LM4912MM/NOPB

LM4912 Stereo 40mW Low Noise Headphone Amplifier General Description Key Specifications The LM4912 is an stereo audio power amplifier capable of delivering 40mW per channel of continuous average power into a 16Ω load or 25mW per channel into a 32Ω load at 1% THD form a 3V power supply. Boomer audio power amplifiers were designed specifically to provide high quality output power with a minimal amount of external components. Since the LM4912 does not require bootstrap capacitors or snubber networks, it is optimally suited for low-power portable systems. The LM4912 features a low-power consumption shutdown mode and a power mute mode that allows for faster turn on time with less than 1mV voltage change at outputs on release. Additionally, the LM4912 features an internal thermal shutdown protection mechanism. The LM4912 is unity gain stable and may be configured with external gain-setting resistors. 65dB (typ) ■ PSRR at 217 Hz and 1kHz ■ Output Power at 1kHz with VDD = 2.4V, 1% THD 25mW +N into a 16Ω load (typ) 40mW +N into a 16Ω load (typ) 1.0µA (max) ■ Shutdown Current ■ Output Voltage change on release from Shutdown 1mV VDD = 2.4V, RL = 16Ω (max) 10µV (typ) ■ Output Noise, 20Hz to 20kHz, A-weighted ■ Output Power at 1kHz with VDD = 3V, 1% THD Features ■ ■ ■ ■ External gain-setting capability Available in space-saving MSOP package Ultra low current shutdown mode Mute mode allows fast turn-on (10ms) with less than 1mV change on outputs ■ 2.0V - 5.5V operation ■ Ultra low noise ■ Operation at low supply voltages Applications ■ Portable CD players ■ PDAs ■ Portable electronics devices Typical Application 20048180 FIGURE 1. Typical Capacitive Coupled Output Configuration Circuit Boomer® is a registered trademark of National Semiconductor Corporation. © 2008 National Semiconductor Corporation 200481 www.national.com LM4912 Stereo 40mW Low Noise Headphone Amplifier June 17, 2008 LM4912 Connection Diagrams MSOP Package 20048179 Top View Order Number LM4912MM See NS Package Number MUB10A MSOP Package 200481a9 Top View G-Boomer Family A3 - LM4912MM www.national.com 2 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) 6.0V −65°C to +150°C -0.3V to VDD + 0.3V Internally Limited 2000V 250V Electrical Characteristics VDD = 5.0V 150°C  θJC (MSOP) 56°C/W  θJA (MSOP) 190°C/W Operating Ratings Temperature Range TMIN ≤ TA ≤ TMAX −40°C ≤ T A ≤ 85°C 2.0V ≤ VDD ≤ 5.5V Supply Voltage (VDD) (Notes 1, 2) The following specifications apply for VDD = 5.0V, RL = 16Ω, CO = 100µ, and CB = 4.7µF unless otherwise specified. Limits apply to TA = 25°C. Symbol Parameter Conditions LM4912 Units (Limits) Typ (Note 6) Limit (Note 7) 2 5 0.1 2.0 µA (max) 2 5 mA (max) IDD Quiescent Power Supply Current VIN = 0V, IO = 0A mA (max) ISD Shutdown Current VSHUTDOWN = GND IM Mute Current VMUTE = VDD VSDIH Shutdown Voltage Input High 1.8 V VSDIL Shutdown Voltage Input Low 0.4 V VMIH Mute Voltage Input High 1.8 V VMIL Mute Voltage Input Low 0.4 V PO Output Power R = 16Ω 145 mW R = 32Ω 80 VNO Output Noise Voltage BW = 20 Hz to 20kHz, A-weighted 10 µV PSRR Power Supply Rejection Ratio VRIPPLE = 200mV sine p-p 65 dB AM Mute Attenuation f = 1kHz 85 dB THD = 1%; f = 1kHz Electrical Characteristics VDD = 3.0V (Notes 1, 2) The following specifications apply for VDD = 3.0V, RL = 16Ω, CO = 100µF, and CB = 4.7µF unless otherwise specified. Limits apply to TA = 25°C. Symbol Parameter Conditions LM4912 Typ (Note 6) Limit (Note 7) Units (Limits) IDD Quiescent Power Supply Current VIN = 0V, IO = 0A 1.5 3 mA (max) ISD Shutdown Current VSHUTDOWN = GND 0.1 2.0 µA (max) IM Mute Current VMUTE = VDD 1.5 3 mA (max) PO Output Power VNO Output Noise Voltage BW = 20 Hz to 20kHz, A-weighted 10 µV PSRR Power Supply Rejection Ratio VRIPPLE = 200mV sine p-p 65 dB AM Mute Attenuation f = 1kHz 80 dB THD = 1%; f = 1kHz 3 R = 16Ω 40 R = 32Ω 25 mW www.national.com LM4912 Junction Temperature Thermal Resistance Absolute Maximum Ratings (Note 2) LM4912 Electrical Characteristics VDD = 2.4V (Notes 1, 2) The following specifications apply for VDD = 2.4V, RL = 16Ω, CO = 100µF, and CB = 4.7µF unless otherwise specified. Limits apply to TA = 25°C. Symbol Parameter Conditions LM4912 Typ (Note 6) Limit (Note 7) Units (Limits) IDD Quiescent Power Supply Current VIN = 0V, IO = 0A 1.5 3 mA (max) ISD Shutdown Current VSHUTDOWN = GND 0.1 2.0 µA (max) IM Mute Current VMUTE = VDD 1.5 3 mA (max) THD = 1%; f = 1kHz PO Output Power R = 16Ω 25 R = 32Ω 12 mW VNO Output Noise Voltage BW = 20 Hz to 20kHz, A-weighted 10 µV PSRR Power Supply Rejection Ratio VRIPPLE = 200mV sine p-p 65 dB TWU Wake Up Time from Shutdown 2 s VOSD Output Voltage Change on Release from Shutdown TUM Time to Un-Mute AM Mute Attenuation 0.01 f = 1kHz 80 1 mV (max) 0.02 s (max) db Note 1: All voltages are measured with respect to the GND 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 Absolute Maximum Ratings, whichever is lower. For the LM4912, see power derating currents for more information. Note 4: Human body model, 100pF discharged through a 1.5kΩ resistor. Note 5: Machine Model, 220pF-240pF 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). Note 8: Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis. Note 9: 10Ω Terminated input. External Components Description See (Figure 1) Components Functional Description 1. RI 2. CI Input coupling capacitor which blocks the DC voltage at the amplifier's input terminals. Also creates a high-pass 3. Rf Feedback resistance which sets the closed-loop gain in conjunction with Ri. 4. CS 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. 5. CB Bypass pin capacitor which provides half-supply filtering. Refer to the section, Proper Selection of Proper Components, for information concerning proper placement and selection of CB 6. Co Output coupling capacitor which blocks the DC voltage at the amplifier's output. Forms a high pass filter with RL at www.national.com 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/(2πRiCi). 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. fo = 1/(2πRLCo) 4 LM4912 Typical Performance Characteristics THD+N vs Frequency THD+N vs Frequency 20048184 20048185 THD+N vs Frequency THD+N vs Frequency 20048186 20048187 THD+N vs Frequency THD+N vs Frequency 20048188 20048189 5 www.national.com LM4912 THD+N vs Output Power THD+N vs Output Power 20048190 20048191 THD+N vs Output Power THD+N vs Output Power 20048108 20048107 THD+N vs Output Power THD+N vs Output Power 20048110 www.national.com 20048109 6 LM4912 Output Resistance vs Load Resistance Output Power vs Supply Voltage 20048113 20048112 Output Power vs Supply Voltage Output Power vs Load Resistance 20048196 20048111 Power Dissipation vs. Output Power Power Dissipation vs. Output Power 20048198 20048199 7 www.national.com LM4912 Power Dissipation vs Output Power Channel Separation 200481a0 200481a1 Channel Separation Channel Separation 200481a2 20048117 Channel Separation Channel Separation 20048119 www.national.com 20048118 8 LM4912 Channel Separation Mute Attenuation 200481b4 20048116 Power Supply Rejection Ratio Power Supply Rejection Ratio 200481a3 200481a4 Power Supply Rejection Ratio Power Supply Rejection Ratio 20048120 20048123 9 www.national.com LM4912 Power Supply Rejection Ratio Power Supply Rejection Ratio 20048121 20048122 Frequency Response vs Input Capacitor Size Open Loop Frequency Response 200481a7 20048127 Supply Voltage vs Supply Current Clipping Voltage vs Supply Voltage 20048124 www.national.com 20048125 10 Shutdown Hysteresis Voltage, Vdd=5V 200481b1 200481a8 Shutdown Hysteresis Voltage, Vdd=3V Power Derating Curve 200481b3 200481b2 11 www.national.com LM4912 Noise Floor LM4912 maximum junction temperature is approximately 144°C provided that device operation is around the maximum power dissipation point. Thus, for typical applications, power dissipation is not an issue. Power dissipation is a function of output power and thus, if typical operation is not around the maximum power dissipation point, the ambient temperature may be increased accordingly. Refer to the Typical Performance Characteristics curves for power dissipation information for lower output powers. Application Information AMPLIFIER CONFIGURATION EXPLANATION As shown in Figure 1, the LM4912 has three operational amplifiers internally. Two of the amplifier's have externally configurable gain while the other amplifier is internally fixed at the bias point acting as a unity-gain buffer. The closed-loop gain of the two configurable amplifiers is set by selecting the ratio of Rf to Ri. Consequently, the gain for each channel of the IC is POWER SUPPLY BYPASSING As with any amplifier, proper supply bypassing is important for low noise performance and high power supply rejection. The capacitor location on the power supply pins should be as close to the device as possible. Typical applications employ a 3V regulator with 10mF tantalum or electrolytic capacitor and a ceramic bypass capacitor which aid in supply stability. This does not eliminate the need for bypassing the supply nodes of the LM4912. A bypass capacitor value in the range of 0.1µF to 1µF is recommended for CS. AVD = -(Rf / Ri) By driving the loads through outputs VoA and VoB with VoC acting as a buffered bias voltage the LM4912 does not require output coupling capacitors. The classical single-ended amplifier configuration where one side of the load is connected to ground requires large, expensive output coupling capacitors. A configuration, such as the one used in the LM4912, has a major advantage over single supply, single-ended amplifiers. Since the outputs VoA, VoB, and VoC are all biased at 1/2 VDD, no net DC voltage exists across each load. This eliminates the need for output coupling capacitors which are required in a single-supply, single-ended amplifier configuration. Without output coupling capacitors in a typical singlesupply, single-ended amplifier, the bias voltage is placed across the load resulting in both increased internal IC power dissipation and possible loudspeaker damage. MICRO POWER SHUTDOWN The voltage applied to the SHUTDOWN pin controls the LM4912's shutdown function. Activate micro-power shutdown by applying a logic-low voltage to the SHUTDOWN pin. When active, the LM4912's micro-power shutdown feature turns off the amplifier's bias circuitry, reducing the supply current. The trigger point varies depending on supply voltage and is shown in the Shutdown Hysteresis Voltage graphs in the Typical Performance Characteristics section. The low 0.1µA(typ) shutdown current is achieved by applying a voltage that is as near as ground as possible to the SHUTDOWN pin. A voltage that is higher than ground may increase the shutdown current. 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 100kΩ pull-up resistor between the SHUTDOWN pin and VDD. Connect the switch between the SHUTDOWN pin and ground. Select normal amplifier operation by opening the switch. Closing the switch connects the SHUTDOWN pin to ground, 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 microcontroller, use a digital output to apply the control voltage to the SHUTDOWN pin. Driving the SHUTDOWN pin with active circuitry eliminates the pull-up resistor. Shutdown enable/disable times are controlled by a combination of CB and VDD. Larger values of CB results in longer turn on/off times from Shutdown. Smaller Vdd values also increase turn on/off time for a given value of CB. Longer shutdown times also improve the LM4912's resistance to click and pop upon entering or returning from shutdown. For a 2.4V supply and CB = 4.7µF, the LM4912 requires about 2 seconds to enter or return from shutdown. This longer shutdown time enables the LM4912 to have virtually zero pop and click transients upon entering or release from shutdown. Smaller values of CB will decrease turn-on time, but at the cost of increased pop and click and reduced PSRR. Since shutdown enable/disable times increase dramatically as supply voltage gets below 2.2V, this reduced turn-on time may be desirable if extreme low supply voltage levels are used as this would offset increases in turn-on time caused by the lower supply voltage. POWER DISSIPATION Power dissipation is a major concern when using any power amplifier and must be thoroughly understood to ensure a successful design. When operating in capacitor-coupled mode, Equation 1 states the maximum power dissipation point for a single-ended amplifier operating at a given supply voltage and driving a specified output load. PDMAX = (VDD) 2 / (2π2RL) (1) Since the LM4912 has two operational amplifiers in one package, the maximum internal power dissipation point is twice that of the number which results from Equation 1. From Equation 1, assuming a 3V power supply and a 32Ω load, the maximum power dissipation point is 14mW per amplifier. Thus the maximum package dissipation point is 28mW. The maximum power dissipation point obtained from Equation 1 must not be greater than the power dissipation that results from Equation 2: PDMAX = (TJMAX - TA) / θJA (2) For package MUB10A, θJA = 190°C/W. TJMAX = 150°C for the LM4912. 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, the load impedance increased or TA reduced. For the typical application of a 3V power supply, with an 32Ω load, the maximum ambient temperature possible without violating the www.national.com 12 AUDIO POWER AMPLIFIER DESIGN A 25mW/32Ω Audio Amplifier Given: Power Output 25mWrms Load Impedance Input Level 32Ω 1Vrms Input Impedance 20kΩ 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. 3V is a standard voltage in most applications, it is chosen for the supply rail. Extra supply voltage creates headroom that allows the LM4912 to reproduce peak in excess of 25mW 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 gain can be determined from Equation 2. 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 LM4912 is tolerant of external component combinations, consideration to component values must be used to maximize overall system quality. The LM4912 is unity-gain stable which gives the designer maximum system flexibility. The LM4912 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 1Vrms are available from sources such as audio codecs. Very large values should not be used for the gain-setting resistors. Values for Ri and Rf should be less than 1MΩ. Please refer to the section, Audio Power Amplifier Design, for a more complete explanation of proper gain selection Besides gain, one of the major considerations is the closedloop bandwidth of the amplifier. To a large extent, the bandwidth is dictated by the choice of external components shown in Figures 2 and 3. 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 and turn-on time. (3) From Equation 4, the minimum AV is 0.89; use AV = 1. Since the desired input impedance is 20kΩ, and with a AV gain of 1, a ratio of 1:1 results from Equation 1 for Rf to Ri. The values are chosen with Ri = 20kΩ and Rf = 20kΩ. The final design step is to address the bandwidth requirements which must be stated as a pair of -3dB frequency points. Five times away from a -3dB point is 0.17dB down from passband response which is better than the required ± 0.25dB specified. fL = 100Hz/5 = 20Hz fH = 20kHz * 5 = 100kHz SELECTION OF INPUT CAPACITOR SIZE Amplifying the lowest audio frequencies requires a high value input coupling capacitor, Ci. A high value capacitor can be expensive and may compromise space efficiency in portable designs. In many cases, however, the headphones used in portable systems have little ability to reproduce signals below 60Hz. Applications using headphones with this limited frequency response reap little improvement by using a high value input capacitor. In addition to system cost and size, turn on time is affected by the size of the input coupling capacitor Ci. A larger input coupling capacitor requires more charge to reach its quiescent DC voltage. This charge comes from the output via the feedback Thus, by minimizing the capacitor size based on nec- As stated in the External Components section, Ri in conjunction with Ci creates a Ci ≥ 1 / (2π * 20kΩ * 20Hz) = 0.397µF; use 0.39µF. The high frequency pole is determined by the product of the desired frequency pole, fH, and the differential gain, AV. With an AV = 1 and fH = 100kHz, the resulting GBWP = 100kHz which is much smaller than the LM4912 GBWP of 10MHz. This figure displays that is a designer has a need to design an amplifier with higher differential gain, the LM4912 can still be used without running into bandwidth limitations. Revision History Rev Date 1.0 7/15/05 Fixed spelling typos. Description 1.01 06/16/08 Fixed a typo. 13 www.national.com LM4912 essary low frequency response, turn-on time can be minimized. A small value of Ci (in the range of 0.1µF to 0.39µF), is recommended. MUTE When in C-CUPL mode, the LM4912 also features a mute function that is independent of load impedance and enables extremely fast turn-on/turn-off with a minimum of output pop and click. The mute function leaves the outputs at their bias level, thus resulting in higher power consumption than shutdown mode, but also provides much faster turn on/off times. Mute mode is enabled by providing a logic high signal on the MUTE pin in the opposite manner as the shutdown function described above. Threshold voltages and activation techniques match those given for the shutdown function as well. Additionally, Mute should not be enabled during shutdown or while entering or returning from shutdown. This is not a valid operation condition and may result in much higher pop and click values. LM4912 Physical Dimensions inches (millimeters) unless otherwise noted MSOP Order Number LM4912MM NS Package Number MUB10A www.national.com 14 LM4912 Notes 15 www.national.com LM4912 Stereo 40mW Low Noise Headphone Amplifier Notes For more National Semiconductor product information and proven design tools, visit the following Web sites at: Products Design Support Amplifiers www.national.com/amplifiers WEBENCH www.national.com/webench Audio www.national.com/audio Analog University www.national.com/AU Clock Conditioners www.national.com/timing App Notes www.national.com/appnotes Data Converters www.national.com/adc Distributors www.national.com/contacts Displays www.national.com/displays Green Compliance www.national.com/quality/green Ethernet www.national.com/ethernet Packaging www.national.com/packaging Interface www.national.com/interface Quality and Reliability www.national.com/quality LVDS www.national.com/lvds Reference Designs www.national.com/refdesigns Power Management www.national.com/power Feedback www.national.com/feedback Switching Regulators www.national.com/switchers LDOs www.national.com/ldo LED Lighting www.national.com/led PowerWise www.national.com/powerwise Serial Digital Interface (SDI) www.national.com/sdi Temperature Sensors www.national.com/tempsensors Wireless (PLL/VCO) www.national.com/wireless THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. 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