LM4924 LM4924 2-Cell Battery, 40mW Per Channel Output Capacitor-Less (OCL) StereoHeadphone Audio Amplifier Literature Number: SNAS272A 2 Cell Battery, 40mW Per Channel Output Capacitor-Less (OCL) Stereo Headphone Audio Amplifier General Description Key Specifications The LM4924 is a Output Capacitor-Less (OCL) stereo headphone amplifier, which when connected to a 3.0V supply, delivers 40mW per channel to a 16 load with less than 1% THD+N. With the LM4924 packaged in the MM and SD packages, the customer benefits include low profile and small size. These packages minimizes PCB area and maximizes output power. The LM4924 features circuitry that reduces output transients ("clicks" and "pops") during device turn-on and turn-off, and Mute On and Off. An externally controlled, low-power consumption, active-low shutdown mode is also included in the LM4924. Boomer audio power amplifiers are designed specifically to use few external components and provide high quality output power in a surface mount packages. OCL output power (RL = 16, VDD = 3.0V, THD+N = 1%) 40mW (typ) Micropower shutdown current 0.1A (typ) Supply voltage operating range 1.5V < VDD < 3.6V PSRR 100Hz, VDD = 3.0V, AV = 2.5 66dB (typ) Features 2-cell 1.5V to 3.6V battery operation OCL mode for stereo headphone operation Unity-gain stable "Click and pop" suppression circuitry for shutdown On and Off transients Active low micropower shutdown Thermal shutdown protection circuitry Applications Portable two-cell audio products Portable two-cell electronic devices Typical Application 20121057 FIGURE 1. Block Diagram Boomer(R) is a registered trademark of National Semiconductor Corporation. (c) 2009 National Semiconductor Corporation 201210 www.national.com LM4924 2 Cell Battery, 40mW Per Channel Output Capacitor-Less (OCL) Stereo Headphone Audio Amplifier June 1, 2009 LM4924 LM4924 Connection Diagrams MSOP Package MSOP Marking 20121006 Z- Plant Code X - Date Code T - Die Traceability G - Boomer Family B7 - LM4924MM 20121058 Top View Order Number LM4924MM See NS Package Number MUB10A for MSOP SD Marking SD Package 20121007 Z - Plant Code X - Date Code T - Die Traceability Bottom Line - Part Number 20121052 Top View Order Number LM4924SD See NS Package Number SDA10A www.national.com 2 LM4924 Typical Connections 20121059 FIGURE 2. Typical OCL Output Configuration Circuit 3 www.national.com LM4924 Small Outline Package Vapor Phase (60sec) 215C Infrared (15 sec) 220C See AN-450 "Surface Mounting and their Effects on Product Reliablilty" for other methods of soldering surface mount devices. Thermal Resistance 175C/W JA (typ) MUB10A Absolute Maximum Ratings (Note 1) 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 2) ESD Susceptibility(Note 3) ESD Susceptibility on pin 7, 8, and 9 (Note 3) ESD Susceptibility (Note 4) Junction Temperature Solder Information 3.8V -65C to +150C -0.3V to VDD +0.3V Internally limited 2000V JA (typ) SDA10A 73C/W Operating Ratings 2kV 200V 150C Temperature Range TMIN TA TMAX Supply Voltage Electrical Characteristics VDD = 3.0V (Notes 1, 5) Symbol Conditions -40C TA +85C 1.5V VDD 3.6V The following specifications apply for the circuit shown in Figure 2, unless otherwise specified. AV = 2.5, RL = 16.Limits apply for TA = 25C. Parameter LM4924 Typical (Note 6) Limit (Note 7) 1.9 IDD Quiescent Power Supply Current VIN = 0V, IO = 0A, RL = (Note 8) 1.5 ISD Shutdown Current VSHUTDOWN = GND 0.1 1 VOS Output Offset Voltage 1 10 PO Output Power (Note 9) 40 30 VNO Output Voltage Noise Units (Limits) mA (max) A (max) mV (max) f = 1kHz, per channel OCL (Figure 2), THD+N = 1% mW (min) 20Hz to 20kHz, A-weighted, Figure 2 13 THD PO = 10mW 0.1 0.5 VRMS % Crosstalk Freq = 1kHz 45 35 dB (min) Freq = 100Hz, OCL 66 58 dB (min) 230 VRIPPLE = 200mVP-P sine wave PSRR Power Supply Rejection Ratio TWAKE-UP Wake-Up Time 1.5V VDD 3.6V, Fig 2 VIH Control Logic High 1.5V VDD 3.6V 0.7VDD V (min) VIL Control Logic Low 1.5V VDD 3.6V 0.3VDD V (max) 70 dB Mute Attenuation 1VPP Reference, RIN = 20k, RFB = 50k Electrical Characteristics VDD = 1.8V msec 90 (Notes 1, 5) The following specifications apply for the circuit shown in Figure 2, unless otherwise specified. AV = 2.5, RL = 16. Limits apply for TA = 25C. Symbol Parameter Conditions LM4924 Typical (Note 6) Limit (Note 7) Units (Limits) IDD Quiescent Power Supply Current VIN = 0V, IO = 0A, RL = (Note 8) 1.4 mA (max) ISD Shutdown Current VSHUTDOWN = GND 0.1 A (max) VOS Output Offset Voltage 1 mV (max) 10 mW f = 1kHz PO Output Power (Note 9) OCL Per channel, Fig. 2, Freq = 1kHz THD+N = 1% VNO Output Voltage Noise 20Hz to 20kHz, A-weighted, Figure 2 10 VRMS THD PO = 5mW 0.1 % Crosstalk Freq = 1kHz 45 dB (min) www.national.com 4 Parameter Conditions LM4924 Typical (Note 6) PSRR Power Supply Rejection Ratio Limit (Note 7) Units (Limits) VRIPPLE = 200mVP-P sine wave Freq = 100Hz, OCL 66 dB Note 1: 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 2: The maximum power dissipation is dictated by TJMAX, JA, and the ambient temperature TA and must be derated at elevated temperatures. The maximum allowable power dissipation is PDMAX = (TJMAX - TA)/JA. For the LM4924, TJMAX = 150C. For the JAs, please see the Application Information section or the Absolute Maximum Ratings section. Note 3: Human body model, 100pF discharged through a 1.5k resistor. Note 4: Machine model, 220pF-240pF discharged through all pins. Note 5: All voltages are measured with respect to the ground (GND) pins unless otherwise specified. Note 6: Typicals are measured at 25C and represent the parametric norm. Note 7: Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis. Note 8: The quiescent power supply current depends on the offset voltage when a practical load is connected to the amplifier. Note 9: Output power is measured at the device terminals. 5 www.national.com LM4924 Symbol LM4924 Typical Performance Characteristics THD+N vs Frequency VDD = 1.8V, PO = 5mW, RL = 16 THD+N vs Frequency VDD = 1.8V, PO = 5mW, RL = 32 20121013 20121014 THD+N vs Frequency VDD = 3.0V, PO = 10mW, RL = 16 THD+N vs Frequency VDD = 3.0V, PO = 10mW, RL = 32 20121015 www.national.com 20121016 6 LM4924 THD+N vs Output Power VDD = 1.8V, RL = 16, f = 1kHz THD+N vs Output Power VDD = 1.8V, RL = 32, f = 1kHz 20121017 20121018 THD+N vs Output Power VDD = 3.0V, RL = 16, f = 1kHz THD+N vs Output Power VDD = 3.0V, RL = 32, f = 1kHz 20121019 20121020 7 www.national.com LM4924 Power Supply Rejection Ratio VDD = 1.8V, RL = 16, Vripple = 200mVp-p, Input Terminated into 10 load Power Supply Rejection Ratio VDD = 3.0V, RL = 16, Vripple = 200mVp-p, Input Terminated into 10 load 20121011 20121012 Noise Floor VDD = 1.8V, RL = 16 Noise Floor VDD = 3.0V, RL = 16 20121009 www.national.com 20121010 8 Output Power vs Load Resistance f = 1kHz. from top to bottom: VDD = 3.0V, 10%THD+N; VDD = 3.0V, 1%THD+N VDD = 1.8V, 10%THD+N; VDD = 1.8V, 1%THD+N 20121008 20121021 Output Power vs Supply Voltage RL = 16, from top to bottom: THD+N = 10%; THD+N = 1% Output Power vs Supply Voltage RL = 32, from top to bottom: THD+N = 10%; THD+N = 1% 20121031 20121022 9 www.national.com LM4924 Channel Sepration RL = 16 LM4924 Power Dissipation vs Output Power VDD = 1.8V, f = 1kHz, from top to bottom: RL = 16; RL = 32 Power Dissipation vs Output Power VDD = 3.0V, f = 1kHz, from top to bottom: RL = 16; RL = 32 20121024 20121025 Supply Current vs Supply Voltage 20121026 www.national.com 10 turn-on time. Here are some typical turn-on times for various values of CBYPASS. ELIMINATING OUTPUT COUPLING CAPACITORS Typical single-supply audio amplifiers that drive single-ended (SE) headphones use a coupling capacitor on each SE output. This output coupling capacitor blocks the half-supply voltage to which the output amplifiers are typically biased and couples the audio signal to the headphones. The signal return to circuit ground is through the headphone jack's sleeve. The LM4924 eliminates these output coupling capacitors. VoC is internally configured to apply a 1/2VDD bias voltage to a stereo headphone jack's sleeve. This voltage matches the quiescent voltage present on the VoA and VoB outputs that drive the headphones. The headphones operate in a manner similar to a bridge-tied-load (BTL). 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. The headphone jack's sleeve is not connected to circuit ground. Using the headphone output jack as a line-level output will place the LM4924's bandgap 1/2VDD bias 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 LM4924 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 LM4924 and the external equipment. AMPLIFIER CONFIGURATION EXPLANATION As shown in Figure 1, the LM4924 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 BYPASS CAPACITOR VALUE SELECTION Besides minimizing the input capacitor size, careful consideration should be paid to value of CBYPASS, the capacitor connected to the BYPASS pin. Since CBYPASS determines how fast the LM4924 settles to quiescent operation, its value is critical when minimizing turn-on pops. The slower the LM4924's outputs ramp to their quiescent DC voltage (nominally VDD/2), the smaller the turn-on pop. Choosing CB equal to 4.7F along with a small value of Ci (in the range of 0.1F to 0.47F), produces a click-less and pop-less shutdown function. As discussed above, choosing Ci no larger than necessary for the desired bandwidth helps minimize clicks and pops. This ensures that output transients are eliminated when power is first applied or the LM4924 resumes operation after shutdown. POWER DISSIPATION Power dissipation is a major concern when designing a successful amplifier. A direct consequence of the increased power delivered to the load by a bridge amplifier is an increase in internal power dissipation. The maximum power dissipation for a given application can be derived from the power dissipation graphs or from Equation 1. AV = -(Rf/Ri) By driving the loads through outputs VO1 and VO2 with VO3 acting as a buffered bias voltage the LM4924 does not require output coupling capacitors. The typical 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 LM4924 has a major advantage over single supply, single-ended amplifiers. Since the outputs VO1, VO2, and VO3 are all biased at 1/2 VDD, no net DC voltage exists across each load. This eliminates the need for output coupling capacitors that 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. PDMAX = 4(VDD) 2 / (2RL) (1) It is critical that the maximum junction temperature TJMAX of 150C is not exceeded. Since the typical application is for headphone operation (16 impedance) using a 3.3V supply the maximum power dissipation is only 138mW. Therefore, power dissipation is not a major concern. OPTIMIZING CLICK AND POP REDUCTION PERFORMANCE The LM4924 contains circuitry that eliminates turn-on and shutdown transients ("clicks and pops"). For this discussion, turn-on refers to either applying the power supply voltage or when the micro-power shutdown mode is deactivated. As the VDD/2 voltage present at the BYPASS pin ramps to its final value, the LM4924's internal amplifiers are configured as unity gain buffers. An internal current source charges the capacitor connected between the BYPASS pin and GND in a controlled, linear manner. Ideally, the input and outputs track the voltage applied to the BYPASS pin. The gain of the internal amplifiers remains unity until the voltage on the bypass pin reaches VDD/2. As soon as the voltage on the bypass pin is stable, the device becomes fully operational and the amplifier outputs are reconnected to their respective output pins. Although the BYPASS pin current cannot be modified, changing the size of CBYPASS alters the device's turn-on time. There is a linear relationship between the size of CBYPASS and the 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 3.0V regulator with 10F 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 LM4924. A bypass capacitor value in the range of 0.1F to 1F is recommended for CS. MICRO POWER SHUTDOWN The voltage applied to the SHUTDOWN pin controls the LM4924's shutdown function. Activate micro-power shutdown by applying a logic-low voltage to the SHUTDOWN pin. When active, the LM4924's micro-power shutdown feature turns off 11 www.national.com LM4924 Application Information LM4924 the amplifier's bias circuitry, reducing the supply current. The trigger point is 0.4V (max) for a logic-low level, and 1.5V (min) for a logic-high level. The low 0.1A (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 100k 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. ential output to eliminate the output coupling capacitors. The result is a headphone jack sleeve that is connected to VO3 instead of GND. For powered speakers that are designed to have single-ended signals at the input, the click and pop circuitry will not be able to eliminate the turn-on/turn-off click and pop. Unless the inputs to the powered speakers are fully differential the turn-on/turn-off click and pop will be very large. AUDIO POWER AMPLIFIER DESIGN A 30mW/32 Audio Amplifier Given: Power Output 32 Input Level 1Vrms Input Impedance 20k 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. Since 3.3V is a standard supply voltage in most applications, it is chosen for the supply rail in this example. Extra supply voltage creates headroom that allows the LM4924 to reproduce peaks in excess of 30mW without producing audible distortion. At this time, the designer must make sure that the power supply choice along with the output impedance does no 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 2. SELECTING EXTERNAL COMPONENTS Selecting proper external components in applications using integrated power amplifiers is critical to optimize device and system performance. While the LM4924 is tolerant of external component combinations, consideration to component values must be used to maximize overall system quality. The LM4924 is unity-gain stable which gives the designer maximum system flexibility. The LM4924 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 1M. 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. 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. (2) From Equation 2, the minimum AV is 0.98; use AV = 1. Since the desired input impedance is 20k, and with AV equal to 1, a ratio of 1:1 results from Equation 1 for Rf to Ri. The values are chosen with Ri = 20k and Rf = 20k. SELECTION OF INPUT CAPACITOR SIZE Amplifiying 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 necessary low frequency response, turn-on time can be minimized. A small value of Ci (in the range of 0.1F to 0.39F), is recommended. The last step in this design example is setting the amplifier's -3dB 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 gain variation for both response limits is 0.17dB, well within the 0.25dB desired limit. The results are an fL = 100Hz/5 = 20Hz (3) fH = 20kHz x 5 = 100kHz (4) and an As mentioned in the Selecting Proper 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 (3). USING EXTERNAL POWERED SPEAKERS The LM4924 is designed specifically for headphone operation. Often the headphone output of a device will be used to drive external powered speakers. The LM4924 has a differ- www.national.com 30mWrms Load Impedance Ci 1/(2R ifL) 12 (5) 1/(2*20k*20Hz) = 0.397F Use a 0.39F capacitor, the closest standard value. The high frequency pole is determined by the product of the desired frequency pole, fH, and the differential gain, AV. With HIGHER GAIN AUDIO AMPLIFIER 20121029 FIGURE 3. 13 www.national.com LM4924 an AV = 1 and fH = 100kHz, the resulting GBWP = 100kHz which is much smaller than the LM4924 GBWP of 11MHz. This figure displays that if a designer has a need to design an amplifier with higher differential gain, the LM4924 can still be used without running into bandwidth limitations. The result is LM4924 The LM4924 is unity-gain stable and requires no external components besides gain-setting resistors, input coupling capacitors, and proper supply bypassing in the typical application. However, if a very large closed-loop differential gain is required, a feedback capacitor (Cf) may be needed to bandwidth limit the amplifier. This feedback capacitor creates a low pass filter that eliminates possible high frequency oscillations. Care should be taken when calculating the -3dB frequency in that an incorrect combination of Rf and Cf will cause frequency response roll off before 20kHz. A typical combination of feedback resistor and capacitor that will not produce audio band high frequency roll off is Rf = 20k and Cf = 25pF. These components result in a -3dB point of approximately 320kHz. REFERENCE DESIGN BOARD and LAYOUT GUIDELINES MSOP & SD BOARDS 20121030 FIGURE 4. (Note: RPU2 is not required. It is used for test measurement purposes only.) www.national.com 14 Single-Point Power / Ground Connections The analog power traces should be connected to the digital traces through a single point (link). A "PI-filter" can be helpful in minimizing high frequency noise coupling between the analog and digital sections. Further, place digital and analog power traces over the corresponding digital and analog ground traces to minimize noise coupling. Minimization of THD PCB trace impedance on the power, ground, and all output traces should be minimized to achieve optimal THD performance. Therefore, use PCB traces that are as wide as possible for these connections. As the gain of the amplifier is increased, the trace impedance will have an ever increasing adverse affect on THD performance. At unity-gain (0dB) the parasitic trace impedance effect on THD performance is reduced but still a negative factor in the THD performance of the LM4924 in a given application. Placement of Digital and Analog Components All digital components and high-speed digital signal traces should be located as far away as possible from analog components and circuit traces. GENERAL MIXED SIGNAL LAYOUT RECOMMENDATION Avoiding Typical Design / Layout Problems Avoid ground loops or running digital and analog traces parallel to each other (side-by-side) on the same PCB layer. When traces must cross over each other do it at 90 degrees. Running digital and analog traces at 90 degrees to each other from the top to the bottom side as much as possible will minimize capacitive noise coupling and cross talk. Power and Ground Circuits For two layer mixed signal design, it is important to isolate the digital power and ground trace paths from the analog power and ground trace paths. Star trace routing techniques (bringing individual traces back to a central point rather than daisy chaining traces together in a serial manner) can greatly enhance low level signal performance. Star trace routing refers 15 www.national.com LM4924 to using individual traces to feed power and ground to each circuit or even device. This technique will require a greater amount of design time but will not increase the final price of the board. The only extra parts required may be some jumpers. PCB LAYOUT GUIDELINES This section provides practical guidelines for mixed signal PCB layout that involves various digital/analog power and ground traces. Designers should note that these are only "rule-of-thumb" recommendations and the actual results will depend heavily on the final layout. LM4924 Physical Dimensions inches (millimeters) unless otherwise noted MSOP Package Order Number LM4924MM NS Package Number MUB10A SD Package Order Number LM4924SD NS Package Number SDA10A www.national.com 16 LM4924 Notes 17 www.national.com LM4924 2 Cell Battery, 40mW Per Channel Output Capacitor-Less (OCL) Stereo Headphone Audio 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(R) Tools www.national.com/webench Audio www.national.com/audio App Notes www.national.com/appnotes Clock and Timing www.national.com/timing Reference Designs www.national.com/refdesigns Data Converters www.national.com/adc Samples www.national.com/samples Interface www.national.com/interface Eval Boards www.national.com/evalboards LVDS www.national.com/lvds Packaging www.national.com/packaging Power Management www.national.com/power Green Compliance www.national.com/quality/green Switching Regulators www.national.com/switchers Distributors www.national.com/contacts LDOs www.national.com/ldo Quality and Reliability www.national.com/quality LED Lighting www.national.com/led Feedback/Support www.national.com/feedback Voltage Reference www.national.com/vref Design Made Easy www.national.com/easy www.national.com/powerwise Solutions www.national.com/solutions Mil/Aero www.national.com/milaero PowerWise(R) Solutions Serial Digital Interface (SDI) www.national.com/sdi Temperature Sensors www.national.com/tempsensors SolarMagicTM www.national.com/solarmagic Wireless (PLL/VCO) www.national.com/wireless www.national.com/training PowerWise(R) Design University THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION ("NATIONAL") PRODUCTS. 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