LM4870 1.1W Audio Power Amplifier with Shutdown Mode General Description Key Specifications The LM4870 is a bridge-connected audio power amplifier capable of delivering 1.1W of continuous average power to an 8Ω load with less than 0.5% THD+N over the audio spectrum from a 5V power supply. Boomer audio power amplifiers were designed specifically to provide high quality output power with a minimal number of external components. Since the LM4870 does not require output coupling capacitors, bootstrap capacitors or snubber networks, it is optimally suited for low-power portable systems. The LM4870 features an externally controlled, low-power consumption shutdown mode, as well as an internal thermal shutdown protection mechanism. It also includes two headphone control inputs and a headphone sense output for external monitoring. The LM4870 is unity-gain stable and the gain is set using external resistors. Typical Application n THD+N at 1W into 8Ω 0.5% (max) n Output power into 8Ω at 1kHz at 10%THD+N 1.5W (typ) n Shutdown Current 0.6µA (typ) Features n No output coupling capacitors, bootstrap capacitors, or snubber circuits are necessary n Small Outline (SOIC) power packaging n Unity-gain stable n External gain configuration capability Applications n Personal computers n Desktop computers n Low voltage audio system Connection Diagram Small Outline Package DS100094-2 Top View Order Number LM4870M See NS Package Number M16A DS100094-1 FIGURE 1. Typical Audio Amplifier Application Circuit Boomer ® is a registered trademark of National Semiconductor Corporation. © 1999 National Semiconductor Corporation DS100094 www.national.com LM4870 1.1W Audio Power Amplifier with Shutdown Mode October 1997 Absolute Maximum Ratings (Note 2) Infrared (15 sec.) 220˚C Thermal Resistance 35˚C/W φJC(typ) 100˚C/W φJA(typ) See AN-450 “Surface Mounting and their Effects on Product Reliability” for other methods of soldering surface mount devices. 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 ESD Susceptibility (Note 4) ESD Susceptibility (Note 5) Junction Temperature Soldering Information Small Outline Package Vapor Phase (60 sec.) 6.0V −65˚C to +150˚C −0.3V to VDD + 0.3V Internally limited 3000V 250V 150˚C Operating Ratings Temperature Range TMIN ≤ TA ≤ TMAX Supply Voltage −20˚C ≤ TA ≤ +85˚C 2.0V ≤ VDD ≤ 5.5V 215˚C Electrical Characteristics (Notes 1, 2) The following specifications apply for VDD = 5V, RL = 8Ω unless otherwise specified. Limits apply for TA = 25˚C. Symbol VDD Parameter Conditions LM4870 Typical Limit (Note 6) (Note 7) Supply Voltage IDD Quiescent Power Supply Current ISD Shutdown Current VOS Output Offset Voltage PO Output Power THD+N Total Harmonic Distortion + Noise PSRR Power Supply Rejection Ratio Vod Output Dropout Voltage VIH HP-IN High Input Voltage VIL HP-IN Low Input Voltage VOH VOL VO = 0V, IO = 0A Vpin2 = VDD VIN = 0V THD+N = 0.5% (max); f = 1 kHz PO = 1 Wrms; 20 Hz ≤ f ≤ 20 kHz VDD = 4.9V to 5.1V Units (Limits) 2.0 V (min) 5.5 V (max) 15.0 mA (max) 10 50.0 mV (max) 1.1 1.0 W (min) 7.0 0.4 µA 0.25 % 60 dB HP-SENSE High Output Voltage VIN = 0V to 5V HP-SENSE = 0V to 4V HP-SENSE = 4V to 0V IO = 500 µA 2.8 2.5 V (min) HP-SENSE Low Output Voltage IO = −500 µA 0.2 0.8 V (max) 0.6 1.0 2.5 V (max) V 2.5 V Note 1: All voltages are measured with respect to the ground pins, 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 − T A)/θJA or the number given in the Absolute Maximum Ratings, whichever is lower. For the LM4870, TJMAX = +150˚C, and the typical junction-to-ambient thermal resistance, when board mounted, is 100˚C/W. Note 4: Human body model, 100 pF discharged through a 1.5 kΩ resistor. Note 5: Machine Model, 200 pF–240 pF 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). www.national.com 2 External Components Description (Figure 1) Components Functional Description 1. Ri Inverting input resistance which sets the closed-loop gain in conjunction with Rf. This resistor also forms a high pass filter with Ci at fC = 1/(2π Ri Ci). 2. Ci Input coupling capacitor which blocks DC voltage at the amplifier’s input terminals. Also creates a highpass filter with Ri at fC = 1/(2π Ri Ci). 3. Rf Feedback resistance which sets closed-loop gain in conjunction with Ri. 4. CS Supply bypass capacitor which provides power supply filtering. Refer to the Application Information section for proper placement and selection of supply bypass capacitor. 5. CB Bypass pin capacitor which provides half supply filtering. Refer to Application Information section for proper placement and selection of bypass capacitor. Typical Performance Characteristics THD+N vs Frequency THD+N vs Frequency DS100094-9 THD+N vs Output Power THD+N vs Frequency DS100094-10 THD+N vs Output Power DS100094-12 Supply Current vs Supply Voltage DS100094-11 THD+N vs Output Power DS100094-13 DS100094-14 LM4870 Noise Floor vs Frequency Power Derating Curve DS100094-17 DS100094-18 DS100094-16 3 www.national.com Typical Performance Characteristics Supply Current Distribution vs Temperature (Continued) Power Dissipation vs Output Power DS100094-19 Output Power vs Supply Voltage Output Power vs Load Resistance DS100094-20 Output Power vs Supply Voltage DS100094-21 Open Loop Frequency Response DS100094-22 DS100094-27 DS100094-23 Power Supply Rejection Ratio DS100094-24 Application Information 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. Consequently, 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 caus- BRIDGE CONFIGURATION EXPLANATION As shown in Figure 1, the LM4870 has two operational amplifiers internally, allowing for a few different amplifier configurations. The first amplifier’s gain is externally configurable, while 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 R i while the second amplifier’s gain is fixed by the two internal 40 kΩ resistors. 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 out of phase 180˚. Consequently, the differential gain for the IC is: Avd = 2 * (Rf/Ri) www.national.com 4 Application Information bypass capacitors, especially CB, is thus dependant upon desired low frequency THD+N, system cost, and size constraints. (Continued) ing excessive clipping which will damage high frequency transducers used in loudspeaker systems, please refer to the Audio Power Amplifier Deslgn section. A bridge configuration, such as the one used in Boomer Audio Power Amplifiers, 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 eliminates the need for an output coupling capacitor which is required in a single supply, single-ended amplifier configuration. Without a large output coupling capacitor in a single supply single-ended amplifier, the half-supply bias across the load would result in both increased internal IC power dissipation. SHUTDOWN FUNCTION In order to reduce power consumption while not in use, the LM4870 contains a shutdown pin to externally turn off the amplifier’s bias circuitry. The shutdown feature turns the amplifier off when a logic high is placed on the shutdown pin. Upon going into shutdown, the output is immediately disconnected from the speaker. There is a built-in threshold which produces a drop in quiescent current to 500 µA typically. For a 5V power supply, this threshold occurs when 2V–3V is applied to the shutdown pin. A typical quiescent current of 0.6 µA results when the supply voltage is applied to the shutdown pin. 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 that when closed, is connected to ground and enables the amplifier. If the switch is open, then a soft pull-up resistor of 47 kΩ will disable the LM4870. There are no soft pull-down resistors inside the LM4870, so a definite shutdown pin voltage must be applied externally, or the internal logic gate will be left floating which could disable the amplifier unexpectedly. 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. PDMAX = 4 * (VDD) 2/(2π2RL) (1) Since the LM4870 has two operational amplifiers in one package, the maximum internal power dissipation is 4 times that of a single-ended amplifier. Even with this substantial increase in power dissipation, the LM4870 does not require heatsinking. 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: PDMAX = (TJMAX − T A)/θJA (2) For the LM4870 surface mount package, θJA = 100˚C/W and TJMAX = 150˚C. Depending on the ambient temperature, TA, of the system surroundings, 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 or the load impedance increased. 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 88˚C, provided that device operation is around the maximum power dissipation point. Power dissipation is a function of output power and thus, 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 lower output powers. HEADPHONE CONTROL INPUTS The LM4870 possesses two headphone control inputs that disable the amplifier and reduce IDD to less than 1 mA when either one or both of these inputs have a logic-high voltage placed on their pins. Unlike the shutdown function, the headphone control function does not provide the level of current conservation that is required for battery powered systems. Since the quiescent current resulting from the headphone control function is 1000 times more than the shutdown function, the residual currents in the device may create a pop at the output when coming out of the headphone control mode. The pop effect may be eliminated by connecting the headphone sensing output to the shutdown pin input as shown in Figure 2. This solution will not only eliminate the output pop, but will also utilize the full current conservation of the shutdown function by reducing IDD to 0.6 µA. The amplifier will then be fully shutdown. This configuration also allows the designer to use the control inputs as either two headphone control pins or a headphone control pin and a shutdown pin where the lowest level of current consumption is obtained from either function. Figure 3 shows the implementation of the LM4870’s headphone control function using a single-supply headphone amplifier. The voltage divider of R1 and R2 sets the voltage at the HP-IN1 pin to be approximately 50 mV when there are no headphones plugged into the system. This logic-low voltage at the HP-IN1 pin enables the LM4870 to amplify AC signals. Resistor R3 limits the amount of current flowing out of the HP-IN1 pin when the voltage at that pin goes below ground resulting from the music coming from the headphone amplifier. The output coupling cap protects the headphones by blocking the amplifier’s half-supply DC voltage. The capacitor also protects the headphone amplifier from the low voltage set up by resistors R1 and R2 when there aren’t any headphones plugged into the system. The tricky point to this setup is that the AC output voltage of the headphone amplifier cannot exceed the 2.0V HP-IN1 voltage threshold when there aren’t any headphones plugged into the system, assuming that R1 and R2 are 100k and 1k, respectively. The LM4870 may not be fully shutdown when this level is ex- POWER SUPPLY BYPASSING As with any power 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 device as possible. As displayed in the Typical Performance CharacterIstIcs section, the effect of a larger half-supply bypass capacitor is improved low frequency THD+N due to increased half-supply stability. Typical applications employ a 5V regulator with a10 µF tantalum and a 0.1 µF film bypass capacitors which aid in supply stability, but do not eliminate the need for bypassing the supply nodes of the LM4870. The selection of 5 www.national.com Application Information In addition, the HP-SENSE pin, although it may be connected to the SHUTDOWN pin as shown in Figure 2, may still be used as a control flag. It is capable of driving the input to another logic gate or approximately 2 mA without serious loading. (Continued) ceeded momentarily, due to the discharging time constant of the bias-pin voltage. This time constant is established by the two 50k resistors (in parallel) with the series bypass capacitor value. When a set of headphones are plugged into the system, the contact pin of the headphone jack is disconnected from the signal pin, interrupting the voltage divider set up by resistors R1 and R2. Resistor R1 then pulls up the HP-IN1 pin, enabling the headphone function and disabling the LM4870 amplifier. The headphone amplifier then drives the headphones, whose impedance is in parallel with resistor R2. Since the typical impedance of headphones are 32Ω, resistor R2 has negligible effect on the output drive capability. Also shown in Figure 3 are the electrical connections for the headphone jack and plug. A 3-wire plug consists of a Tip, Ring, and Sleave, where the Tip and Ring are signal carrying conductors and the Sleave is the common ground return. One control pin contact for each headphone jack is sufficient to indicate to control inputs that the user has inserted a plug into a jack and that another mode of operation is desired. For a system implementation where the headphone amplifier is designed using a split supply, the output coupling cap, CC and resistor R2 of Figure 3, can be eliminated. The functionality described earlier remains the same, however. DS100094-7 FIGURE 2. HP-SENSE Pin to SHUTDOWN Pin Connection DS100094-8 FIGURE 3. Typical Headphone Control Input Circuitry AUDIO POWER AMPLIFIER DESIGN Input Impedance: 20 kΩ Bandwidth: 100 Hz-20 kHz ± 0.25 dB A designer must first determine the needed supply rail to obtain the specified output power. The Output Power vs Supply Voltage graphs in the Typical Performance Characteristics section show how much power the LM4870 will output given different supply voltage. According to the graph, 4.6V Design a 500 mW/8Ω Audio Amplifier Given: Power Output: 1.0Wrms Load Impedance: 8Ω Input Level: 1 Vrms(max) www.national.com 6 Application Information put (Ri) resistor is 1.5:1. If Ri is set to the required input impedance of 20 kΩ, then Rf = 30 kΩ. The final design step is to address the bandwidth requirements which must be stated as a pair of −3 dB frequency points. Five times away from a −3 dB point is 0.17 dB down from passband response which is better than the required ± 0.25 dB specified. fL = 100 Hz/5 = 20 Hz fH = 20 kHz5 = 100 HzAs stated in the External Components section, Ri in conjunction with Ci create a highpass filter. Ci ≥ 1/(2π * 20 kΩ * 20 Hz) = 0.397 µF; use 0.39 µF. (Continued) are required for 1.0W of output power. Since 5V rails are common, it is chosen to be the supply voltage. Supply voltage (up to a maximum of 5.5V) above the required 4.6V will allow the LM4870 to reproduce transient signals at greater than 1.0W of output power. The extra voltage will also increase the device power dissipation, so the designer must make sure that the conditions explained in the Power Dissipationsection are not violated. Once the power dissipation issues have been addressed, the required differential gain can be determined from Equation 4: From equation 4, the minimum Avd is A 3. vd The high frequency pole is determined by the product of the desired high frequency pole, fH, and the differential gain, A vd. With a Avd = 3 and fH = 100 kHz, the resulting GBWP = 150 kHz which is much smaller than the LM4870 GBWP of 4 MHz. This figure displays that if a designer has a need to design an amplifier with a higher differential gain, the LM4870 can still be used without running into bandwidth problems. = 2.83; use Avd = Since the amplifier output is bridged, giving a gain of 2, and the desired system gain is 3, the ratio of feedback (Rf) to in- 7 www.national.com LM4870 1.1W Audio Power Amplifier with Shutdown Mode Physical Dimensions inches (millimeters) unless otherwise noted Small Outline Package (M) Order Number LM4870M NS Package Number M16A 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 Corporation Americas Tel: 1-800-272-9959 Fax: 1-800-737-7018 Email: [email protected] www.national.com National Semiconductor Europe Fax: +49 (0) 1 80-530 85 86 Email: [email protected] Deutsch Tel: +49 (0) 1 80-530 85 85 English Tel: +49 (0) 1 80-532 78 32 Français Tel: +49 (0) 1 80-532 93 58 Italiano Tel: +49 (0) 1 80-534 16 80 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. 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