LM4873MT

LM4873 Dual 2.1W Audio Amplifier Plus Stereo Headphone Function April 2000 LM4873 Dual 2.1W Audio Amplifier Plus Stereo Headphone Function General Description The LM4873 is a dual bridge-connected audio power amplifier which, when connected to a 5V supply, will deliver 2.1W to a 4Ω load (Note 1) or 2.4W to a 3Ω load (Note 2)with less than 1.0% THD+N. In addition, the headphone input pin allows the amplifiers to operate in single-ended mode to drive stereo headphones. A Mux Control pin toggles between the two stereo sets of amplifier inputs, allowing for two selectable amplifier closed-loop responses. Boomer audio power amplifiers were designed specifically to provide high quality output power from a surface mount package while requiring few external components. To simplify audio system design, the LM4873 combines dual bridge speaker amplifiers and stereo headphone amplifiers on one chip. The LM4873 features an externally controlled, low-power consumption shutdown mode, a stereo headphone amplifier mode, and thermal shutdown protection. It also utilizes circuitry to reduce “clicks and pops” during device turn-on. Note 1: An LM4873MTE-1 which has been properly mounted to the circuit board will deliver 2.1W into 4Ω. The other package options for the LM4873 will deliver 1.1W into 8Ω. See the Application Information section for LM4873MTE-1 usage information. Note 2: An LM4873MTE-1 which has been properly mounted to the circuit board and forced-air cooled will deliver 2.4W into 3Ω. Key Specifications n PO at 1% THD+N into 3Ω (LM4873MTE-1) into 4Ω (LM4873MTE-1) into 4Ω (LM4873MTE) into 8Ω (LM4873) n Single-ended mode - THD+N at 75mW into 32Ω n Shutdown current 2.4W(typ) 2.1W(typ) 1.9W(typ) 1.1W(typ) 0.5%(max) 0.7µA(typ) Features n n n n n Input mux control and two separate inputs per channel Stereo headphone amplifier mode “Click and pop” suppression circuitry Thermal shutdown protection circuitry Exposed-DAP TSSOP and TSSOP packaging available Applications n Multimedia monitors n Portable and desktop computers n Portable audio systems Typical Application DS100993-31 * Refer to the section Proper Selection of External Components, for a detailed discussion of CB size. FIGURE 1. Typical Audio Amplifier Application Circuit Boomer ® is a registered trademark of National Semiconductor Corporation. © 2000 National Semiconductor Corporation DS100993 www.national.com LM4873 Connection Diagram DS100993-30 Top View Order Number LM4873MTE-1 See NS Package Number MXA28A for Exposed-DAP TSSOP Connection Diagram DS100993-2 Top View Order Number LM4873MT, LM4873MTE See NS Package Number MTC20 for TSSOP See NS Package Number MXA20A for Exposed-DAP TSSOP www.national.com 2 LM4873 Absolute Maximum Ratings (Note 4) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Supply Voltage 6.0V Storage Temperature −65˚C to +150˚C Input Voltage −0.3V to VDD +0.3V Power Dissipation (Note 14) Internally limited ESD Susceptibility (Note 15) 2000V ESD Susceptibility (Note 16) 200V Junction Temperature 150˚C Solder Information Small Outline Package Vapor Phase (60 sec.) 215˚C Infrared (15 sec.) 220˚C See AN-450 “Surface Mounting and their Effects on Product Reliablilty” for other methods of soldering surface mount devices. Thermal Resistance 20˚C/W θJC (typ) — M16B θJA (typ) — M16B θJC (typ) — N16A θJA (typ) — N16A θJC (typ) — MTC20 θJA (typ) — MTC20 θJC (typ) — MXA20A θJA (typ) — MXA20A θJA (typ) — MXA20A θJA (typ) — MXA20A θJC (typ) — MXA28A θJA (typ) — MXA28A θJA (typ) — MXA28A θJA (typ) — MXA28A 80˚C/W 20˚C/W 63˚C/W 20˚C/W 80˚C/W 2˚C/W 41˚C/W (Note 5) 51˚C/W (Note 6) 90˚C/W (Note 7) 2˚C/W 41˚C/W (Note 8) 51˚C/W (Note 9) 90˚C/W (Note 10) Operating Ratings Temperature Range TMIN ≤ TA ≤ TMAX Supply Voltage −40˚C ≤ TA ≤ 85˚C 2.0V ≤ VDD ≤ 5.5V Electrical Characteristics for Entire IC (Notes 3, 4) The following specifications apply for VDD = 5V unless otherwise noted. Limits apply for TA = 25˚C. Symbol Parameter Conditions LM4873 Typical (Note 17) VDD IDD ISD VIH VIL Supply Voltage Quiescent Power Supply Current Shutdown Current Headphone High Input Voltage Headphone Low Input Voltage VIN = 0V, IO = 0A (Note 19) , HP-IN = 0V VIN = 0V, IO = 0A (Note 19) , HP-IN = 4V VPIN1 = VDD 7.5 5.8 0.7 Limit (Note 18) 2 5.5 15 6 2 4 0.8 V (min) V (max) mA (max) mA (min) µA (min) V (min) V (max) Units (Limits) Electrical Characteristics for Bridged-Mode Operation (Notes 3, 4) The following specifications apply for VDD = 5V unless otherwise specified. Limits apply for TA = 25˚C. Symbol Parameter Conditions LM4873 Typical (Note 17) VOS PO Output Offset Voltage Output Power (Note 13) VIN = 0V THD = 1%, f = 1 kHz LM4873MTE-1, RL = 3Ω (Note 11) LM4873MTE, RL = 3Ω (Note 11) LM4873MTE-1, RL = 4Ω (Note 12) LM4873MTE, RL = 4Ω (Note 12) LM4873, RL = 8Ω THD+N = 10%, f = 1 kHz LM4873MTE-1, RL = 3Ω (Note 11) LM4873MTE-1, RL = 4Ω (Note 12) LM4873, RL = 8Ω THD+N = 1%, f = 1 kHz, RL = 32Ω 3.0 2.6 1.5 0.34 W W W 5 2.4 2.2 2.1 1.9 1.1 1.0 Limit (Note 18) 50 mV (max) W W W W W (min) Units (Limits) 3 www.national.com LM4873 Electrical Characteristics for Bridged-Mode Operation (Notes 3, 4) The following specifications apply for VDD = 5V unless otherwise specified. Limits apply for TA = 25˚C. Symbol Parameter Conditions (Continued) LM4873 Typical (Note 17) Limit (Note 18) Units (Limits) THD+N Total Harmonic Distortion+Noise 20 Hz ≤ f ≤ 20 kHz, AVD = 2 LM4873MTE-1, RL = 4Ω, PO = 2W LM4873, RL = 8Ω, PO = 1W VDD = 5V, VRIPPLE = 200 mVRMS, RL = 8Ω, CB = 1.0 µF f = 1 kHz, CB = 1.0 µF VDD = 5V, PO = 1.1W, RL = 8Ω 0.3 0.3 67 80 97 % dB dB dB PSRR XTALK SNR Power Supply Rejection Ratio Channel Separation Signal To Noise Ratio Electrical Characteristics for Single-Ended Operation (Notes 3, 4) The following specifications apply for VDD = 5V unless otherwise specified. Limits apply for TA = 25˚C. Symbol Parameter Conditions LM4873 Typical (Note 17) VOS PO Output Offset Voltage Output Power VIN = 0V THD = 0.5%, f = 1 kHz, RL = 32Ω THD+N = 1%, f = 1 kHz, RL = 8Ω THD+N = 10%, f = 1 kHz, RL = 8Ω THD+N PSRR XTALK SNR Total Harmonic Distortion+Noise Power Supply Rejection Ratio Channel Separation Signal To Noise Ratio AV = −1, PO = 75 mW, 20 Hz ≤ f ≤ 20 kHz, RL = 32Ω CB = 1.0 µF, VRIPPLE = 200 mV f = 1 kHz f = 1 kHz, CB = 1.0 µF VDD = 5V, PO = 340mW, RL = 8Ω RMS, Limit (Note 18) 50 75 Units (Limits) 5 85 340 440 0.2 52 60 94 mV (max) mW (min) mW mW % dB dB dB Note 3: All voltages are measured with respect to the ground pins, 2, 7, and 15, unless otherwise specified. Note 4: 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 5: The θJA given is for an MXA20A package whose exposed-DAP is soldered to an exposed 2in2 piece of 1 ounce printed circuit board copper. Note 6: The θJA given is for an MXA20A package whose exposed-DAP is soldered to an exposed 1in2 piece of 1 ounce printed circuit board copper. Note 7: The θJA given is for an MXA20A package whose exposed-DAP is not soldered to any copper. Note 8: The θJA given is for an MXA28A package whose exposed-DAP is soldered to an exposed 2in2 piece of 1 ounce printed circuit board copper. Note 9: The θJA given is for an MXA28A package whose exposed-DAP is soldered to an exposed 1in2 piece of 1 ounce printed circuit board copper. Note 10: The θJA given is for an MXA28A package whose exposed-DAP is not soldered to any copper. Note 11: When driving 3Ω loads from a 5V supply, the LM4873MTE or LM4873MTE-1 must be mounted to the circuit board and forced-air cooled (450 linear-feet per minute). Note 12: When driving 4Ω loads from a 5V supply, the LM4873MTE or LM4873MTE-1 must be mounted to the circuit board. Note 13: Output power is measured at the device terminals. Note 14: The maximum power dissipation must be derated at elevated temperatures and is dictated by TJMAX, θ JA, and the ambient temperature TA. The maximum allowable power dissipation is PDMAX = (TJMAX − T A)/θJA. For the LM4873, TJMAX = 150˚C. For the θJAs for different packages, please see the Application Information section or the Absolute Maximum Ratings section. Note 15: Human body model, 100 pF discharged through a 1.5 kΩ resistor. Note 16: Machine model, 220 pF–240 pF discharged through all pins. Note 17: Typicals are measured at 25˚C and represent the parametric norm. Note 18: Limits are guaranteed to National’s AOQL (Average Outgoing Quality Level). Note 19: The quiescent power supply current depends on the offset voltage when a practical load is connected to the amplifier. www.national.com 4 LM4873 Truth Table for Logic Inputs SHUTDOWN Low Low Low Low High HP-IN Low Low High High X INPUT SELECT Low High Low High X LM4873 MODE (INPUT #) Bridged (1) Bridged (2) Single-Ended (1) Single-Ended (2) Shutdown 5 www.national.com LM4873 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 C i at fc = 1/(2πRiCi). Input coupling capacitor which blocks the DC voltage at the amplifier’s input terminals. Also creates a highpass filter with Ri at fc = 1/(2πRiCi). 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 MTE (20 pin)Specific Characteristics LM4873MTE THD+N vs Output Power LM4873MTE THD+N vs Frequency DS100993-34 DS100993-33 LM4873MTE THD+N vs Frequency LM4873MTE Power Dissipation vs Power Output LM4873MTE(Note 20) Power Derating Curve DS100993-36 DS100993-90 DS100993-95 Note 20: These curves show the thermal dissipation ability of the LM4873MTE at different ambient temperatures given these conditions: 500LFPM + JEDEC board: The part is soldered to a 1S2P 20-lead exposed-DAP TSSOP test board with 500 linear feet per minute of forced-air flow across it. Board information - copper dimensions: 74x74mm, copper coverage: 100% (buried layer) and 12% (top/bottom layers), 16 vias under the exposed-DAP. 500LFPM + 2.5in2: The part is soldered to a 2.5in2, 1 oz. copper plane with 500 linear feet per minute of forced-air flow across it. 2.5in2: The part is soldered to a 2.5in2, 1oz. copper plane. Not Attached: The part is not soldered down and is not forced-air cooled. www.national.com 6 LM4873 Typical Performance Characteristics MTE-1 (28 pin) Specific Characteristics LM4873MTE-1 THD+N vs Output Power LM4873MTE-1 THD+N vs Frequency LM4873MTE-1 THD+N vs Output Power DS100993-97 DS100993-99 DS100993-96 LM4873MTE-1 THD+N vs Frequency LM4873MTE-1 Power Dissipation vs Power Output LM4873MTE-1(Note 21) Power Derating Curve DS100993-98 DS100993-90 DS100993-A0 Note 21: These curves show the thermal dissipation ability of the LM4835MTE at different ambient temperatures given these conditions: 500LFPM + 2in2: The part is soldered to a 2in2, 1 oz. copper plane with 500 linear feet per minute of forced-air flow across it. 2in2on bottom: The part is soldered to a 2in2, 1oz. copper plane that is on the bottom side of the PC board through 21 8 mil vias. 2in2: The part is soldered to a 2in2, 1oz. copper plane. 1in2: The part is soldered to a 1in2, 1oz. copper plane. Not Attached: The part is not soldered down and is not forced-air cooled. Non-MTE Specific Characteristics THD+N vs Frequency THD+N vs Frequency THD+N vs Frequency DS100993-3 DS100993-4 DS100993-5 7 www.national.com LM4873 Non-MTE Specific Characteristics THD+N vs Output Power (Continued) THD+N vs Output Power THD+N vs Output Power DS100993-6 DS100993-7 DS100993-8 THD+N vs Output Power THD+N vs Frequency THD+N vs Output Power DS100993-87 DS100993-89 DS100993-86 THD+N vs Frequency Output Power vs Load Resistance Power Dissipation vs Supply Voltage DS100993-88 DS100993-84 DS100993-85 Output Power vs Supply Voltage Output Power vs Supply Voltage Output Power vs Supply Voltage DS100993-9 DS100993-10 DS100993-11 www.national.com 8 LM4873 Non-MTE Specific Characteristics Output Power vs Load Resistance (Continued) Output Power vs Load Resistance Power Dissipation vs Output Power DS100993-12 DS100993-13 DS100993-14 Dropout Voltage vs Supply Voltage Power Derating Curve Power Dissipation vs Output Power DS100993-15 DS100993-16 DS100993-17 Noise Floor Channel Separation Channel Separation DS100993-18 DS100993-19 DS100993-20 9 www.national.com LM4873 Non-MTE Specific Characteristics Power Supply Rejection Ratio (Continued) Open Loop Frequency Response Supply Current vs Supply Voltage DS100993-23 DS100993-21 DS100993-22 Application Information PIN OUT COMPATIBILITY WITH THE LM4863 The LM4873 pin out was designed to simplify replacing the LM4863: except for the four bottom pins, which the implement the LM4873’s extra functionality, the LM4873MT/MTE and LM4863MT/MTE pin outs match.(Note 22) Note 22: If the LM4873 replaces an LM4863 and the input mux circuitry is not being used, the LM4873 Mux Control pin must be tied to VDD or GND. source or the feedback network, an audible click may be generated during the transition from one mux input to the other. For example, in the above example circuit, if the two gains are markedly different, then, when a transition is made between mux states, a click may be heard as the feedback network, and therefore the gain, is suddenly changed. EXPOSED-DAP MOUNTING CONSIDERATIONS The exposed-DAP package of the LM4873MTE requires special attention to thermal design. If thermal design issues are not properly addressed, an LM4873MTE driving 4Ω will go into thermal shutdown. The exposed-DAP on the bottom of the LM4873MTE should be soldered down to a copper pad on the circuit board. Heat is conducted away from the exposed-DAP by a copper plane. If the copper plane is not on the top surface of the circuit board, 8 to 10 vias of 0.013 inches or smaller in diameter should be used to thermally couple the exposed-DAP to the plane. For good thermal conduction, the vias must be plated-through and solder-filled. The copper plane used to conduct heat away from the exposed-DAP should be as large as pratical. If the plane is on the same side of the circuit board as the exposed-DAP, 2.5in2 is the minimum for 5V operation into 4Ω. If the heat sink plane is buried or not on the same side as the exposedDAP, 5in2 is the minimum for 5V operation into 4Ω. If the ambient temperature is higher than 25˚C, a larger copper plane or forced-air cooling will be required to keep the LM4873MTE junction temperature below the thermal shutdown temperature (150˚C). See the power derating curve for the LM4873MTE for derating information. The LM4873MTE requires forced-air cooling when operating into 3Ω. With the part attached to 2.5in2 of exposed copper, with a 3Ω load, and with an ambient temperature of 25˚C, 450 linear-feet per minute kept the part out of thermal shutdown. In higher ambient temperatures, higher airflow rates and/or larger copper areas will be required to keep the part out of thermal shutdown. See DEMOBOARD CIRCUIT LAYOUT for an example of an exposed-DAP TSSOP circuit board layout. 3Ω & 4Ω LAYOUT CONSIDERATIONS With low impedance loads, the output power at the loads is heavily dependent on trace resistance from the output pins of the LM4873. Traces from the output of the LM4873MTE to the load or load connectors should be as wide as practical. Any resistance in the output traces will reduce the power de10 INPUT MUX DS100993-70 FIGURE 2. Input Mux Example The has two inputs per channel. The Mux Control pin controls which input is active. As shown in the Truth Table for Logic Inputs, if the Mux Control is held low, input 1 is active. If the Mux Control is held high, input 2 is active. Figure 2 shows an example usage of the Mux Control circuit. Mux input 1 is connected to a feedback network that increases gain at low frequencies (bass boost). Mux input 2 is connected to a simple gain circuit. The example circuit has mux input 1 used to equalize the internal speaker and mux input 2 used for line-out or headphone driving. In this case, the Mux Control and HP In pins would be tied together, so that when the headphone was plugged in, the feedback network would automatically be changed. If the HP In and Mux Control pins are not connected, the example circuit be used for user-selectable bass-boost, so that independent of the HP In state, the user could select bass-boost. Since the Mux Control switches between the two inverting inputs of the amplifier, thereby changing the input signal www.national.com LM4873 Application Information (Continued) livered to the load. For example, with a 4Ω load and 0.1Ω of trace resistance in each output, output power at the load drops from 2.1W to 2.0W Output power is also dependent on supply regulation. To keep the supply voltage from sagging under full output power conditions, the supply traces should be as wide as practical. BRIDGE CONFIGURATION EXPLANATION As shown in Figure 1, the LM4873 has two pairs of 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 20 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 each channel of the IC is AVD = 2 * (Rf/R i) By driving the load differentially through outputs +OutA and −OutA or +OutB and −OutB, 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 the 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. A bridge configuration, such as the one used in LM4873, also creates a second advantage over single-ended amplifiers. Since the differential outputs, +OutA, −OutA, +OutB, and −OutB, 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. If an output coupling capacitor is not used in a single-ended configuration, the half-supply bias across the load would result in both increased internal IC power dissipation as well as permanent loudspeaker damage. POWER DISSIPATION Whether the power amplifier is bridged or single-ended, power dissipation is a major concern when designing the amplifier. Equation 1 states the maximum power dissipation point for a single-ended amplifier operating at a given supply voltage and driving a specified load. PDMAX = (VDD)2/(2π2RL): Single-Ended (1) However, a direct consequence of the increased power delivered to the load by a bridge amplifier is an increase in internal power dissipation. Equation 2 states the maximum power dissipation point for a bridge amplifier operating at the same given conditions. PDMAX = 4 * (VDD)2/(2π2RL): Bridge Mode (2) 11 Since the LM4873 is a dual channel power amplifier, the maximum internal power dissipation is 2 times that of Equation 1 or Equation 2 depending on the mode of operation. Even with this substantial increase in power dissipation, the LM4873 does not require heatsinking. The power dissipation from Equation 2, assuming a 5V power supply and an 8Ω load, must not be greater than the power dissipation that results from Equation 3: (3) PDMAX = (TJMAX − TA)/θJA For packages M16A and MTC20, θJA = 80˚C/W, and for package N16A, θJA = 63˚C/W. TJMAX = 150˚C for the LM4873. Depending on the ambient temperature, TA, of the system surroundings, Equation 3 can be used to find the maximum internal power dissipation supported by the IC packaging. If the result of Equation 2 is greater than that of Equation 3, then either the supply voltage must be decreased, the load impedance increased, or the ambient temperature reduced. For the typical application of a 5V power supply, with an 8Ω bridged load, the maximum ambient temperature possible without violating the maximum junction temperature is approximately 48˚C provided that device operation is around the maximum power dissipation point and assuming surface mount packaging. 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. 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. The effect of a larger half supply bypass capacitor is improved PSRR 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 filtering. This does not eliminate the need for bypassing the supply nodes of the LM4873. The selection of bypass capacitors, especially C B, is thus dependent upon desired PSRR requirements, click and pop performance as explained in the section, Proper Selection of External Components, system cost, and size constraints. SHUTDOWN FUNCTION In order to reduce power consumption while not in use, the LM4873 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 the supply VDD to provide maximum device performance. By switching the shutdown pin to VDD, the LM4873 supply current draw will be minimized in idle mode. While the device will be disabled with shutdown pin voltages less than VDD, the idle current may be greater than the typical value of 0.7 µA. 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 rewww.national.com LM4873 Application Information (Continued) sistor will disable the LM4873. This scheme guarantees that the shutdown pin will not float, thus preventing unwanted state changes. HP-IN FUNCTION The LM4873 possesses a headphone control pin that turns off the amplifiers which drive +OutA and +OutB so that single-ended operation can occur and a bridged connected load is muted. Quiescent current consumption is reduced when the IC is in this single-ended mode. Figure 3 shows the implementation of the LM4873’s headphone control function using a single-supply headphone amplifier. The voltage divider of R1 and R2 sets the voltage at the HP-IN pin (pin 16) to be approximately 50 mV when there are no headphones plugged into the system. This logic-low voltage at the HP-IN pin enables the LM4873 and places it in bridged mode operation. Resistor R4 limits the amount of current flowing out of the HP-IN pin when the voltage at that pin goes below ground resulting from the music coming from the headphone amplifier. The output coupling capacitors protect the headphones by blocking the amplifier’s half supply DC voltage. When there are no headphones plugged into the system and the IC is in bridged mode configuration, both loads are essentially at a 0V DC potential. Since the HP-IN threshold is set at 4V, even in an ideal situation, the output swing cannot cause a false single-ended trigger. 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-IN pin, enabling the headphone function. This disables the second side of the amplifier thus muting the bridged speakers. The amplifier then drives the headphones, whose impedance is in parallel with resistors R2 and R3. Resistors R2 and R3 have negligible effect on output drive capability since the typical impedance of headphones are 32Ω. 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. The LM4873 can be used to drive both a pair of bridged 8Ω speakers and a pair of 32Ω headphones without using the HP-IN pin. In this case the HP-IN would not be connected to the headphone jack but to a microprocessor or a switch. By enabling the HP-IN pin, the 8Ω speakers can be muted. 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 LM4873 is tolerant to a variety of external component combinations, consideration to component values must be used to maximize overall system quality. DS100993-24 FIGURE 3. Headphone Circuit The LM4873 is unity-gain stable, giving the designer maximum system performance. The LM4873 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 closed-loop 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 12 LM4873 Application Information CLICK AND POP CIRCUITRY (Continued) CB 0.01 µF 0.1 µF 0.22 µF 0.47 µF 1.0 µF TON 20 ms 200 ms 420 ms 840 ms 2 Sec The LM4873 contains circuitry to minimize turn-on transients or “clicks and pops”. In this case, turn-on refers to either power supply turn-on or the device coming out of shutdown mode. When the device is turning on, the amplifiers are internally configured as unity gain buffers. An internal current source ramps up the voltage of the bypass pin. Both the inputs and outputs ideally track the voltage at the bypass pin. The device will remain in buffer mode until the bypass pin has reached its half supply voltage, 1/2 VDD. As soon as the bypass node is stable, the device will become fully operational, where the gain is set by the external resistors. Although the bypass pin current source cannot be modified, the size of CB can be changed to alter the device turn-on time and the amount of “clicks and pops”. By increasing amount of turn-on pop can be reduced. However, the tradeoff for using a larger bypass capacitor is an increase in turn-on time for this device. There is a linear relationship between the size of CB and the turn-on time. Here are some typical turn-on times for a given CB: In order eliminate “clicks and pops”, all capacitors must be discharged before turn-on. Rapid on/off switching of the device or the shutdown function may cause the “click and pop” circuitry to not operate fully, resulting in increased “click and pop” noise. In a single-ended configuration, the output coupling capacitor, C O, is of particular concern. This capacitor discharges through the internal 20 kΩ resistors. Depending on the size of CO, the time constant can be relatively large. To reduce transients in single-ended mode, an external 1 kΩ–5 kΩ resistor can be placed in parallel with the internal 20 kΩ resistor. The tradeoff for using this resistor is an increase in quiescent current. The value of CI will also reflect turn-on pops. Clearly, a certain size for CI is needed to couple in low frequencies without excessive attenuation. But in many cases, the speakers used in portable systems, whether integral or external, have little ability to reproduce signals below 100 Hz to 150 Hz. In this case, using a large input and output capacitor may not increase system performance. In most cases, choosing a small value of CI in the range of 0.1 µF to 0.33 µF), along with CB equal to 1.0 µF should produce a virtually clickless and popless turn-on. In cases where CI is larger than 0.33 µF, it may be advantageous to increase the value of CB. Again, it should be understood that increasing the value of CB will reduce the “clicks and pops” at the expense of a longer device turn-on time. 13 www.national.com LM4873 Application Information NO-LOAD DESIGN CONSIDERATIONS (Continued) Ci ≥ 1/(2π*20 kΩ*20 Hz) = 0.397 µF; use 0.33 µF If the outputs of the LM4873 have a load higher than 10kΩ, the LM4873 may show a small oscillation at high output levels. To prevent this oscillation, place 5kΩ resistors from the power outputs to ground. AUDIO POWER AMPLIFIER DESIGN Design a 1W/8Ω Bridged Audio Amplifier Given: Power Output: Load Impedance: Input Level: Input Impedance: 1 Wrms 8Ω 1 Vrms 20 kΩ 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 LM4873 GBWP of 3.5 MHz. This figure displays that if a designer has a need to design an amplifier with a higher differential gain, the LM4873 can still be used without running into bandwidth problems. DEMOBOARD CIRCUIT LAYOUT The demoboard circuit layout is provided here as an example of a circuit using the LM4873. If an LM4873MTE is used with this layout, the exposed-DAP is soldered down to the copper pad beneath the part. Heat is conducted away from the part by the two large copper pads in the upper corners of the demoboard. This demoboard provides enough heat dissipation ability to allow an LM4873MTE to output 1.9W into 4Ω at 25˚C. Bandwidth: 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 dropout voltage. Using this method, the minimum supply voltage would be (Vopeak + (2 * Vod)), where Vod is extrapolated from the Dropout Voltage vs Supply Voltage curve in the Typical Performance Characteristics section. (4) Using the Output Power vs Supply Voltage graph for an 8Ω load, the minimum supply rail is 3.9V. But since 5V is a standard supply voltage in most applications, it is chosen for the supply rail. Extra supply voltage creates headroom that allows the LM4873 to reproduce peaks in excess of 1W without producing audible distortion. At this time, the designer must make sure that the power supply choice along with the output impedance does not violate the conditions explained in the Power Dissipation section. Once the power dissipation equations have been addressed, the required differential gain can be determined from Equation 4. DS100993-93 Silk Screen Layer (5) use AVD = 3 Since the desired input impedance was 20 kΩ, and with a AVD of 3, a ratio of 1.5:1 of Rf to Riresults in an allocation of Ri = 20 kΩ and R f = 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 pole gives 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 kHz x 5 = 100 kHz As stated in the External Components section, Ri in conjunction with Ci create a highpass filter. Rf/R i = AVD/2 From equation 4, the minimum AVD is 2.83; (6) DS100993-91 Component-side Copper Layers www.national.com 14 LM4873 Application Information (Continued) DS100993-92 Solder-side Copper Layers 15 www.national.com LM4873 Physical Dimensions inches (millimeters) unless otherwise noted 20-Lead MOLDED PKG, TSSOP, JEDEC, 4.4mm BODY WIDTH Order Number LM4873MT NS Package Number MTC20 www.national.com 16 LM4873 Physical Dimensions inches (millimeters) unless otherwise noted (Continued) 20-Lead MOLDED TSSOP, EXPOSED PAD, 6.5x4.4x0.9mm Order Number LM4873MTE NS Package Number MXA20A 17 www.national.com LM4873 Physical Dimensions inches (millimeters) unless otherwise noted (Continued) 28-Lead MOLDED TSSOP, EXPOSED PAD, 9.7x4.4x0.9mm Order Number LM4873MTE-1 NS Package Number MXA28A www.national.com 18 LM4873 Dual 2.1W Audio Amplifier Plus Stereo Headphone Function 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 Corporation Americas Tel: 1-800-272-9959 Fax: 1-800-737-7018 Email: support@nsc.com www.national.com National Semiconductor Europe Fax: +49 (0) 180-530 85 86 Email: europe.support@nsc.com 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 Response Group Tel: 65-2544466 Fax: 65-2504466 Email: ap.support@nsc.com National Semiconductor Japan Ltd. Tel: 81-3-5639-7560 Fax: 81-3-5639-7507 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.