LM4923 1.1 Watt Fully Differential Audio Power Amplifier With Shutdown Select General Description Key Specifications The LM4923 is a fully differential audio power amplifier primarily designed for demanding applications in mobile phones and other portable communication device applications. It is capable of delivering 1.1 watt of continuous average power to an 8Ω BTL 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 LM4923 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 LM4923 features a low-power consumption shutdown mode. To facilitate this, Shutdown may be enabled by logic low. Additionally, the LM4923 features an internal thermal shutdown protection mechanism. The LM4923 contains advanced pop & click circuitry which eliminates noises which would otherwise occur during turn-on and turn-off transitions. ■ Improved PSRR at 217Hz 85dB(typ) ■ Power Output at 5.0V @ 1% THD+N 1.1W(typ) ■ Power Output at 3.3V @ 1% THD+N 400mW(typ) ■ Shutdown Current 0.1µA(typ) Features ■ ■ ■ ■ ■ Fully differential amplification Available in space-saving LLP package 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 ■ 2.4 - 5.5V operation ■ No output coupling capacitors, snubber networks or bootstrap capacitors required Applications ■ Mobile phones ■ PDAs ■ Portable electronic devices Connection Diagrams LQ Package 8 Pin LQ Marking 20071302 X − Date Code TT − Die Traceability G − Boomer B2 − LM4923LQ 20071330 Top View Order Number LM4923LQ See NS Package Number LQB08A 8 Pin MSOP Package 200713a1 Top View See Order Number LM4923MM See NS Package Number MUA08A Boomer® is a registered trademark of National Semiconductor Corporation. © 2007 National Semiconductor Corporation 200713 www.national.com LM4923 1.1 Watt Fully Differential Audio Power Amplifier With Shutdown Select December 17, 2007 LM4923 Typical Application 20071313 FIGURE 1. Typical Audio Amplifier Application Circuit www.national.com 2 If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Supply Voltage Storage Temperature Input Voltage Power Dissipation (Note 3) ESD Susceptibility (Note 4) ESD Susceptibility (Note 5) Junction Temperature Thermal Resistance θJA (LLP) 140°C/W θJA (MM) 210°C/W θJC (MM) Soldering Information See AN-1187 6.0V −65°C to +150°C −0.3V to VDD +0.3V Internally Limited 2000V 200V 150°C 56°C/W Operating Ratings Temperature Range TMIN ≤ TA ≤ TMAX Supply Voltage −40°C ≤ TA ≤ 85°C 2.4V ≤ VDD ≤ 5.5V 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. LM4923 Symbol IDD Quiescent Power Supply Current Units (Limits) Typical Limit (Note 6) (Note 7) VIN = 0V, RL = 8Ω 4 4 9 9 mA (max) 0.1 1 µA (max) 1 Parameter Conditions VIN = 0V, no load ISD Shutdown Current VSHUTDOWN = GND Po Output Power THD = 1% (max); f = 1 kHz LM4923, RL = 8Ω 1.1 THD+N Total Harmonic Distortion+Noise Po = 0.4 Wrms; f = 1kHz 0.02 PSRR Power Supply Rejection Ratio Vripple = 200mV sine p-p % f = 217Hz (Note 8) 85 73 f = 1kHz (Note 8) 85 73 50 dB CMRR Common_Mode Rejection Ratio f = 217Hz, VCM = 200mVpp VOS Output Offset VIN = 0V 4 mV VSDIH Shutdown Voltage Input High 0.9 V VSDIL Shutdown Voltage Input Low 0.7 V 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. LM4923 Symbol IDD Quiescent Power Supply Current Units (Limits) Typical Limit (Note 6) (Note 7) VIN = 0V, RL = 8Ω 3 3 5.5 5.5 mA (max) 0.1 1 µA (max) Parameter Conditions VIN = 0V, no load ISD Shutdown Current VSHUTDOWN = GND Po Output Power THD = 1% (max); f = 1kHz LM4923, RL = 8Ω 0.375 W THD+N Total Harmonic Distortion+Noise Po = 0.25Wrms; f = 1kHz 0.02 % PSRR Power Supply Rejection Ratio Vripple = 200mV sine p-p f = 217Hz (Note 8) 85 f = 1kHz (Note 8) 85 73 CMRR Common-Mode Rejection Ratio f = 217Hz VCM = 200mVpp 50 dB VOS Output Offset VIN = 0V 4 mV 3 www.national.com LM4923 Absolute Maximum Ratings (Note 2) LM4923 LM4923 Symbol Parameter Conditions Typical Limit (Note 6) (Note 7) Units (Limits) VSDIH Shutdown Voltage Input High 0.8 V VSDIL Shutdown Voltage Input Low 0.6 V 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 LM4923, 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: Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis. Note 8: 10Ω terminated input. External Components Description (Figure 1) Components Functional Description 1. Ri Inverting input resistance which sets the closed-loop gain in conjunction with Rf. 2. Rf Feedback resistance which sets the closed-loop gain in conjunction with Ri. 3. 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. 4. CB Bypass pin capacitor which provides half-supply filtering. Refer to the section, Proper Selection of External Components, for information concerning proper placement and selection of CB. www.national.com 4 LM4923 Typical Performance Characteristics THD+N vs Frequency VDD = 2.6V, RL = 8Ω, PO = 150mW THD+N vs Frequency VDD = 2.6V, RL = 4Ω, PO = 150mW 20071306 20071305 THD+N vs Frequency VDD = 5V, RL = 8Ω, PO = 400mW THD+N vs Frequency VDD = 3V, RL = 8Ω, PO = 275mW 20071309 20071308 THD+N vs Frequency VDD = 3V, RL = 4Ω, PO = 225mW THD+N vs Output Power VDD = 2.6V, RL = 8Ω 20071307 20071311 5 www.national.com LM4923 THD+N vs Output Power VDD = 2.6V, RL = 4Ω THD+N vs Output Power VDD = 5V, RL = 8Ω 20071310 20071315 THD+N vs Output Power VDD = 3V, RL = 8Ω THD+N vs Output Power VDD = 3V, RL = 4Ω 20071314 20071312 PSRR vs Frequency VDD = 5V, RL = 8Ω, Input terminated PSRR vs Frequency VDD = 3V, RL = 8Ω, Input terminated 20071304 www.national.com 20071303 6 LM4923 Output Power vs Supply Voltage RL = 8Ω CMRR vs Frequency VDD = 5V, RL = 8Ω 20071301 20071317 CMRR vs Frequency VDD = 3V, RL = 8Ω PSRR vs Common Mode Voltage VDD = 3V, RL = 8Ω, f = 217Hz 20071326 20071322 PSRR vs Common Mode Voltage VDD = 5V, RL = 8Ω, f = 217Hz Power Dissipation vs Output Power VDD = 2.6V, RL = 8Ω and 4Ω 20071316 20071321 7 www.national.com LM4923 Power Dissipation vs Output Power VDD = 5V, RL = 8Ω Power Dissipation vs Output Power VDD = 3V, RL = 8Ω 20071319 20071325 Power Derating Curve Noise Floor VDD = 5V 20071318 20071324 Noise Floor VDD = 3V Clipping Voltage vs Supply Voltage 20071323 20071320 www.national.com 8 Supply Current Shutdown Voltage 20071328 20071327 Application Information A bridged configuration, such as the one used in the LM4923, also creates a second advantage over single-ended 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 single-supply, 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 LM4923 include increased power supply rejection ratio, common-mode noise reduction, and click and pop reduction. DIFFERENTIAL AMPLIFIER EXPLANATION The LM4923 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. This results in a differential gain of AVD = -RF/Ri LM4923 Output Power vs Load Resistance (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 LM4923 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 LM4923 simply amplifies the difference between them. EXPOSED-DAP PACKAGE PCB MOUNTING CONSIDERATIONS The LM4923'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 LM4923'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 a thermal via. The via diameter should be 0.012in 0.013in. Ensure efficient thermal conductivity by platingthrough and solder-filling the vias. 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 single-ended 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. 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 LM4923's thermal shutdown protection. The LM4923's power de-rating curve in the Typical Performance Characteristics shows the maximum power dissipation versus temperature. Example PCB layouts are shown in 9 www.national.com LM4923 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, the maximum ambient temperature possible without violating the maximum junction temperature is approximately 62°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 LM4923 can operate at higher ambient temperatures. Refer to the Typical Performance Characteristics curves for power dissipation information. 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. 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. 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 LM4923. The LM4923 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, but PSRR decreases at frequencies below 1kHz. The issue of CB selection is thus dependant upon desired PSRR and click and pop performance as explained in the section Proper Selection of External Components. 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 singleended. 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. PDMAX = (VDD)2 / (2π2RL) Single-Ended SHUTDOWN FUNCTION In order to reduce power consumption while not in use, the LM4923 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. (2) 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 (3) Since the LM4923 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 LM4923 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. The maximum power dissipation point obtained from Equation 3 must not be greater than the power dissipation results from Equation 4: PDMAX = (TJMAX - TA) / θJA 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 LM4923 is tolerant to a variety of external component combinations, consideration of component values must be made when maximizing overall system quality. The LM4923 is unity-gain stable, giving the designer maximum system flexibility. The LM4923 should be used in low closed-loop gain configurations to minimize THD+N values (4) The LM4923's θJA in an LQB08A package is 140°C/W. Depending on the ambient temperature, TA, of the system surroundings, Equation 4 can be used to find the maximum internal power dissipation supported by the IC packaging. If the result of Equation 3 is greater than that of Equation 4, then www.national.com 10 VCMi < (VDD-1.2)*((Rf+(Ri)/(Rf)-VDD*(Ri / 2Rf) (5) -RF / RI = AVD (6) 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. (7) Using the Output Power vs Supply Voltage graph for an 8Ω load, the minimum supply rail just about 5V. Extra supply voltage creates headroom that allows the LM4923 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. Special care must be taken to match the values of the feedback resistors (RF1 and RF2) to each other as well as matching the input resistors (Ri1 and Ri2) to each other (see Figure 1) more in front. 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, and possibly damaging the loudspeaker. The chart below demonstrates this problem by showing the effects of differing values between the feedback resistors while assuming that the input 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 RF1 RF2 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% R 0 R 0 (8) Rf / Ri = AVD From Equation 7, the minimum AVD is 2.83. Since the desired input impedance was 20kΩ, a ratio of 2.83:1 of Rf to Ri results in an allocation of Ri = 20kΩ for both input resistors and Rf = 60kΩ for both feedback resistors. 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. 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 = 150kHz which is much smaller than the LM4923 GBWP of 10MHz. This figure displays that if a designer has a need to design an amplifier with a higher differential gain, the LM4923 can still be used without running into bandwidth limitations. Similar results would occur if the input resistors were not carefully matched. Adding input coupling capacitors 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. AUDIO POWER AMPLIFIER DESIGN Design a 1W/8Ω Audio Amplifier Given: Power Output Load Impedance Input Level Input Impedance Bandwidth 1Wrms 8Ω 1Vrms 20kΩ 100Hz–20kHz ± 0.25dB 11 www.national.com LM4923 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 LM4923 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 5: LM4923 Revision History Rev Date 1.0 09/28/07 Added the MSOP package, then released. 1.01 12/17/07 Updated the mktg outline LQB08A into the rev B. www.national.com Description 12 LM4923 Physical Dimensions inches (millimeters) unless otherwise noted LQ Package Order Number LM4923LQ NS Package Number LQB08A MSOP Package Order Number LM4923MM NS Package Number MUA08A 13 www.national.com LM4923 1.1 Watt Fully Differential Audio Power Amplifier With Shutdown Select Notes For more National Semiconductor product information and proven design tools, visit the following Web sites at: Products Design Support Amplifiers www.national.com/amplifiers WEBENCH www.national.com/webench Audio www.national.com/audio Analog University www.national.com/AU Clock Conditioners www.national.com/timing App Notes www.national.com/appnotes Data Converters www.national.com/adc Distributors www.national.com/contacts Displays www.national.com/displays Green Compliance www.national.com/quality/green Ethernet www.national.com/ethernet Packaging www.national.com/packaging Interface www.national.com/interface Quality and Reliability www.national.com/quality LVDS www.national.com/lvds Reference Designs www.national.com/refdesigns Power Management www.national.com/power Feedback www.national.com/feedback Switching Regulators www.national.com/switchers LDOs www.national.com/ldo LED Lighting www.national.com/led PowerWise www.national.com/powerwise Serial Digital Interface (SDI) www.national.com/sdi Temperature Sensors www.national.com/tempsensors Wireless (PLL/VCO) www.national.com/wireless THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. 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