LM4929 Stereo 40mW Low Noise Headphone Amplifier with OCL Output General Description Key Specifications The LM4929 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+N from 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 LM4929 does not require bootstrap capacitors or snubber networks, it is optimally suited for low-power portable systems. The LM4929 is configured for OCL (Output Capacitor-Less) outputs, operating with no DC blocking capacitors on the outputs. The LM4929 features a low-power consumption shutdown mode with a faster turn on time. Additionally, the LM4929 features an internal thermal shutdown protection mechanism. The LM4929 is unity gain stable and may be configured with external gain-setting resistors. j PSRR at 217Hz and 1kHz 65dB (typ) j Output Power at 1kHz with VDD = 2.4V, 1% THD+N into a 16Ω load 25mW (typ) j Output Power at 1kHz with VDD = 3V, 1% THD+N into a 16Ω load j Shutdown current 40mW (typ) 2.0µA (max) j Output Voltage change on release 1mV (max) from Shutdown VDD = 2.4V, RL = 16Ω Features n n n n n n OCL outputs — No DC Blocking Capacitors External gain-setting capability Available in space-saving MSOP package Ultra low current shutdown mode 2V - 5.5V operation Ultra low noise Applications n Portable CD players n PDAs n Portable electronics devices Block Diagram 20132441 FIGURE 1. Block Diagram Boomer ® is a registered trademark of National Semiconductor Corporation. © 2004 National Semiconductor Corporation DS201324 www.national.com LM4929 Stereo 40mW Low Noise Headphone Amplifier with OCL Output December 2004 LM4929 Typical Application 20132481 FIGURE 2. Typical OCL Output Configuration Circuit Connection Diagram MSOP Package 20132428 Top View (Note 10) Order Number LM4929MM See NS Package Number MUB10A www.national.com 2 Junction Temperature If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Thermal Resistance Supply Voltage 150˚C θJC (MSOP) 56˚C/W θJA (MSOP) 190˚C/W 6.0V Storage Temperature −65˚C to +150˚C Operating Ratings -0.3V to VDD + 0.3V Input Voltage Power Dissipation (Note 3) Internally Limited ESD Susceptibility (Note 4) 2000V ESD Susceptibility (Note 5) Temperature Range TMIN ≤ TA ≤ TMAX −40˚C ≤ T 200V A ≤ 85˚C 2V ≤ VDD ≤ 5.5V Supply Voltage Electrical Characteristics VDD = 5V (Notes 1, 2) The following specifications apply for VDD = 5V, RL = 16Ω, and CB = 4.7µF unless otherwise specified. Limits apply to TA = 25˚C. Pin 3 connected to GND. Symbol Parameter Conditions LM4929 Typ (Note 6) IDD Quiescent Power Supply Current VIN = 0V, IO = 0A ISD Shutdown Current VSHUTDOWN = GND VSDIH Shutdown Voltage Input High VSDIL Shutdown Voltage Input Low Limit (Note 7) Units (Limits) 2 5 mA (max) 0.1 2.0 µA(max) 1.8 V 0.4 V 80 mW THD = 1%; f = 1 kHZ PO Output Power RL= 16Ω VNO Output Noise Voltage BW = 20Hz to 20kHz, A-weighted 10 µV PSRR Power Supply Rejection Ratio VRIPPLE = 200mV sine p-p 65 dB RL = 32Ω 80 Electrical Characteristics VDD = 3.0V (Notes 1, 2) The following specifications apply for VDD = 3.0V, RL = 16Ω, and CB = 4.7µF unless otherwise specified. Limits apply to TA = 25˚C. Pin 3 connected to GND. Symbol Parameter Conditions LM4929 Typ (Note 6) Limit (Note 7) Units (Limits) IDD Quiescent Power Supply Current VIN = 0V, IO = 0A 1.5 3.5 mA (max) ISD Shutdown Current VSHUTDOWN = GND 0.1 2.0 µA(max) THD = 1%; f = 1kHz Output Power PO R = 16Ω 40 R = 32Ω 25 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 Electrical Characteristics VDD = 2.4V (Notes 1, 2) The following specifications apply for VDD = 2.4V, RL = 16Ω, and CB = 4.7µF unless otherwise specified. Limits apply to TA = 25˚C. Pin 3 connected to GND. Symbol Parameter Conditions LM4929 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) PO Output Power THD = 1%; f = 1kHz 3 R = 16Ω 25 R = 32Ω 12 mW www.national.com LM4929 Absolute Maximum Ratings (Note 2) LM4929 Electrical Characteristics VDD = 2.4V (Notes 1, 2) (Continued) The following specifications apply for VDD = 2.4V, RL = 16Ω, and CB = 4.7µF unless otherwise specified. Limits apply to TA = 25˚C. Pin 3 connected to GND. Symbol Parameter Conditions LM4929 Typ (Note 6) Limit (Note 7) Units (Limits) 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 OCL 0.5 s 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 LM4929, 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. Note 10: Pin 3 (NC) should be connected to GND for proper part operation. External Components Description Components (Figure 2) 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/(2πRiCi). 2. CI Input coupling capacitor which blocks the DC voltage at the amplifier’s input terminals. Also creates a high-pass 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. 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 www.national.com 4 LM4929 Typical Performance Characteristics THD+N vs Frequency THD+N vs Frequency 20132483 20132482 THD+N vs Frequency THD+N vs Frequency 20132406 20132403 THD+N vs Frequency THD+N vs Frequency 20132405 20132404 5 www.national.com LM4929 Typical Performance Characteristics (Continued) THD+N vs Output Power THD+N vs Output Power 20132492 20132493 Output Power vs Load Resistance Output Power vs Supply Voltage 20132413 20132494 Output Power vs Supply Voltage Output Power vs Load Resistance 20132495 www.national.com 20132497 6 LM4929 Typical Performance Characteristics (Continued) Power Supply Rejection Ratio Power Supply Rejection Ratio 201324A5 201324A6 Frequency Response vs Input Capacitor Size Open Loop Frequency Response 20132426 201324A7 Supply Voltage vs Supply Current Clipping Voltage vs Supply Voltage 20132424 20132425 7 www.national.com LM4929 Typical Performance Characteristics (Continued) Shutdown Hysteresis Voltage, VDD = 5V Shutdown Hysteresis Voltage, VDD = 3V 201324B1 201324B2 Power Dissipation vs Output Power VDD = 3V Power Dissipation vs Output Power VDD = 5V 20132401 20132402 THD+N vs Output Power VDD = 3V, RL = 32Ω Power Dissipation vs Output Power VDD = 2.4V 20132414 www.national.com 20132415 8 LM4929 Typical Performance Characteristics (Continued) THD+N vs Output Power VDD = 2.4V, RL = 32Ω THD+N vs Output Power VDD = 3V, RL = 16Ω 20132438 20132439 Power Derating Curve THD+N vs Output Power VDD = 2.4V, RL = 16Ω 20132440 20132429 9 www.national.com LM4929 Since the LM4929 has three operational amplifiers in one package, the maximum power dissipation increases due to the use of the third amplifier as a buffer and is given in Equation 2: Application Information AMPLIFIER CONFIGURATION EXPLANATION As shown in Figure 1, the LM4929 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 closedloop 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 The maximum power dissipation point obtained from Equation 2 must not be greater than the power dissipation that results from Equation 3: AVD = -(Rf / Ri) PDMAX = (TJMAX - TA) / θJA By driving the loads through outputs VoA and VoB with VoC acting as a buffered bias voltage the LM4929 does not require output coupling capacitors. The classical singleended amplifier configuration where one side of the load is connected to ground requires large, expensive output coupling capacitors. For package MUB10A, θJA = 190˚C/W. TJMAX = 150˚C for the LM4929. 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 TA reduced. For the typical application of a 3V power supply, with a 32Ω load, the maximum ambient temperature possible without violating the 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. PDMAX = 4(VDD) A configuration such as the one used in the LM4929 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. The LM4929 eliminates these output coupling capacitors by running in OCL mode. Unless shorted to ground, VoC is internally configured to apply a 1/2 VDD bias voltage to a stereo headphone jack’s sleeve. This voltage matches the bias voltage present on VoA and VoB outputs that drive the headphones. The headphones operate in a manner similar to a bridge-tied load (BTL). Because the same DC voltage is applied to both headphone speaker terminals this results in no net DC current flow through the speaker. AC current flows through a headphone speaker as an audio signal’s output amplitude increases on the speaker’s terminal. www.national.com / (2π2RL) (2) (3) MICRO POWER SHUTDOWN The voltage applied to the SHUTDOWN pin controls the LM4929’s shutdown function. Activate micro-power shutdown by applying a logic-low voltage to the SHUTDOWN pin. When active, the LM4929’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. 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. 2 / (2π2RL) 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 LM4929. A bypass capacitor value in the range of 0.1µF to 1µF is recommended for CS. The headphone jack’s sleeve is not connected to circuit ground when used in OCL mode. Using the headphone output jack as a line-level output will place the LM4929’s 1/2 VDD bias voltage on a plug’s sleeve connection. This presents no difficulty when the external equipment uses capacitively coupled inputs. For the very small minority of equipment that is DC coupled, the LM4929 monitors the current supplied by the amplifier that drives the headphone jack’s sleeve. If this current exceeds 500mAPK, the amplifier is shutdown, protecting the LM4929 and the external equipment. PDMAX = (VDD) 2 (1) 10 cent DC voltage. This charge comes from the output via the feedback Thus, by minimizing the capacitor size based on necessary 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. (Continued) 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. AUDIO POWER AMPLIFIER DESIGN A 25mW/32Ω AUDIO AMPLIFIER 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 LM4929’s resistance to click and pop upon entering or returning from shutdown. For a 2.4V supply and CB = 4.7µF, the LM4929 requires about 2 seconds to enter or return from shutdown. This longer shutdown time enables the LM4929 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. This technique is not recommended for OCL mode since shutdown enable/disable times are very fast (0.5s) independent of supply voltage. Given: Power Output Load Impedance Input Level 25mWrms 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 LM4929 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 LM4929 is tolerant of external component combinations, consideration to component values must be used to maximize overall system quality. (4) The LM4929 is unity-gain stable which gives the designer maximum system flexibility. The LM4929 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 Figure 2. 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. 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 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 LM4929 GBWP of 10MHz. This figure displays that is a designer has a need to design an amplifier with higher differential gain, the LM4929 can still be used without running into bandwidth limitations. Figure 3 shows an optional resistor connected between the amplifier output that drives the headphone jack sleeve and ground. This resistor provides a ground path that supressed power supply hum. Thishum may occur in applications such as notebook computers in a shutdown condition and connected to an external powered speaker. The resistor’s 100Ω value is a suggested starting point. Its final value must be 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 quies11 www.national.com LM4929 Application Information LM4929 Application Information (Continued) determined based on the tradeoff between the amount of noise suppression that may be needed and minimizing the additional current drawn by the resistor (25mA for a 100Ω resistor and a 5V supply). ESD PROTECTION As stated in the Absolute Maximum Ratings, the LM4929 has a maximum ESD susceptibility rating of 2000V. For higher ESD voltages, the addition of a PCDN042 dual transil (from California Micro Devices), as shown in Figure 3, will provide additional protection. 201324B4 FIGURE 3. The PCDN042 provides additional ESD protection beyond the 2000V shown in the Absolute Maximum Ratings for the VOC output www.national.com 12 inches (millimeters) unless otherwise noted MSOP Order Number LM4929MM NS Package Number MUB10A 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. For the most current product information visit us at www.national.com. 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. 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. BANNED SUBSTANCE COMPLIANCE National Semiconductor certifies that the products and packing materials meet the provisions of the Customer Products Stewardship Specification (CSP-9-111C2) and the Banned Substances and Materials of Interest Specification (CSP-9-111S2) and contain no ‘‘Banned Substances’’ as defined in CSP-9-111S2. National Semiconductor Americas Customer Support Center Email: [email protected] Tel: 1-800-272-9959 www.national.com National Semiconductor Europe Customer Support Center Fax: +49 (0) 180-530 85 86 Email: [email protected] Deutsch Tel: +49 (0) 69 9508 6208 English Tel: +44 (0) 870 24 0 2171 Français Tel: +33 (0) 1 41 91 8790 National Semiconductor Asia Pacific Customer Support Center Email: [email protected] National Semiconductor Japan Customer Support Center Fax: 81-3-5639-7507 Email: [email protected] Tel: 81-3-5639-7560 LM4929 Stereo 40mW Low Noise Headphone Amplifier with OCL Output Physical Dimensions