LM4861 1.1W Audio Power Amplifier with Shutdown Mode General Description Key Specifications The LM4861 is a bridge-connected audio power amplifier capable of delivering 1.1W of continuous average power to an 8Ω load with 1% (THD) using a 5V power supply. Boomer audio power amplifiers were designed specifically to provide high quality output power with a minimal amount of external components using surface mount packaging. Since the LM4861 does not require output coupling capacitors, bootstrap capacitors, or snubber networks, it is optimally suited for low-power portable systems. The LM4861 features an externally controlled, low-power consumption shutdown mode, as well as an internal thermal shutdown protection mechanism. The unity-gain stable LM4861 can be configured by external gain-setting resistors. n THD at 1 kHz at 1W continuous average output power into 8Ω 1.0% (max) n Output power at 10% THD+N at 1 kHz into 8Ω 1.5W (typ) n Shutdown Current 0.6 µA (typ) Features n No output coupling capacitors, bootstrap capacitors, or snubber circuits are necessary n Small Outline (SO) packaging n Compatible with PC power supplies n Thermal shutdown protection circuitry n Unity-gain stable n External Gain Configuration Capability Applications n n n n Typical Application Personal computers Portable consumer products Self-powered speakers Toys and games Connection Diagram Small Outline Package DS011986-2 Top View Order Number LM4861M See NS Package Number M08A DS011986-1 FIGURE 1. Typical Audio Amplifier Application Circuit Boomer ® is a registered trademark of National Semiconductor Corporation. © 1999 National Semiconductor Corporation DS011986 www.national.com LM4861 1.1W Audio Power Amplifier with Shutdown Mode May 1997 Absolute Maximum Ratings (Note 2) 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 (Note 3) ESD Susceptibility (Note 4) ESD Susceptibility (Note 5) Junction Temperature Soldering Information Small Outline Package Vapor Phase (60 sec.) Infrared (15 sec.) Operating Ratings 6.0V −65˚C to +150˚C −0.3V to VDD + 0.3V Internally limited 3000V 250V 150˚C Temperature Range TMIN ≤ TA ≤ TMAX Supply Voltage Thermal Resistance θJC (typ) — M08A θJA (typ) — M08A θJC (typ) — N08E θJA (typ) — N08E −40˚C ≤ TA ≤ +85˚C 2.0V ≤ VDD ≤ 5.5V 35˚C/W 140˚C/W 37˚C/W 107˚C/W 215˚C 220˚C Electrical Characteristics (Note 1) (Note 2) The following specifications apply for VDD = 5V, unless otherwise specified. Limits apply for TA = 25˚C. LM4861 Symbol VDD Parameter Conditions Typical Limit (Note 6) (Note 7) Supply Voltage Units (Limits) 2.0 V (min) 5.5 V (max) mA (max) IDD Quiescent Power Supply Current VIN = 0V, IO = 0A (Note 8) 6.5 10.0 ISD Shutdown Current Vpin1 = VDD 0.6 10.0 µA (max) VOS Output Offset Voltage VIN = 0V 5.0 50.0 mV (max) PO Output Power THD = 1% (max); f = 1 kHz 1.1 1.0 W (min) THD+N Total Harmonic Distortion + Noise PO = 1Wrms; 20 Hz ≤ f ≤ 20 kHz 0.72 % PSRR Power Supply Rejection Ratio VDD = 4.9V to 5.1V 65 dB 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 the Absolute Maximum Ratings, whichever is lower. For the LM4861, TJMAX = 150˚C, and the typical junction-to-ambient thermal resistance, when board mounted, is 140˚C/W. Note 4: Human body model, 100 pF discharged through a 1.5 kΩ resistor. Note 5: Machine Model, 220 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 Nationai’s AOQL (Average Outgoing Quality Level). Note 8: The quiescent power supply current depends on the offset voltage when a practical load is connected to the amplifier. www.national.com 2 High Gain Application Circuit DS011986-3 FIGURE 2. Audio Ampiifier with AVD = 20 Single Ended Application Circuit DS011986-4 *CS and CB size depend on specific application requirements and constraints. Typical vaiues of CS and CB are 0.1 µF. **Pin 1 should be connected to VDD to disable the amplifier or to GND to enable the amplifier. This pin should not be left floating. ***These components create a “dummy” load for pin 8 for stability purposes. FIGURE 3. Single-Ended Amplifier with AV = −1 3 www.national.com External Components Description (Figures 1, 2) 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 the Application Information section for proper placement and selection of bypass capacitor. 6. Cf (Note 9) Cf in conjunction with Rf creates a low-pass filter which bandwidth limits the amplifier and prevents possible high frequency oscillation bursts. fC = 1/(2π Rf Cf) Note 9: Optional component dependent upon specific design requirements. Refer to the Application Information section for more information. Typical Performance Characteristics THD+N vs Frequency THD+N vs Frequency DS011986-5 THD+N vs Output Power DS011986-6 THD+N vs Output Power DS011986-8 www.national.com THD+N vs Frequency THD+N vs Output Power DS011986-9 4 DS011986-7 DS011986-10 Typical Performance Characteristics Output Power vs Load Resistance (Continued) Output Power vs Supply Voltage Power Dissipation vs Output Power DS011986-17 DS011986-18 Supply Current Distribution vs Temperature Noise Floor vs Frequency Supply Current vs Supply Voltage DS011986-15 DS011986-14 Power Supply Rejection Ratio Power Derating Curve DS011986-16 DS011986-12 Open Loop Frequency Response DS011986-13 DS011986-20 5 DS011986-19 www.national.com 625 mW.The maximum power dissipation point obtained from Equation 1 must not be greater than the power dissipation that results from Equation 2: Application Information BRIDGE CONFIGURATION EXPLANATION As shown in Figure 1 , the LM4861 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 Ri 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) PDMAX = (TJMAX − TA)/θJA (2) For the LM4861 surface mount package, θJA = 140˚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 62.5˚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. 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 causing excessive clipping which will damage high frequency transducers used in loudspeaker systems, please refer to the Audio Power Amplifier Design 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, singleended amplifier configuration. Without an output coupling capacitor in a single supply, single-ended amplifier, the halfsupply bias across the load would result in both increased internal IC power dissipation and also permanent loudspeaker damage. An output coupling capacitor forms a high pass filter with the load requiring that a large value such as 470 µF be used with an 8Ω load to preserve low frequency response. This combination does not produce a flat response down to 20 Hz, but does offer a compromise between printed circuit board size and system cost, versus low frequency response. 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 10 µF and a 0.1 µF bypass capacitors which aid in supply stability, but do not eliminate the need for bypassing the supply nodes of the LM4861. The selection of bypass capacitors, especially CB, is thus dependant upon desired low frequency THD + N, system cost, and size constraints. SHUTDOWN FUNCTION In order to reduce power consumption while not in use, the LM4861 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. 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 LM4861. There are no soft pull-down resistors inside the LM4861, 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 singleended. 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) HIGHER GAIN AUDIO AMPLIFIER The LM4861 is unity-gain stable and requires no external components besides gain-setting resistors, an input coupling capacitor, and proper supply bypassing in the typical application. However, if a closed-loop differential gain of greater than 10 is required, a feedback capacitor may be needed, as shown in Figure 2, to bandwidth limit the amplifier. This feedback capacitor creates a low pass filter that eliminates possible high frequency oscillations. Care should be taken when calculating the −3 dB frequency in that an incorrect combina- Since the LM4861 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 LM4861 does not require heatsinking. From Equation 1, assuming a 5V power supply and an 8Ω load, the maximum power dissipation point is www.national.com 6 Application Information AUDIO POWER AMPLIFIER DESIGN (Continued) Design a 1W / 8Ω Audio Amplifier tion of Rf and Cf will cause rolloff before 20 kHz. A typical combination of feedback resistor and capacitor that will not produce audio band high frequency rolloff is Rf = 100 kΩ and Cf = 5 pF. These components result in a −3 dB point of approximately 320 kHz. Once the differential gain of the amplifier has been calculated, a choice of Rf will result, and Cf can then be calculated from the formula stated in the External Components Description section. Given: Power Output Load Impedance Input Level Input Impedance 1 Wrms 8Ω 1 Vrms 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. By extrapolating from the Output Power vs Supply Voltage graph 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 dropout voltage. Using this method, the minimum supply voltage would be (Vopeak + VOD , where VOD is typically 0.6V. VOICE-BAND AUDIO AMPLIFIER Many applications, such as telephony, only require a voiceband frequency response. Such an application usually requires a flat frequency response from 300 Hz to 3.5 kHz. By adjusting the component values of Figure 2, this common application requirement can be implemented. The combination of Ri and Ci form a highpass filter while Rf and Cf form a lowpass filter. Using the typical voice-band frequency range, with a passband differential gain of approximately 100, the following values of Ri, Ci, Rf, and Cf follow from the equations stated in the External Components Description section. Ri = 10 kΩ, Rf = 510k ,Ci = 0.22 µF, and Cf = 15 pF (3) For 1W of output power into an 8Ω load, the required Vopeak is 4.0V. A minumum supply rail of 4.6V results from adding Vopeak and Vod. But 4.6V is not a standard voltage that exists in many applications and for this reason, a supply rail of 5V is designated. Extra supply voltage creates dynamic headroom that allows the LM4861 to reproduce peaks in excess of 1Wwithout clipping the signal. 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 4. Five times away from a −3 dB point is 0.17 dB down from the flatband response. With this selection of components, the resulting −3 dB points, fL and fH, are 72 Hz and 20 kHz, respectively, resulting in a flatband frequency response of better than ± 0.25 dB with a rolloff of 6 dB/octave outside of the passband. If a steeper rolloff is required, other common bandpass filtering techniques can be used to achieve higher order filters. SINGLE-ENDED AUDIO AMPLIFIER Although the typical application for the LM4861 is a bridged monoaural amp, it can also be used to drive a load singleendedly in applications, such as PC cards, which require that one side of the load is tied to ground. Figure 3 shows a common single-ended application, where VO1 is used to drive the speaker. This output is coupled through a 470 µF capacitor, which blocks the half-supply DC bias that exists in all singlesupply amplifier configurations. This capacitor, designated CO in Figure 3, in conjunction with RL, forms a highpass filter. The −3 dB point of this high pass filter is 1/(2πRLCO), so care should be taken to make sure that the product of RL and CO is large enough to pass low frequencies to the load. When driving an 8Ω load, and if a full audio spectrum reproduction is required, CO should be at least 470 µF. VO2, the output that is not used, is connected through a 0.1 µF capacitor to a 2 kΩ load to prevent instability. While such an instability will not affect the waveform of VO1, it is good design practice to load the second output. (4) (5) Rf/Ri = AVD/2 From equation 4, the minimum Avd is 2.83: Avd = 3 Since the desired input impedance was 20 kΩ, and with a Avd of 3, a ratio of 1:1.5 of Rf to Ri results in an allocation of Ri = 20 kΩ, 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. This fact results in a low and high frequency pole of 20 Hz and 100 kHz respectively. As 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. The high frequency pole is determined by the product of the desired high frequency pole, fH, and the differential gain, Avd. With a Avd = 2 and fH = 100 kHz, the resulting GBWP = 100 kHz which is much smaller than the LM4861 GBWP of 4 MHz. This figure displays that if a designer has a need to design an amplifier with a higher differential gain, the LM4861 can still be used without running into bandwidth problems. 7 www.national.com LM4861 1.1W Audio Power Amplifier with Shutdown Mode Physical Dimensions inches (millimeters) unless otherwise noted 8-Lead (0.150" Wide) Molded Small Outllne Package, JEDEC (M) Order Number LM4861 NS Package Number M08A 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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