20160019 THD+N vs Frequency VDD = 5V, RL = 8Ω, PO = 1W THD+N vs Frequency VDD = 5V, RL = 4Ω, PO = 1W 20160020 www.national.com 20160021 6 (Note 10) THD+N vs Output Power VDD = 2.6V, RL = 4Ω LM4928 Typical Performance Characteristics (Continued) THD+N vs Output Power VDD = 2.6V, RL = 8Ω 20160022 20160023 THD+N vs Output Power VDD = 3V, RL = 8Ω THD+N vs Output Power VDD = 3V, RL = 4Ω 20160024 20160025 THD+N vs Output Power VDD = 5V, RL = 8Ω THD+N vs Output Power VDD = 5V, RL = 4Ω 20160026 20160027 7 www.national.com LM4928 Typical Performance Characteristics (Note 10) PSRR vs Common Mode Voltage VDD = 3V, RL = 8Ω, f = 217Hz (Continued) PSRR vs Common Mode Voltage VDD = 5V, RL = 8Ω, f = 217Hz 201600C8 201600C9 PSRR vs Frequency VDD = 5V, RL = 8Ω Input Terminated to GND, BW = 500kHz PSRR vs Frequency VDD = 3V, RL = 8Ω Input Terminated to GND, BW = 500kHz 201600D1 201600D0 Output Power vs Supply Voltage RL = 8Ω Output Power vs Supply Voltage RL = 4Ω 201600D2 www.national.com 201600C5 8 (Note 10) CMRR vs Frequency VDD = 3V, RL = 8Ω LM4928 Typical Performance Characteristics (Continued) CMRR vs Frequency VDD = 5V, RL = 8Ω 201600C0 201600C1 Crosstalk vs Frequency VDD = 5V, RL = 4Ω, PO = 1W Top = Vin Left driven, Vout Right measured Bot = Vin Right driven, Vout Left measured Crosstalk vs Frequency VDD = 5V, RL = 8Ω, PO = 1W Top = Vin Left driven, Vout Right measured Bot = Vin Right driven, Vout Left measured 20160069 20160072 Crosstalk vs Frequency VDD = 3V, RL = 8Ω, PO = 250mW Top = Vin Left driven, Vout Right measured Bot = Vin Right driven, Vout Left measured Crosstalk vs Frequency VDD = 3V, RL = 4Ω, PO = 500mW Top = Vin Left driven, Vout Right measured Bot = Vin Right driven, Vout Left measured 20160071 20160070 9 www.national.com LM4928 Typical Performance Characteristics (Note 10) Power Dissipation vs Output Power VDD = 3V (Continued) Power Dissipation vs Output Power VDD = 5V 201600C6 201600C7 Noise Floor VDD = 5V Noise Floor VDD = 3V 201600C3 201600C2 Output Power vs Load Resistance Clipping Voltage vs Supply Voltage 201600C4 20160030 www.national.com 10 (Note 10) Power Derating Curve (SD Package) fin = 1kHz, RL = 8Ω (Continued) Power Derating Curve (SD Package) fin = 1kHz, RL = 4Ω 20160068 20160067 Power Derating Curve (TL Package) fin = 1kHz, RL = 8Ω 20160066 11 www.national.com LM4928 Typical Performance Characteristics LM4928 EXPOSED-DAP PACKAGE PCB MOUNTING CONSIDERATIONS The LM4928’s exposed-DAP (die attach paddle) package (LLP) provide a low thermal resistance between the die and the PCB to which the part is mounted and soldered. This allows rapid heat transfer from the die to the surrounding PCB copper traces, ground plane and, finally, surrounding air. Failing to optimize thermal design may compromise the LM4928’s high power performance and activate unwanted, though necessary, thermal shutdown protection. The LLP package must have its DAP soldered to a copper pad on the PCB. The DAP’s PCB copper pad is connected to a large plane of continuous unbroken copper. This plane forms a thermal mass and heat sink and radiation area. Place the heat sink area on either outside plane in the case of a two-sided PCB, or on an inner layer of a board with more than two layers. Connect the DAP copper pad to the inner layer or backside copper heat sink area with at least 4 vias thermal via. The via diameter should be 0.012in - 0.013in. Ensure efficient thermal conductivity by plating-through and solder-filling the vias. Application Information DIFFERENTIAL AMPLIFIER EXPLANATION The LM4928 is a fully differential audio amplifier that features differential input and output stages. Internally this is accomplished by two circuits: a differential amplifier and a common mode feedback amplifier that adjusts the output voltages so that the average value remains VDD / 2. When setting the differential gain, the amplifier can be considered to have "halves". Each half uses an input and feedback resistor (Ri1 and RF1) to set its respective closed-loop gain (see Figure 1). With Ri1 = Ri2 and RF1 = RF2, the gain is set at -RF / Ri for each half per channel. This results in a differential gain of AVD = -RF/Ri (1) It is extremely important to match the input resistors to each other, as well as the feedback resistors to each other for best amplifier performance. See the Proper Selection of External Components section for more information. A differential amplifier works in a manner where the difference between the two input signals is amplified. In most applications, this would require input signals that are 180˚ out of phase with each other. The LM4928 can be used, however, as a single ended input amplifier while still retaining its fully differential benefits. In fact, completely unrelated signals may be placed on the input pins. The LM4928 simply amplifies the difference between them. Best thermal performance is achieved with the largest practical copper heat sink area. In all circumstances and conditions, the junction temperature must be held below 150˚C to prevent activating the LM4928’s thermal shutdown protection. The LM4928’s power de-rating curve in the Typical Performance Characteristics shows the maximum power dissipation versus temperature. Example PCB layouts are shown in the Demonstration Board Layout section. Further detailed and specific information concerning PCB layout, fabrication, and mounting an LLP package is available from National Semiconductor’s package Engineering Group under application note AN1187. All of these applications provide what is known as a "bridged mode" output (bridge-tied-load, BTL). This results in output signals at Vo1 and Vo2 that are 180˚ out of phase with respect to each other. Bridged mode operation is different from the single-ended amplifier configuration that connects the load between the amplifier output and ground. A bridged amplifier design has distinct advantages over the singleended configuration: it provides differential drive to the load, thus doubling maximum possible output swing for a specific supply voltage. Four times the output power is possible compared with a single-ended amplifier under the same conditions. This increase in attainable output power assumes that the amplifier is not current limited or clipped. In order to choose an amplifier’s closed-loop gain without causing excess clipping, please refer to the Audio Power Amplifier Design section. PCB LAYOUT AND SUPPLY REGULATION CONSIDERATIONS FOR DRIVING 4Ω LOADS Power dissipated by a load is a function of the voltage swing across the load and the load’s impedance. As load impedance decreases, load dissipation becomes increasingly dependent on the interconnect (PCB trace and wire) resistance between the amplifier output pins and the load’s connections. Residual trace resistance causes a voltage drop, which results in power dissipated in the trace and not in the load as desired. This problem of decreased load dissipation is exacerbated as load impedance decreases. Therefore, to maintain the highest load dissipation and widest output voltage swing, PCB traces that connect the output pins to a load must be as wide as possible. A bridged configuration, such as the one used in the LM4928, also creates a second advantage over singleended amplifiers. Since the differential outputs, Vo1 and Vo2, are biased at half-supply, no net DC voltage exists across the load. This assumes that the input resistor pair and the feedback resistor pair are properly matched (see Proper Selection of External Components). BTL configuration eliminates the output coupling capacitor required in singlesupply, single-ended amplifier configurations. If an output coupling capacitor is not used in a single-ended output configuration, the half-supply bias across the load would result in both increased internal IC power dissipation as well as permanent loudspeaker damage. Further advantages of bridged mode operation specific to fully differential amplifiers like the LM4928 include increased power supply rejection ratio, common-mode noise reduction, and click and pop reduction. www.national.com Poor power supply regulation adversely affects maximum output power. A poorly regulated supply’s output voltage decreases with increasing load current. Reduced supply voltage causes decreased headroom, output signal clipping, and reduced output power. Even with tightly regulated supplies, trace resistance creates the same effects as poor supply regulation. Therefore, making the power supply traces as wide as possible helps maintain full output voltage swing. POWER DISSIPATION Power dissipation is a major concern when designing a successful amplifer, whether the amplifier is bridged or single-ended. Equation 2 states the maximum power dissipation point for a single-ended amplifier operating at a given supply voltage and driving a specified output load. 12 but PSRR decreases at frequencies below 1kHz. The issue of CB selection is thus dependant upon desired PSRR and click and pop performance. (Continued) PDMAX = (VDD)2 / (2π2RL) Single-Ended (2) OPTIMIZING RF IMMUNITY The internal circuitry of the LM4928 suppresses the amount of RF signal that is coupled into the chip. However, certain external factors, such as output trace length, output trace orientation, distance between the chip and the antenna, antenna strength, speaker type, and type of RF signal, may affect the RF immunity of the LM4928. In general, the RF immunity of the LM4928 is application specific. Nevertheless, optimal RF immunity can be achieved by using short output traces and increasing the distance between the LM4928 and the antenna. However, a direct consequence of the increased power delivered to the load by a bridge amplifier is an increase in internal power dissipation versus a single-ended amplifier operating at the same conditions. PDMAX = 4(VDD)2/(2π2RL) Bridge Mode per channel (3) PDMAX = 8(VDD)2/(2π2RL) Bridge Mode both channel (4) Since the LM4928 has bridged outputs, the maximum internal power dissipation is 4 times that of a single-ended amplifier. Even with this substantial increase in power dissipation, the LM4928 does not require additional heatsinking under most operating conditions and output loading. From Equation 3, assuming a 5V power supply and an 8Ω load, the maximum power dissipation point is 625mW per channel. Then multiply by two or use equation 4 to get 1.25W total power dissipation for both channels. The maximum power dissipation point obtained from Equation 4 must not be greater than the power dissipation results from Equation 5: PDMAX = (TJMAX - TA) / θJA SHUTDOWN FUNCTION In order to reduce power consumption while not in use, the LM4928 contains shutdown circuitry that is used to turn off the amplifier’s bias circuitry. The device may then be placed into shutdown mode by toggling the Shutdown Select pin to logic low. The trigger point for shutdown is shown as a typical value in the Supply Current vs Shutdown Voltage graphs in the Typical Performance Characteristics section. It is best to switch between ground and supply for maximum performance. While the device may be disabled with shutdown voltages in between ground and supply, the idle current may be greater than the typical value of 0.1µA. In either case, the shutdown pin should be tied to a definite voltage to avoid unwanted state changes. (5) Depending on the ambient temperature, TA, of the system surroundings, Equation 5 can be used to find the maximum internal power dissipation supported by the IC packaging. If the result of Equation 4 is greater than that of Equation 5, then either the supply voltage must be decreased, the load impedance increased, the ambient temperature reduced, or the θJA reduced with heatsinking. In many cases, larger traces near the output, VDD, and GND pins can be used to lower the θJA. The larger areas of copper provide a form of heatsinking allowing higher power dissipation. For the typical application of a 5V power supply, with an 8Ω load in the LLP package, the maximum ambient temperature possible without violating the maximum junction temperature is approximately 85˚C provided that device operation is around the maximum power dissipation point. Recall that internal power dissipation is a function of output power. If typical operation is not around the maximum power dissipation point, the LM4928 can operate at higher ambient temperatures. Refer to the Typical Performance Characteristics curves for power dissipation information. In many applications, a microcontroller or microprocessor output is used to control the shutdown circuitry, which provides a quick, smooth transition to shutdown. Another solution is to use a single-throw switch in conjunction with an external pull-up resistor. This scheme guarantees that the shutdown pin will not float, thus preventing unwanted state changes. PROPER SELECTION OF EXTERNAL COMPONENTS Proper selection of external components in applications using integrated power amplifiers is critical when optimizing device and system performance. Although the LM4928 is tolerant to a variety of external component combinations, consideration of component values must be made when maximizing overall system quality. The LM4928 is unity-gain stable, giving the designer maximum system flexibility. The LM4928 should be used in low closed-loop gain configurations to minimize THD+N values and maximize signal to noise ratio. Low gain configurations require large input signals to obtain a given output power. Input signals equal to or greater than 1Vrms are available from sources such as audio codecs. Please refer to the Audio Power Amplifier Design section for a more complete explanation of proper gain selection. When used in its typical application as a fully differential power amplifier the LM4928 does not require input coupling capacitors for input sources with DC common-mode voltages of less than VDD. Exact allowable input common-mode voltage levels are actually a function of VDD, Ri, and Rf and may be determined by Equation 6: POWER SUPPLY BYPASSING As with any power amplifier, proper supply bypassing is critical for low noise performance and high power supply rejection ratio (PSRR). The capacitor location on both the bypass and power supply pins should be as close to the device as possible. A larger half-supply bypass capacitor improves PSRR because it increases half-supply stability. Typical applications employ a 5V regulator with 10µF and 0.1µF bypass capacitors that increase supply stability. This, however, does not eliminate the need for bypassing the supply nodes of the LM4928. The LM4928 will operate without the bypass capacitor CB, although the PSRR may decrease. A 1µF capacitor is recommended for CB. This value maximizes PSRR performance. Lesser values may be used, VCMi < (VDD-1.2)(Ri+Rf)/Rf-VDD/2(Ri/Rf) 13 (6) www.national.com LM4928 Application Information LM4928 Application Information A designer must first determine the minimum supply rail to obtain the specified output power. The supply rail can easily be found by extrapolating from the Output Power vs Supply Voltage graphs in the Typical Performance Characteristics section. A second way to determine the minimum supply rail is to calculate the required VOPEAK using Equation 7 and add the dropout voltages. Using this method, the minimum supply voltage is (Vopeak + (VDO TOP + VDO BOT), where VDO BOT and VDO TOP are extrapolated from the Dropout Voltage vs Supply Voltage curve in the Typical Performance Characteristics section. (Continued) Special care must be taken to match the values of the input resistors (Ri1 and Ri2) and (Rf1 and Rf2) to each other. Because of the balanced nature of differential amplifiers, resistor matching differences can result in net DC currents across the load. This DC current can increase power consumption, internal IC power dissipation, reduce PSRR, CMRR, and possibly damaging the loudspeaker. The chart below demonstrates this problem by showing the effects of differing values between the input resistors while assuming that the feedback resistors are perfectly matched. The results below apply to the application circuit shown in Figure 1, and assumes that VDD = 5V, RL = 8Ω, and the system has DC coupled inputs tied to ground. Tolerance Ri1 Ri2 V02 - V01 ILOAD 20% 0.8R 1.2R -0.500V 62.5mA 10% 0.9R 1.1R -0.250V 31.25mA 5% 0.95R 1.05R -0.125V 15.63mA 1% 0.99R 1.01R -0.025V 3.125mA 0 0 0% R R (7) Using the Output Power vs Supply Voltage graph for an 8Ω load, the minimum supply rail just about 4.5V. Extra supply voltage creates headroom that allows the LM4928 to reproduce peaks in excess of 1W without producing audible distortion. At this time, the designer must make sure that the power supply choice along with the output impedance does not violate the conditions explained in the Power Dissipation section. Once the power dissipation equations have been addressed, the required differential gain can be determined from Equation 8. Similar results would occur if the feedback resistors were not carefully matched. Adding input coupling resistors in between the signal source and the input resistors will eliminate this problem, however. To achieve best performance with minimum component count, it is highly recommended that both the feedback and input resistors matched to 1% tolerance or better for best performance. (8) Rf / Ri = AVD From Equation 8, the minimum AVD is 2.83. With Rf = 40kΩ, a ratio of Rf to Ri of 2.83 gives Ri = 14kΩ. The final design step is to address the bandwidth requirement which must be stated as a single -3dB frequency point. Five times away from a -3dB point is 0.17dB down from passband response which is better than the required ± 0.25dB specified. AUDIO POWER AMPLIFIER DESIGN Design a 1W/8Ω Audio Amplifier Given: Power Output Load Impedance Maximum Input Level Maximum Input Impedance Bandwidth www.national.com fH = 20kHz * 5 = 100kHz The high frequency pole is determined by the product of the desired frequency pole, fH , and the differential gain, AVD . With a AVD = 2.83 and fH = 100kHz, the resulting GBWP = 283kHz which is much smaller than the LM4928 GBWP of 10MHz. This figure displays that if a designer has a need to design an amplifier with a higher differential gain, the LM4928 can still be used without running into bandwidth limitations. 1Wrms 8Ω 1Vrms 20kΩ 100Hz–20kHz ± 0.25dB 14 LM4928 LM4928 Demo Board Schematic 20160073 15 www.national.com LM4928 LM4928 LLP Demo Board Artwork Top Silkscreen Top Layer 20160008 20160009 Bottom Layer and Ground Plane 20160007 www.national.com 16 LM4928 LM4928 microSMD Board Artwork Top Silkscreen Top Layer 20160012 20160013 Middle Layer Bottom Layer and Ground Plane 20160010 20160011 17 www.national.com LM4928 Revision History www.national.com Rev Date Description 1.0 7/13/05 Input first set of edits. 1.1 10/3/05 More edits input. 1.2 10/10/05 Input few text edits. 1.3 10/25/04 Added the Typ Perf section. 1.4 11/02/05 Added the X1, X2, and X3 values on the TLA1611A mktg outline. 1.5 11/15/05 Added 3 more curves (66, 67, and 68) and some texts edits. 1.6 11/16/05 Texts edits. 1.7 12/13/05 Added 4 more curves (69, 70, 71, and 72) and did some texts edits. 1.8 12/14/05 First WEB released (per Kashif). 1.9 12/16/05 Coded the LM4928TL ( Future Product ) for it will be released soon ( early January, 2006) per Kashif. Re-released D/S to the WEB. 2.0 01/04/06 Released the TL package to the WEB. 2.1 01/09/06 Edited B7 and B8 (now 73), then re-released D/S to the WEB (per Kashif). 2.2 02/01/06 Text edits, then re-released D/S to the WEB. 18 LM4928 Physical Dimensions inches (millimeters) unless otherwise noted LLP Package Order Number LM4928SD NS Package Number SDA14A 16–bump micro SMD Order Number LM4928TL NS Package Number TLA1611A X1 = 1.970 ± 0.03, X2 = 1.970 ± 0.03, X3 = 0.600 ± 0.075 19 www.national.com LM4928 1.2 Watt Stereo Fully Differential Audio Power Amplifier with RF Suppression and Shutdown Low Notes National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the 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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