LM4991 3W Audio Power Amplifier with Shutdown Mode General Description Key Specifications The LM4991 is a mono bridged audio power amplifier capable of delivering 3W of continuous average power into a 3Ω load with less than 10% THD when powered by a 5V power supply (Note 1). To conserve power in portable applications, the LM4991’s micropower shutdown mode (ISD = 0.1µA, typ) is activated when VDD is applied to the SHUTDOWN pin. n Improved PSRR at 217kHz and 1kHz 64dB (typ) n PO at VDD = 5.0V, 10% THD, 1kHz n LM4991LD (only), 3Ω, 4Ω 3W (typ), 2.5W (typ) n All packages, 8Ω load 1.5W (typ) n Shutdown current 0.1µA (typ) Boomer audio power amplifiers are designed specifically to provide high power, high fidelity audio output. They require few external components and operate on low supply voltages from 2.2V to 5.5V. Since the LM4991 does not require output coupling capacitors, bootstrap capacitors, or snubber networks, it is ideally suited for low-power portable systems that require minimum volume and weight. Additional LM4991 features include thermal shutdown protection, unity-gain stability, and external gain set. Note 1: An LM4991LD that has been properly mounted to a circuit board will deliver 3W into 3Ω (at 10% THD). The other package options for the LM4991 will deliver 1.5W into 8Ω (at 10% THD). See the Application Information sections for further information concerning the LM4991LD and LM4991M. Features Available in space-saving LLP and MA packages Ultra low current shutdown mode Can drive capacitive loads up to 500pF Improved pop & click circuitry reduces noises during turn-on and turn-off transitions n 2.2 - 5.5V operation n No output coupling capacitors, snubber networks, bootstrap capacitors or gain-setting resistors required n Unity-gain stable n n n n Applications n n n n Wireless and cellular handsets PDA’s Portable computers Desktop computers Connection Diagrams Small Outline Package LLP Package 20074002 Top View Order Number LM4991MA See NS Package Number M08A 20074039 Top View Order Number LM4991LD See NS Package Number LDC08A Boomer ® is a registered trademark of National Semiconductor Corporation. © 2003 National Semiconductor Corporation DS200740 www.national.com LM4991 3W Audio Power Amplifier with Shutdown Mode May 2003 LM4991 Typical Application 20074001 FIGURE 1. Typical Audio Amplifier Application Circuit www.national.com 2 Thermal Resistance (Note 2) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Supply Voltage 6.0V Supply Temperature θJC (LD) (Note 9) 4.3˚C/W θJA (LD) 56˚C/W θJC (MA) 35˚C/W θJA (MA) 140˚C/W −65˚C to +150˚C −0.3V to VDD to +0.3V Input Voltage Power Dissipation (Note 4) Internally Limited ESD Susceptibility (Note 5) 2000V ESD Susceptibility (Note 6) 200V Junction Temperature Operating Ratings Temperature Range TMIN ≤ TA ≤ TMAX −40˚C ≤ TA ≤ +85˚C 2.2V ≤ VDD ≤ 5.5V Supply Voltage 150˚C Electrical Characteristics VDD = 5V (Notes 2, 3) The following specifications apply for VDD = 5V and RL = 8Ω unless otherwise specified. Limits apply for TA = 25˚C. LM4991 Symbol Parameter Conditions Typical (Note 7) Limit (Note 8) IDD Quiescent Power Supply Current VIN = 0V, no Load 3 7 VIN = 0V, RL = 8Ω 4 10 ISD Shutdown Current VSHUTDOWN = VDD 0.1 2.0 VSDIH Shutdown Voltage mA (max) µA (max) 1.5 VSDIL VOS Units (Limits) V 1.3 Output Offset Voltage V 5 35 LM4991LD, RL = 3Ω (Note 10) LM4991LD, RL = 4Ω (Note 10) LM4991, RL = 8Ω 2.38 2.1 1.3 0.9 THD+N = 10%, f = 1kHz LM4991LD, RL = 3Ω (Note 10) LM4991LD, RL = 4Ω (Note 10) LM4991, RL = 8Ω 3 2.5 1.5 mV (max) THD = 1% (max), f = 1kHz Po THD+N PSRR Output Power Total Harmonic Distortion+Noise Power Supply Rejection Ratio PO = 0.5W, f = 1kHz VRIPPLE = 200mV sine p-p, Input terminated with 10Ω, f = 1kHz W (min) W 0.2 % 64 55 dB (min) Electrical Characteristics VDD = 3V (Notes 2, 3) The following specifications apply for VDD = 3V and RL = 8Ω unless otherwise specified. Limits apply for TA = 25˚C. LM4991 Symbol Parameter Conditions Typical (Note 7) Limit (Note 8) IDD Quiescent Power Supply Current VIN = 0V, no Load 3 7 VIN = 0V, RL = 8Ω 4 7 VSHUTDOWN = VDD 0.1 2.0 ISD Shutdown Current VSDIH Shutdown Voltage Input High 1.1 VSDIL Shutdown Voltage Input Low 0.9 VOS Output Offset Voltage Po Output Power 5 Total Harmonic Distortion+Noise mA (max) µA (max) V V 35 mV (max) THD = 1% (max), f = 1kHz RL = 4Ω RL = 8Ω THD+N Units (Limits) 600 425 PO = 0.25W, f = 1kHz 3 0.1 mW % www.national.com LM4991 Absolute Maximum Ratings LM4991 Electrical Characteristics VDD = 3V (Notes 2, 3) The following specifications apply for VDD = 3V and RL = 8Ω unless otherwise specified. Limits apply for TA = 25˚C. (Continued) LM4991 Symbol PSRR Parameter Power Supply Rejection Ratio Conditions VRIPPLE = 200mV sine p-p, Input terminated with 10Ω, f = 1kHz Typical (Note 7) Limit (Note 8) Units (Limits) 68 dB Electrical Characteristics VDD = 2.6V (Notes 2, 3) The following specifications apply for VDD = 2.6V and RL = 8Ω unless otherwise specified. Limits apply for TA = 25˚C. LM4991 Symbol Parameter Conditions Typical (Note 7) Limit (Note 8) Units (Limits) IDD Quiescent Power Supply Current VIN = 0V, no Load 2 VIN = 0V, RL = 8Ω 3 ISD Shutdown Current VSHUTDOWN = VDD 0.1 µA(max) VSDIH Shutdown Voltage Input High 1 V VSDIL Shutdown Voltage Input Low 0.9 VOS Output Offset Voltage 5 mA (max) V 35 mV (max) THD = 1% (max), f = 1kHz Po Output Power THD+N Total Harmonic Distortion+Noise PSRR Power Supply Rejection Ratio 400 300 RL = 4Ω RL = 8Ω PO = 0.15W, f = 1kHz VRIPPLE = 200mV sine p-p, Input terminated with 10Ω, f = 1kHz 0.1 51 mW % dB 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: All voltages are measured with respect to the ground pin, unless otherwise specified. Note 4: 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 LM4991, TJMAX = 150˚C. For the θJA’s for different packages, please see the Application Information section or the Absolute Maximum Ratings section. Note 5: Human body model, 100pF discharged through a 1.5kΩ resistor. Note 6: Machine Model, 220pF–240pF discharged through all pins. Note 7: Typicals are specified at 25˚C and represent the parametric norm. Note 8: Limits are guaranteed to National’s AOQL (Average Outgoing Quality Level). Note 9: The given θJA is for an LM4991 packaged in an LDC08A with the Exposed–DAP soldered to an exposed 1in2 area of 1oz printed circuit board copper. Note 10: When driving 3Ω or 4Ω loads from a 5V supply, the LM4991LD must be mounted to a circuit board. www.national.com 4 LM4991 External Components Description (Figure 1) Components Functional Description 1. Ri Inverting input resistance that sets the closed-loop gain in conjunction with Rf. This resistor also forms a high pass filter with Ci at fC= 1/(2π RiCi). 2. Ci Input coupling capacitor that blocks the DC voltage at the amplifiers input terminals. Also creates a highpass filter with Ri at fc = 1/(2π RiCi). Refer to the section, Proper Selection of External Components, for an explanation of how to determine the value of Ci. 3. Rf Feedback resistance that sets the closed-loop gain in conjunction with Ri. 4. CS Supply bypass capacitor that provides power supply filtering. Refer to the Power Supply Bypassing section for information concerning proper placement and selection of the supply bypass capacitor. 5. CB Bypass pin capacitor that provides half-supply filtering. Refer to the section, Proper Selection of External Components, for information concerning proper placement and selection of CB. Typical Performance Characteristics LD and MH Specific Characteristics THD+N vs Output Power VDD = 5V, RL = 4Ω, and f = 1 kHz THD+N vs Frequency VDD = 5V, RL = 4Ω, and PO = 1W 20074041 20074042 Typical Performance Characteristics THD+N vs Frequency VDD = 3V, RL = 4Ω, and PO = 500mW THD+N vs Frequency VDD = 5V, RL = 8Ω, and PO = 500mW 20074043 20074044 5 www.national.com LM4991 Typical Performance Characteristics (Continued) THD+N vs Frequency VDD = 3V, RL = 8Ω, and PO = 250mW THD+N vs Frequency VDD = 2.6V, RL = 4Ω, and PO = 150mW 20074045 20074046 THD+N vs Output Power VDD = 5V, RL = 8Ω, and f = 1kHz THD+N vs Frequency VDD = 2.6V, RL = 8Ω, and PO = 150mW 20074047 20074048 THD+N vs Output Power VDD = 3V, RL = 8Ω, and f = 1kHz THD+N vs Output Power VDD = 3V, RL = 4Ω, and f = 1kHz 20074049 www.national.com 20074050 6 LM4991 Typical Performance Characteristics (Continued) THD+N vs Output Power VDD = 2.6V, RL = 4Ω, and f = 1kHz THD+N vs Output Power VDD = 2.6V, RL = 8Ω, and f = 1kHz 20074051 20074052 Power Supply Rejection Ratio (PSRR) vs Frequency VDD = 5V, RL = 8Ω, input floating Power Supply Rejection Ratio (PSRR) vs Frequency VDD = 5V, RL = 8Ω, input 10Ω terminated 20074053 20074054 Power Supply Rejection Ratio (PSRR) vs Frequency VDD = 3V, RL = 8Ω, input floating Power Supply Rejection Ratio (PSRR) vs Frequency VDD = 3V, RL = 8Ω, input 10Ω terminated 20074055 20074056 7 www.national.com LM4991 Typical Performance Characteristics (Continued) Power Supply Rejection Ratio (PSRR) vs Frequency VDD = 2.6V, RL = 8Ω, input 10Ω terminated Power Supply Rejection Ratio (PSRR) vs Frequency VDD = 2.6V, RL = 8Ω, Input Floating 20074057 20074058 Noise Floor, 5V, 8Ω 80kHz Bandwidth, Input to GND Open Loop Frequency Response, 5V 20074059 20074060 Power Dissipation vs Output Power, VDD = 3V Power Dissipation vs Output Power, VDD = 5V 20074061 www.national.com 20074062 8 LM4991 Typical Performance Characteristics (Continued) Power Dissipation vs Output Power, VDD = 2.6V Shutdown Hysteresis Voltage VDD = 5V, SD Mode = VDD 20074072 20074063 Shutdown Hysteresis Voltage VDD = 3V, SD Mode = VDD Shutdown Hysteresis Voltage VDD = 2.6V, SD Mode = VDD 20074073 20074074 Output Power vs Supply Voltage, RL = 8Ω Output Power vs Supply Voltage, RL = 4Ω 20074067 20074068 9 www.national.com LM4991 Typical Performance Characteristics (Continued) Output Power vs Supply Voltage, RL = 16Ω Output Power vs Supply Voltage, RL = 32Ω 20074069 20074070 Frequency Response vs Input Capacitor Size 20074071 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 4(2x2) vias. The via diameter should be 0.012in-0.013in with a 1.27mm pitch. Ensure efficient thermal conductivity by plating through the vias. Best thermal performance is achieved with the largest practical heat sink area. If the heatsink and amplifier share the same PCB layer, a nominal 2.5in2 area is necessary for 5V operation with a 4Ω load. Heatsink areas not placed on the same PCB layer as the LM4991 should be 5in2 (min) for the same supply voltage and load resistance. The last two area recommendations apply for 25˚C ambient temperature. Increase the area to compensate for ambient temperatures above 25˚C. The LM4991’s power de-rating curve in the Typical Performance Characteristics shows the maximum power dissipation versus temperature. An example PCB layout for the LD package is shown in the Demonstration Board Layout section. Further detailed and specific information concerning PCB layout, fabrication, and mounting an Application Information EXPOSED-DAP PACKAGE PCB MOUNTING CONSIDERATION The LM4991’s exposed-DAP (die attach paddle) package (LD) provides 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 surrounding air. The result is a low voltage audio power amplifier that produces 2W at ≤ 1% THD with a 4Ω load. This high power is achieved through careful consideration of necessary thermal design. Failing to optimize thermal design may compromise the LM4991’s high power performance and activate unwanted, though necessary, thermal shutdown protection. The LD 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, heat sink, and radiation area. Place the heat sink area on either outside plane in the case of a www.national.com 10 amplifier’s half-supply bias voltage across the load. The current flow created by the half-supply bias voltage increases internal IC power dissipation and my permanently damage loads such as speakers. (Continued) LD (LLP) package is available from National Semiconductor’s Package Engineering Group under application note AN1187. POWER DISSIPATION Power dissipation is a major concern when designing a successful amplifier, whether the amplifier is bridged or single-ended. A direct consequence of the increased power delivered to the load by a bridge amplifier is an increase in internal power dissipation. Equation 1 states the maximum power dissipation point for a bridge amplifier operating at a given supply voltage and driving a specified output load. PCB LAYOUT AND SUPPLY REGULATION CONSIDERATIONS FOR DRIVING 3Ω AND 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 dependant 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. For example, 0.1Ω trace resistance reduces the output power dissipated by a 4Ω load from 2.0W to 1.95W. 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. 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. PDMAX = 4*(VDD)2/(2π2RL) (1) Since the LM4991 has two operational amplifiers in one package, the maximum internal power dissipation is 4 times that of a single-ended ampifier. Even with this substantial increase in power dissipation, the LM4991 does not require heatsinking under most operating conditions and output loading. From Equation 1, assuming a 5V power supply and an 8Ω load, the maximum power dissipation point is 625 mW. The maximum power dissipation point obtained from Equation 1 must not be greater than the power dissipation that results from Equation 2: (2) PDMAX = (TJMAX–TA)/θJA For the SO package, θJA = 140˚C/W. For the LD package soldered to a DAP pad that expands to a copper area of 1.0in2 on a PCB, the LM4991’s θJA is 56˚C/W. TJMAX = 150˚C for the LM4991. The θJA can be decreased by using some form of heat sinking. The resultant θJA will be the summation of the θJC, θCS, and θSA. θJC is the junction to case of the package (or to the exposed DAP, as is the case with the LD package), θCS is the case to heat sink thermal resistance and θSA is the heat sink to ambient thermal resistance. By adding additional copper area around the LM4991, the θJA can be reduced from its free air value for the SO package. Increasing the copper area around the LD package from 1.0in2 to 2.0in2 area results in a θJA decrease to 46˚C/W. Depending on the ambient temperature, TA, and the θJA, Equation 2 can be used to find the maximum internal power dissipation supported by the IC packaging. If the result of Equation 1 is greater than that of Equation 2, then either the supply voltage must be decreased, the load impedance increased, the θJA decreased, or the ambient temperature reduced. For the typical application of a 5V power supply, with an 8Ω load, and no additional heatsinking, the maximum ambient temperature possible without violating the maximum junction temperature is approximately 61˚C provided that device operation is around the maximum power dissipation point and assuming surface mount packaging. For the LD package in a typical application of a 5V power supply, with a 4Ω load, and 1.0in2 copper area soldered to the exposed DAP pad, the maximum ambient temperature is approximately 77˚C providing device operation is around the maximum power dissipation point. Internal power dissipation is a function of output power. If typical operation is not around the maximum power dissipation point, the ambient temperature can be increased. Refer to the Typical Performance Characteristics curves for power dissipation information for different output powers and output loading. BRIDGE CONFIGURATION EXPLANATION As shown in Figure 1, the LM4991 has two operational amplifiers internally, allowing for a few different amplifier configurations. The first amplifier’s gain is externally configurable; the second amplifier is internally fixed in a unity-gain, inverting configuration. The closed-loop gain of the first amplifier is set by selecting the ratio of Rf to Ri while the second amplifier’s gain is fixed. Figure 1 shows that the output of amplifier one serves as the input to amplifier two, which results in both amplifiers producing signals identical in magnitude, but 180˚ out of phase. Consequently, the differential gain for the IC is AVD= 2 *(Rf/Ri) By driving the load differentially through outputs Vo1 and Vo2, an amplifier configuration commonly referred to as “bridged mode” is established. Bridged mode operation is different from the classical single-ended amplifier configuration where one side of its load is connected to ground. A bridge amplifier design has a few distinct advantages over the single-ended configuration, as it provides differential drive to the load, thus doubling output swing for a specified supply voltage. Four times the output power is possible as compared to 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 excessive clipping, please refer to the Audio Power Amplifier Design section. Another advantage of the differential bridge output is no net DC voltage across load. This results from biasing VO1 and VO2 at the same DC voltage, in this case VDD/2 . This eliminates the coupling capacitor that single supply, singleended amplifiers require. Eliminating an output coupling capacitor in a single-ended configuration forces a single supply POWER SUPPLY BYPASSING As with any amplifier, proper supply bypassing is critical for low noise performance and high power supply rejection. The capacitor location on both the bypass and power supply pins should be as close to the LM4991 as possible. The capacitor 11 www.national.com LM4991 Application Information LM4991 Application Information Selection Of Input Capacitor Size (Continued) Large input capacitors are both expensive and space hungry for portable designs. Clearly, a certain sized capacitor is needed to couple in low frequencies without severe attenuation. But in many cases the speakers used in portable systems, whether internal or external, have little ability to reproduce signals below 100Hz to 150Hz. Thus, using a large input capacitor may not increase actual system performance. connected between the bypass pin and ground improves the internal bias voltage’s stability, producing improved PSRR. The improvements to PSRR increase as the bypass pin capacitor increases. Typical applications employ a 5V regulator with 10µF and a 0.1µF bypass capacitors which aid in supply stability. This does not eliminate the need for bypassing the supply nodes of the LM4991 with a 1µF tantalum capacitor. The selection of bypass capacitors, especially CB, is dependent upon PSRR requirements, click and pop performance as explained in the section, Proper Selection of External Components, system cost, and size constraints. In addition to system cost and size, click and pop performance is effected by the size of the input coupling capacitor, Ci. A larger input coupling capacitor requires more charge to reach its quiescent DC voltage (nominally 1/2 VDD). This charge comes from the output via the feedback and is apt to create pops upon device enable. Thus, by minimizing the capacitor size based on necessary low frequency response, turn-on pops can be minimized. Besides minimizing the input capacitor size, careful consideration should be paid to the bypass capacitor value. Bypass capacitor, CB, is the most critical component to minimize turn-on pops since it determines how fast the LM4991 turns on. The slower the LM4991’s outputs ramp to their quiescent DC voltage (nominally 1/2 VDD), the smaller the turn-on pop. Choosing CB equal to 1.0µF along with a small value of Ci (in the range of 0.1µF to 0.39µF), should produce a virtually clickless and popless shutdown function. While the device will function properly, (no oscillations or motorboating), with CB equal to 0.1µF, the device will be much more susceptible to turn-on clicks and pops. Thus, a value of CB equal to 1.0µF is recommended in all but the most cost sensitive designs. SHUTDOWN FUNCTION In order to reduce power consumption while not in use, the LM4991 contains a shutdown pin to externally turn off the amplifier’s bias circuitry. This shutdown feature turns the amplifier off when a logic high is placed on the shutdown pin. The trigger point between a logic low and logic high level is typically half- supply. It is best to switch between ground and supply to provide maximum device performance. By switching the shutdown pin to VDD, the LM4991 supply current draw will be minimized in idle mode. While the device will be disabled with shutdown pin voltages less then VDD, 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. In many applications, a microcontroller or microprocessor output is used to control the shutdown circuitry which provides a quick, smooth transition into shutdown. Another solution is to use a single-pole, single-throw switch in conjunction with an external pull-up resistor. When the switch is closed, the shutdown pin is connected to ground and enables the amplifier. If the switch is open, then the external pull-up resistor will disable the LM4991. This scheme guarantees that the shutdown pin will not float thus preventing unwanted state changes. AUDIO POWER AMPLIFIER DESIGN Design a 1W/8Ω Audio Amplifier Given: Power Output Load Impedance PROPER SELECTION OF EXTERNAL COMPONENTS Proper selection of external components in applications using integrated power amplifiers is critical to optimize device and system performance. While the LM4991 is tolerant of external component combinations, consideration to component values must be used to maximize overall system quality. The LM4991 is unity-gain stable which gives a designer maximum system flexibility. The LM4991 should be used in low gain configurations to minimize THD+N values, and maximize the signal to noise ratio. Low gain configurations require large input signals to obtain a given output power. Input signals equal to or greater than 1 Vrms are available from sources such as audio codecs. Please refer to the section, Audio Power Amplifier Design, for a more complete explanation of proper gain selection. Besides gain, one of the major considerations is the closedloop bandwidth of the amplifier. To a large extent, the bandwidth is dictated by the choice of external components shown in Figure 1. The input coupling capacitor, Ci, forms a first order high pass filter which limits low frequency response. This value should be chosen based on needed frequency response for a few distinct reasons. www.national.com Input Level Input Impedance Bandwidth 1 Wrms 8Ω 1 Vrms 20 kΩ 100 Hz–20 kHz ± 0.25 dB A designer must first determine the minimum supply rail to obtain the specified output power. By extrapolating from the Output Power vs Supply Voltage graphs in the Typical Performance Characteristics section, the supply rail can be easily found. A second way to determine the minimum supply rail is to calculate the required Vopeak using Equation 3 and add the output voltage. Using this method, the minimum supply voltage would be (Vopeak + (VODTOP + VODBOT)), where VODBOT and VODTOP are extrapolated from the Dropout Voltage vs Supply Voltage curve in the Typical Performance Characteristics section. (3) Using the Output Power vs Supply Voltage graph for an 8Ω load, the minimum supply rail is 4.6V. But since 5V is a standard voltage in most applications, it is chosen for the supply rail. Extra supply voltage creates headroom that allows the LM4991 to reproduce peaks in excess of 1W without producing audible distortion. At this time, the designer 12 stated as a pair of −3dB frequency points. Five times away from a −3dB point is 0.17dB down from passband response which is better than the required ± 0.25dB specified. (Continued) 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 4. fL = 100Hz/5 = 20Hz fH = 20kHz * 5 = 100kHz As stated in the External Components section, Ri in conjunction with Ci create a highpass filter. Ci ≥ 1/(2π*20kΩ*20Hz) = 0.397µF; use 0.39µF The high frequency pole is determined by the product of the desired frequency pole, fH, and the differential gain, AVD. With a AVD = 3 and fH = 100kHz, the resulting GBWP = 150kHz which is much smaller than the LM4991 GBWP of 4MHz. This figure displays that if a designer has a need to design an amplifier with a higher differential gain, the LM4991 can still be used without running into bandwidth limitations. (4) (5) Rf/Ri = AVD/2 From Equation 4, the minimum AVD is 2.83; use AVD = 3. Since the desired input impedance was 20kΩ, and with a AVD impedance of 2, a ratio of 1.5:1 of Rf to Ri results in an allocation of Ri = 20kΩ and Rf = 30kΩ. The final design step is to address the bandwidth requirements which must be 13 www.national.com LM4991 Application Information LM4991 Physical Dimensions inches (millimeters) unless otherwise noted Order Number LM4991LD See NS Package Number LDC08A Order Number LM4991MA NS Package Number M08A www.national.com 14 LM4991 3W Audio Power Amplifier with Shutdown Mode Notes LIFE SUPPORT POLICY NATIONAL’S PRODUCTS 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. National Semiconductor Americas Customer Support Center Email: [email protected] Tel: 1-800-272-9959 www.national.com National Semiconductor Europe Customer Support Center Fax: +49 (0) 180-530 85 86 Email: [email protected] Deutsch Tel: +49 (0) 69 9508 6208 English Tel: +44 (0) 870 24 0 2171 Français 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 Asia Pacific Customer Support Center Email: [email protected] National Semiconductor Japan Customer Support Center Fax: 81-3-5639-7507 Email: [email protected] Tel: 81-3-5639-7560 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.