LM4876

LM4876 1.1W Audio Power Amplifier with Logic Low Shutdown March 2003 LM4876 1.1W Audio Power Amplifier with Logic Low Shutdown General Description The LM4876 is a single 5V supply bridge-connected audio power amplifier capable of delivering 1.1W (typ) of continuous average power to an 8Ω load with 0.5% THD+N. Like other audio amplifiers in the Boomer series, the LM4876 is designed specifically to provide high quality output power with a minimal amount of external components. The LM4876 does not require output coupling capacitors, bootstrap capacitors, or snubber networks. It is perfectly suited for lowpower portable systems. The LM4876 features an active low externally controlled, micro-power shutdown mode. Additionally, the LM4876 features an internal thermal shutdown protection mechanism. For PCB space efficiency, the LM4876 is available in MSOP and SO surface mount packages. The unity-gain stable LM4876’s closed loop gain is set using external resistors. Key Specifications j THD+N at 1kHz for 1W continuous average output power into 8Ω j Output power at 1kHz into 8Ω 0.5% (max) 1.5W (typ) 0.01µA (typ) 2.0V to 5.5V with 10% THD+N j Shutdown current j Supply voltage range Features n Does not require output coupling capacitors, bootstrap capacitors, or snubber circuits n 10-pin MSOP and 8-pin SO packages n Unity-gain stable n External gain set Applications n n n n Mobile Phones Portable Computers Desktop Computers Low-Voltage Audio Systems Typical Application 10129901 FIGURE 1. Typical LM4876 Audio Amplifier Application Circuit. Numbers in ( ) are specific to the 10-pin MSOP package Boomer ® is a registered trademark of National Semiconductor Corporation. © 2003 National Semiconductor Corporation DS101299 www.national.com LM4876 Connection Diagrams Mini Small Outline MSOP Package 10129925 Top View Order Number LM4876MM See NS Package Number MUB10A Small Outline SO Package 10129902 Top View Order Number LM4876M See NS Package Number M08A www.national.com 2 LM4876 Absolute Maximum Ratings (Note 2) Infrared (15 sec.) See AN-450 "Surface Mounting and their Effects on Product Reliability" for other methods of soldering surface mount devices. θJC (typ) — MUB10A θJA (typ) — MUB10A θJC (typ) — M08A θJA (typ) — M08A 220˚C 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.) 215˚C 6.0V −65˚C to +150˚C −0.3V to VDD +0.3V Internally Limited 2500V 250V 150˚C 56˚C/W 210˚C/W 35˚C/W 140˚C/W Operating Ratings Temperature Range TMIN ≤ TA ≤ TMAX Supply Voltage −40˚C ≤ TA ≤ 85˚C 2.0V ≤ VDD ≤ 5.5V Electrical Characteristics (Notes 1, 2) The following specifications apply for VDD = 5V unless otherwise specified. Limits apply for TA = 25˚C. LM4876 Symbol VDD IDD ISD VOS Po Parameter Supply Voltage Quiescent Power Supply Current Shutdown Current Output Offset Voltage Output Power VIN = 0V, Io = 0A VPIN1 = 0V VIN = 0V THD = 0.5% (max); f = 1 kHz; RL = 8Ω THD+N = 10%; f = 1 kHz; RL = 8Ω THD+N PSRR Total Harmonic Distortion+Noise Power Supply Rejection Ratio Po = 1 Wrms; AVD = 2; 20 Hz ≤ f ≤ 20 kHz; RL = 8Ω VDD = 4.9V to 5.1V 6.5 0.01 5 1.10 1.5 0.25 65 Conditions Typical (Note 6) Limit (Note 7) 2.0 5.5 10.0 2 50 1.0 Units (Limits) V (min) V (max) mA (max) µA (max) mV (max) W (min) W % 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 that guarantee specific performance limits. This assumes that the device operates within the Operating Ratings. Specifications are not guaranteed for parameters where no limit is given. The typical value, however, 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 LM4876, TJMAX = 150˚C. The typical junction-to-ambient thermal resistance is 140˚C/W for the M08A package and 210˚C/W for the MUB10A package. 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 National’s AOQL (Average Outgoing Quality Level). Electrical Characteristics VDD = 5/3.3/2.6V LM4876 Symbol VIH VIL Parameter Shutdown Input Voltage High Shutdown Input Voltage Low Conditions Typical (Note 6) Limit (Note 7) 1.2 0.4 Units (Limits) V(min) V(max) 3 www.national.com LM4876 External Components Description (Figure 1) Components 1. 2. Ri Ci Functional Description 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 RiCi). Input coupling capacitor which 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. Feedback resistance which sets the closed-loop gain in conjunction with Ri. 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. 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. 3. 4. 5. Rf CS CB Typical Performance Characteristics THD+N vs Frequency THD+N vs Frequency 10129903 10129904 THD+N vs Frequency THD+N vs Output Power 10129905 10129906 www.national.com 4 LM4876 Typical Performance Characteristics THD+N vs Output Power (Continued) THD+N vs Output Power 10129907 10129908 Output Power vs Supply Voltage Output Power vs Supply Voltage 10129909 10129910 Output Power vs Supply Voltage Output Power vs Supply Voltage 10129911 10129911 5 www.national.com LM4876 Typical Performance Characteristics Output Power vs Load Resistance (Continued) Power Dissipation vs Output Power 10129912 10129913 Power Derating Curve Clipping Voltage vs Supply Voltage 10129915 10129914 Noise Floor Frequency Response vs Input Capacitor Size 10129916 10129917 www.national.com 6 LM4876 Typical Performance Characteristics Power Supply Rejection Ratio (Continued) Open Loop Frequency Response 10129918 10129919 Supply Current vs Supply Voltage Supply Current vs Shutdown Voltage LM4876 @ VDD = 5/3.3/2.6Vdc 10129920 10129923 7 www.national.com LM4876 Application Information BRIDGE CONFIGURATION EXPLANATION As shown in Figure 1, the LM4876 consists of two operational amplifiers. External resistors Rf and Ri set the closedloop gain of Amp1, whereas two internal 40kΩ resistors set Amp2’s gain at -1. The LM4876 drives a load, such as a speaker, connected between the two amplifier outputs, Vo1 and Vo2 . Figure 1 shows that the Amp1 output serves as the Amp2 input, which results in both amplifiers producing signals identical in magnitude, but 180˚ out of phase. Taking advantage of this phase difference, a load is placed between Vo1 and Vo2 and driven differentially (commonly referred to as "bridge mode"). This results in a differential gain of (1) AVD = 2 * (Rf/Ri) Bridge mode is different from single-ended amplifiers that drive loads connected between a single amplifier’s output and ground. For a given supply voltage, bridge mode has a distinct advantage over the single-ended configuration: its differential output doubles the voltage swing across the load. This results in four times the output power when 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 that the output signal is not clipped. To ensure minimum output signal clipping when choosing an amplifier’s closed-loop gain, refer to the Audio Power Amplifier Design section. Another advantage of the differential bridge output is no net DC voltage across the load. This results from biasing Vo1 and Vo2 at half-supply. This eliminates the coupling capacitor that single supply, single-ended amplifiers require. Eliminating an output coupling capacitor in a single-ended configuration forces a single-supply 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 may permanently damage loads such as speakers. POWER DISSIPATION Power dissipation is a major concern when designing a successful bridged or single-ended amplifier. Equation (2) states the maximum power dissipation point for a singleended amplifier operating at a given supply voltage and driving a specified output load. (2) PDMAX = (VDD)2 /(2π2 RL) Single-Ended However, a direct consequence of the increased power delivered to the load by a bridge amplifier is higher internal power dissipation for the same conditions. The LM4876 has two operational amplifiers in one package and the maximum internal power dissipation is four times that of a single-ended amplifier. Equation (3) states the maximum power dissipation for a bridge amplifier. However, even with this substantial increase in power dissipation, the LM4876 does not require heatsinking. From Equation (3), assuming a 5V power supply and an 8Ω load, the maximum power dissipation point is 633mW. (3) PDMAX = 4*(VDD)2 /(2π2 RL ) Bridge Mode The maximum power dissipation point given by Equation (3) must not exceed the power dissipation given by Equation (4): (4) PDMAX = (TJMAX -TA) /θJA The LM4876’s TJMAX = 150˚C. In the M08A package, the LM4876’s θJA is 140˚C/W. At any given ambient temperature TA, use Equation (4) to find the maximum internal power www.national.com 8 dissipation supported by the IC packaging. Rearranging Equation (4) results in Equation (5). This equation gives the maximum ambient temperature that still allows maximum power dissipation without violating the LM4876’s maximum junction temperature. TA = TJMAX - PDMAX θJA (5) For a typical application with a 5V power supply and an 8W load, the maximum ambient temperature that allows maximum power dissipation without exceeding the maximum junction temperature is approximately 61˚C. (6) TJMAX = PDMAX θJA + TA For the MSOP10A package, θJA = 210˚C/W. Equation (6) shows that TJMAX , for the MSOP10 package, is 158˚C for an ambient temperature of 25˚C and using the same 5V power supply and an 8Ω load. This violates the LM4876’s 150˚C maximum junction temperature when using the MSOP10A package. Reduce the junction temperature by reducing the power supply voltage or increasing the load resistance. Further, allowance should be made for increased ambient temperatures. To achieve the same 61˚C maximum ambient temperature found for the MO8 package, the MSOP10 packaged part should operate on a 4.1V supply voltage when driving an 8Ω load. Alternatively, a 5V supply can be used when driving a load with a minimum resistance of 12Ω for the same 61˚C maximum ambient temperature. Fully charged Li-ion batteries typically supply 4.3V to portable applications such as cell phones. This supply voltage allows the LM4876 to drive loads with a minimum resistance of 9Ω without violating the maximum junction temperature when the maximum ambient temperature is 61˚C. The above examples assume that a device is a surface mount part operating around the maximum power dissipation point. Since internal power dissipation is a function of output power, higher ambient temperatures are allowed as output power or duty cycle decreases. If the result of Equation (3) is greater than that of Equation (4), then decrease the supply voltage, increase the load impedance, or reduce the ambient temperature. If these measures are insufficient, a heat sink can be added to reduce θJA. The heat sink can be created using additional copper area around the package, with connections to the ground pin(s), supply pin and amplifier output pins. When adding a heat sink, the θJA is the sum of θJC, θCS, and θSA. ( θJC is the junction-to-case thermal impedance, θCS is the case-to-sink thermal impedance, and θSA is the sink-toambient thermal impedance.) Refer to the Typical Performance Characteristics curves for power dissipation information at lower output power levels. POWER SUPPLY BYPASSING As with any power amplifier, proper supply bypassing is critical for low noise performance and high power supply rejection. Applications that employ a 5V regulator typically use a 10µF in parallel with a 0.1µF filter capacitors to stabilize the regulator’s output, reduce noise on the supply line, and improve the supply’s transient response. However, their presence does not eliminate the need for local bypass capacitance at the LM4876’s supply pins. Keep the length of leads and traces that connect capacitors between the LM4876’s power supply pin and ground as short as possible. Connecting a 1µF capacitor between the BYPASS pin and ground improves the internal bias voltage’s stability and improves the amplifier’s PSRR. The PSRR improvements increase as the bypass pin capacitor value increases. Too large, however, and the amplifier’s click and pop perfor- LM4876 Application Information (Continued) mance can be compromised. The selection of bypass capacitor values, especially CB, depends on desired PSRR requirements, click and pop performance (as explained in the section, Proper Selection of External Components), system cost, and size constraints. MICRO-POWER SHUTDOWN The voltage applied to the SHUTDOWN pin controls the LM4876’s shutdown function. Activate micro-power shutdown by applying a voltage below 400mV to the SHUTDOWN pin. When active, the LM4876’s micro-power shutdown feature turns off the amplifier’s bias circuitry, reducing the supply current. Though the LM4876 is in shutdown when 400mV is applied to the SHUTDOWN pin, the supply current may be higher than 0.01µA (typ) shutdown current. Therefore, for the lowest supply current during shutdown, connect the SHUTDOWN pin to ground. The relationship between the supply voltage, the shutdown current, and the voltage applied to the SHUTDOWN pin is shown in Typical Performance Characteristics curves. There are a few ways to control the micro-power shutdown. These include using a single-pole, single-throw switch, a microprocessor, or a microcontroller. When using a switch, connect an external pull-down resistor between the SHUTDOWN pin and GND. Connect the switch between the SHUTDOWN pin and VCC. Select normal amplifier operation by closing the switch. Opening the switch connects the SHUTDOWN pin to GND through the pull-down resistor, activating micro-power shutdown. The switch and resistor guarantee that the SHUTDOWN pin will not float. This prevents unwanted state changes. In a system with a microprocessor or a microcontroller, use a digital output to apply the control voltage to the SHUTDOWN pin. Driving the SHUTDOWN pin with active circuitry eliminates the pull down resistor. SELECTING POWER EXTERNAL COMPONENTS Optimizing the LM4876’s performance requires properly selecting external components. Though the LM4876 operates well when using external components with wide tolerances, best performance is achieved by optimizing component values. The LM4876 is unity-gain stable, giving a designer maximum design flexibility. The gain should be set to no more than a given application requires. This allows the amplifier to achieve minimum THD+N and maximum signal-to-noise ratio. These parameters are compromised as the closed-loop gain increases. However, low gain demands input signals with greater voltage swings to achieve maximum output power. Fortunately, many signal sources such as audio CODECs have outputs of 1VRMS (2.83VP-P). Please refer to the Audio Power Amplifier Design section for more information on selecting the proper gain. Input Capacitor Value Selection Amplifying the lowest audio frequencies requires high value input coupling capacitor (Ci in Figure 1). A high value capacitor can be expensive and may compromise space efficiency in portable designs. In many cases, however, the speakers used in portable systems, whether internal or external, have little ability to reproduce signals below 150 Hz. Applications using speakers with this limited low frequency response reap little improvement by using a large input capacitor. Besides affecting system cost and size, Ci also affects the LM4876’s click and pop performance. When the supply voltage is first applied, a transient (pop) is created as the charge on the input capacitor changes from zero to a quiescent state. The magnitude of the pop is directly proportional to the input capacitor’s size. Higher value capacitors need more time to reach a quiescent DC voltage (usually VCC/2) when charged with a fixed current. The amplifier’s output charges the input capacitor through the feedback resistor, Rf. Thus, pops can be minimized by selecting an input capacitor value that is no higher than necessary to meet the desired -3dB frequency. As shown in Figure 1, the input resistor (RI) and the input capacitor, CI produce a -3dB high pass filter cutoff frequency that is found using Equation (7). (7) f-3dB = 2πRINCI As an example when using a speaker with a low frequency limit of 150Hz, Equation (7) gives a value of Ci equal to 0.1µF. The 0.22µF Ci shown in Figure 1 allows for a speaker whose response extends down to 75Hz. Bypass Capacitor Value Selection Besides minimizing the input capacitor size, careful consideration should be paid to value of, CB, the capacitor connected to the BYPASS pin. Since CB determines how fast the LM4876 settles to quiescent operation, its value is critical when minimizing turn-on pops. The slower the LM4876’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), produces a click-less and pop-less shutdown function. As discussed above, choosing Ci as small as possible helps minimize clicks and pops. 9 www.national.com LM4876 Application Information AUDIO POWER AMPLIFIER DESIGN (Continued) (10) Thus, a minimum gain of 2.83 allows the LM4876’s to reach full output swing and maintain low noise and THD+N performance. For this example, let AVD = 3. The amplifier’s overall gain is set using the input (Ri) and feedback (Rf) resistors. With the desired input impedance set at 20kΩ, the feedback resistor is found using Equation (11). (11) Rf/Ri = AVD/2 The value of Rf is 30kΩ. The last step in this design example is setting the amplifier’s -3dB low frequency bandwidth. To achieve the desired ± 0.25dB pass band magnitude variation limit, the low frequency response must extend to at least one-fifth the lower bandwidth limit and the high frequency response must extend to at least five times the upper bandwidth limit. The results is an fL = 100 Hz/5 = 20Hz and an FH = 20 kHz*5 = 100kHz As mentioned in the External Components section, Ri and Ci create a highpass filter that sets the amplifier’s lower bandpass frequency limit. Find the coupling capacitor’s value using Equation (12). (12) Ci ≥ 1/(2πRifL) The result is 1/(2π*20kΩ*20Hz) = 0.398µF. Use a 0.39µF capacitor, the closest standard value. The product of the desired high frequency cutoff (100kHz in this example) and the differential gain, AVD, determines the upper passband response limit. With AVD = 3 and fH = 100kHz, the closed-loop gain bandwidth product (GBWP) is 150kHz. This is less than the LM4876’s 4MHz GBWP. With this margin, the amplifier can be used in designs that require more differential gain and avoid performance-restricting bandwidth limitations. Audio Amplifier Design: Driving 1W into an 8Ω Load The following are the desired operational parameters: Power Output Load Impedance Input Level Input Impedance Bandwidth 1WRMS 8Ω 1VRMS 20kΩ 100Hz–20kHz ± 0.25dB The design begins by specifying the minimum supply voltage necessary to obtain the specified output power. One way to find the minimum supply voltage is to use the Output Power vs Supply Voltage curve in the Typical Performance Characteristics section. Another way, using Equation (8), is to calculate the peak output voltage necessary to achieve the desired output power for a given load impedance. To account for the amplifier’s dropout voltage, two additional voltages, based on the Dropout Voltage vs Supply Voltage in the Typical Performance Characteristics curves, must be added to the result obtained by Equation (8). This results in Equation (9). (8) (9) VCC ≥ (VOUTPEAK + (VODTOP + VODBOT)) The Output Power vs Supply Voltage graph for an 8Ω load indicates a minimum supply voltage of 4.6V. This is easily met by the commonly used 5V supply voltage. The additional voltage creates the benefit of headroom, allowing the LM4876 to produce peak output power in excess of 1W without clipping or other audible distortion. The choice of supply voltage must also not create a violation of maximum power dissipation as explained above in the Power Dissipation section. After satisfying the LM4876’s power dissipation requirements, the minimum differential gain is found using Equation (10). www.national.com 10 LM4876 Physical Dimensions unless otherwise noted inches (millimeters) Order Number LM4876MM NS Package Number MUB10A Order Number LM4876M NS Package Number M08A 11 www.national.com LM4876 1.1W Audio Power Amplifier with Logic Low Shutdown 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. 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