LM4926 Ground-Referenced, Ultra Low Noise, Fixed Gain, 80mW Stereo Headphone Amplifier General Description Key Specifications The LM4926 is a ground referenced, fixed-gain audio power amplifier capable of delivering 80mW of continuous average power into a 16Ω single-ended load with less than 1% THD+N from a 3V power supply. The LM4926 features a new circuit technology that utilizes a charge pump to generate a negative reference voltage. This allows the outputs to be biased about ground, thereby eliminating output-coupling capacitors typically used with normal single-ended loads. The LM4926 features an Automatic Standby Mode circuitry (patent pending). In the absence of an input signal, after approximately 12 seconds, the LM4926 goes into low current standby mode. The LM4926 recovers into full power operating mode immediately after a signal is applied to either the left or right input pins. This feature saves power supply current in battery operated applications. Boomer audio power amplifiers were designed specifically to provide high quality output power with a minimal amount of external components. The LM4926 does not require output coupling capacitors or bootstrap capacitors, and therefore is ideally suited for mobile phone and other low voltage applications where minimal power consumption is a primary requirement. The LM4926 features a low-power consumption shutdown mode selectable for either channel separately. This is accomplished by driving either the SD_RC (Shutdown Right Channel) or SD_LC (Shutdown Left Channel) (or both) pins with logic low, depending on which channel is desired shutdown. Additionally, the LM4926 features an internal thermal shutdown protection mechanism. The LM4926 contains advanced pop & click circuitry that eliminates noises which would otherwise occur during turn-on and turn-off transitions. The LM4926 has an internal fixed gain of 1.5V/V. j Improved PSRR at 217Hz 70dB (typ) j Power Output at VDD = 3V, RL = 16Ω, THD % 1% 80mW (typ) j Shutdown Current 0.01µA (typ) j Internal Fixed Gain 1.5V/V (typ) j Operating Voltage 1.6V to 4.2V Features Ground referenced outputs High PSRR Available in space-saving micro SMD package Ultra low current shutdown mode Improved pop & click circuitry eliminates noises during turn-on and turn-off transitions n No output coupling capacitors, snubber networks, bootstrap capacitors, or gain-setting resistors required n Shutdown either channel independently n n n n n Applications n n n n n Notebook PCs Mobile Phone PDAs Portable electronic devices MP3 Players Boomer ® is a registered trademark of National Semiconductor Corporation. © 2005 National Semiconductor Corporation DS201161 www.national.com LM4926 Ground-Referenced, Ultra Low Noise, Fixed Gain, 80mW Stereo Headphone Amplifier June 2005 LM4926 Typical Application 201161B8 FIGURE 1. Typical Audio Amplifier Application Circuit www.national.com 2 LM4926 Connection Diagrams microSMD Package 16 – Bump TL Marking 20116178 Top View XY – Date Code TT – Lot Traceability G – Boomer Family E8 – LM4926TL 20116109 Top View Order Number LM4926TL See NS Package Number TLE1411A Pin Descriptions Pin Name Function A1 R_IN Right Channel Input A2 SGND Signal Ground A3 CPVDD Charge Pump Power Supply A4 CCP+ Positive Terminal - Charge Pump Flying Capacitor B1 SD_RC Active-Low Shutdown, Right Channel B2 SD_LC Active-Low Shutdown, Left Channel B4 PGND Power Ground C1 L_IN Left Channel Input C2 R_OUT Right Channel Input C4 CCP- Negative Terminal - Charge Pump Flying Capacitor D1 +AVDD Positive Power Supply - Amplifier D2 L_OUT Left Channel Output D3 -AVDD Negative Power Supply - Amplifier D4 VCP_OUT Charge Pump Power Output 3 www.national.com LM4926 Absolute Maximum Ratings (Note 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 θJA (typ) TLE1411A (Note 11) 4.5V Storage Temperature Temperature Range -0.3V to VDD + 0.3V Power Dissipation (Note 3) Internally Limited ESD Susceptibility (Note 4) 2000V ESD Susceptibility (Note 5) 200V 86˚C/W Operating Ratings −65˚C to +150˚C Input Voltage 150˚C TMIN ≤ TA ≤ TMAX −40˚C ≤ TA ≤ 85˚C Supply Voltage (VDD) 1.6V ≤ VDD ≤ 4.2V Electrical Characteristics VDD = 3V (Note 1) The following specifications apply for VDD = 3V and 16Ω load unless otherwise specified. Limits apply to TA = 25˚C. Symbol Parameter Conditions LM4926 Typ (Note 6) Quiescent Power Supply Current VIN = 0V, inputs terminated Auto Standby Mode both channels enabled IDD VIN = 0V, inputs terminated Quiescent Power Supply Current both channels enabled Full Power Mode VIN = 0V, inputs terminated one channel enabled Limit (Notes 7, 8) 2.3 7 Units (Limits) mA 10 5 mA (max) mA ISD Shutdown Current VSD_LC = VSD_RC = GND 0.1 1.8 µA (max) VOS Output Offset Voltage RL = 32Ω, VIN = 0V 0.7 5 mV (max) AV Voltage Gain ∆AV Gain Match RIN PO THD+N –1.5 Input Resistance Output Power Total Harmonic Distortion + Noise V/V 1 20 % 15 25 kΩ (min) kΩ (max) THD+N = 1% (max); f = 1kHz, RL = 16Ω, one channel 80 mW THD+N = 1% (max); f = 1kHz, RL = 32Ω, one channel 65 mW THD+N = 1% (max); f = 1kHz, RL = 16Ω, (two channels in phase) 43 38 mW (min) THD+N = 1% (max); f = 1kHz, RL = 32Ω, (two channels in phase) 50 45 mW (min) PO = 60mW, f = 1kHz, RL = 16Ω single channel 0.04 PO = 50mW, f = 1kHz, RL = 32Ω single channel 0.03 % VRIPPLE = 200mVp-p, Input Referred PSRR Power Supply Rejection Ratio Full Power Mode SNR Signal-to-Noise Ratio VIH f = 217Hz 70 f = 1kHz 65 f = 20kHz 50 RL = 32Ω, POUT = 20mW, (A-weighted) f = 1kHz, BW = 20Hz to 22kHz dB 100 dB Shutdown Input Voltage High VIH = 0.7*CPVDD V VIL Shutdown Input Voltage Low VIL = 0.3*CPVDD V TWU Wake Up Time From Shutdown 5 µs www.national.com 4 LM4926 Electrical Characteristics VDD = 3V (Note 1) (Continued) The following specifications apply for VDD = 3V and 16Ω load unless otherwise specified. Limits apply to TA = 25˚C. Symbol Parameter Conditions LM4926 Typ (Note 6) XTALK Crosstalk RL = 16Ω, PO = 1.6mW, f = 1kHz ZOUT Output Impedance Input Terminated Input not terminated IL Input Leakage VIN THRESH Input Voltage Threshold 60 ∞ 60 Limit (Notes 7, 8) Units (Limits) dB kΩ ± 0.1 nA 2.8 mVp 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 that 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 LM4926, see power de-rating 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: If the product is in shutdown mode and VDD exceeds 4.2V (to a max of 4.5V VDD), then most of the excess current will flow through the ESD protection circuits. If the source impedance limits the current to a max of 10mA, then the part will be protected. If the part is enabled when VDD is above 4.5V, circuit performance will be curtailed or the part may be permanently damaged. Note 10: Human body model, 100pF discharged through a 1.5kΩ resistor. Note 11: θJA value is measured with the device mounted on a PCB with a 3” x 1.5”, 1oz copper heatsink. External Components Description Components (Figure 1) Functional Description 1. 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. 2. C1 Flying capacitor. Low ESR ceramic capacitor (≤100mΩ) 3. C2 Output capacitor. Low ESR ceramic capacitor (≤100mΩ) 4. C3 Tantalum capacitor. 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. C4 Ceramic capacitor. 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 www.national.com LM4926 Typical Performance Characteristics THD+N vs Frequency VDD = 1.6V, RL = 32Ω, PO = 1mW THD+N vs Frequency VDD = 1.6V, RL = 16Ω, PO = 1mW 20116128 20116129 THD+N vs Frequency VDD = 1.8V, RL = 32Ω, PO = 5mW THD+N vs Frequency VDD = 1.8V, RL = 16Ω, PO = 5mW 20116130 www.national.com 20116131 6 LM4926 Typical Performance Characteristics (Continued) THD+N vs Frequency VDD = 3V, RL = 16Ω, PO = 50mW THD+N vs Frequency VDD = 3V, RL = 32Ω, PO = 50mW 20116132 20116133 THD+N vs Frequency VDD = 3.6V, RL = 32Ω, PO = 100mW THD+N vs Frequency VDD = 3.6V, RL = 16Ω, PO = 100mW 20116134 20116135 7 www.national.com LM4926 Typical Performance Characteristics (Continued) THD+N vs Frequency VDD = 4.2V, RL = 16Ω, PO = 150mW THD+N vs Frequency VDD = 4.2V, RL = 32Ω, PO = 150mW 20116136 20116137 THD+N vs Output Power VDD = 1.6V, RL = 32Ω, f = 1kHz One channel enabled THD+N vs Output Power VDD = 1.6V, RL = 16Ω, f = 1kH One channel enabled 20116149 20116147 www.national.com 8 LM4926 Typical Performance Characteristics (Continued) THD+N vs Output Power VDD = 1.6V, RL = 16Ω, f = 1kHz Two channels in phase THD+N vs Output Power VDD = 1.6V, RL = 32Ω, f = 1kHz Two channels in phase 20116153 20116151 THD+N vs Output Power VDD = 1.8V, RL = 32Ω, f = 1kHz One channel enabled THD+N vs Output Power VDD = 1.8V, RL = 16Ω, f = 1kHz One channel enabled 20116159 20116161 9 www.national.com LM4926 Typical Performance Characteristics (Continued) THD+N vs Output Power VDD = 1.8V, RL = 16Ω, f = 1kHz Two channels in phase THD+N vs Output Power VDD = 1.8V, RL = 32Ω, f = 1kHz Two channels in phase 20116163 20116165 THD+N vs Output Power VDD = 3.0V, RL = 32Ω, f = 1kHz One channel enabled THD+N vs Output Power VDD = 3.0V, RL = 16Ω, f = 1kHz One channel enabled 201161E1 201161G2 www.national.com 10 LM4926 Typical Performance Characteristics (Continued) THD+N vs Output Power VDD = 3.0V, RL = 16Ω, f = 1kHz Two channels in phase THD+N vs Output Power VDD = 3.0V, RL = 32Ω, f = 1kHz Two channels in phase 201161E5 201161G4 THD+N vs Output Power VDD = 3.6V, RL = 32Ω, f = 1kHz One channel enabled THD+N vs Output Power VDD = 3.6V, RL = 16Ω, f = 1kHz One channel enabled 201161F1 201161F3 11 www.national.com LM4926 Typical Performance Characteristics (Continued) THD+N vs Output Power VDD = 3.6V, RL = 16Ω, f = 1kHz Two channels in phase THD+N vs Output Power VDD = 3.6V, RL = 32Ω, f = 1kHz two channels in phase 201161F5 201161F7 THD+N vs Output Power VDD = 4.2V, RL = 32Ω, f = 1kHz One channel enabled THD+N vs Output Power VDD = 4.2V, RL = 16Ω, f = 1kHz One channel enabled 20116173 www.national.com 20116180 12 LM4926 Typical Performance Characteristics (Continued) THD+N vs Output Power VDD = 4.2V, RL = 16Ω, f = 1kHz Two channels in phase THD+N vs Output Power VDD = 4.2V, RL = 32Ω, f = 1kHz Two channels in phase 20116182 20116184 PSRR vs Frequency VDD = 1.6V, RL = 32Ω PSRR vs Frequency VDD = 1.6V, RL = 16Ω 20116140 20116141 13 www.national.com LM4926 Typical Performance Characteristics (Continued) PSRR vs Frequency VDD = 3V, RL = 16Ω PSRR vs Frequency VDD = 3V, RL = 32Ω 20116143 20116142 PSRR vs Frequency VDD = 4.2V, RL = 32Ω PSRR vs Frequency VDD = 4.2V, RL = 16Ω 20116144 www.national.com 20116145 14 LM4926 Typical Performance Characteristics (Continued) Output Power vs Supply Voltage RL = 16Ω, one channel Output Power vs Supply Voltage RL = 32Ω, one channel 20116139 20116138 Output Power vs Supply Voltage RL = 32Ω, 2 channels in phase Output Power vs Supply Voltage RL = 16Ω, 2 channels in phase 201161G8 201161G9 15 www.national.com LM4926 Typical Performance Characteristics (Continued) Supply Current vs Supply Voltage RL = 16Ω Representation of Automatic Standby Mode Behavior VDD = 3V 20116119 20116189 www.national.com 16 Power dissipation is a major concern when using any power amplifier and must be thoroughly understood to ensure a successful design. 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. SUPPLY VOLTAGE SEQUENCING It is a good general practice to first apply the supply voltage to a CMOS device before any other signal or supply on other pins. This is also true for the LM4926 audio amplifier which is a CMOS device. PDMAX = (VDD) Before applying any signal to the inputs or shutdown pins of the LM4926, it is important to apply a supply voltage to the VDD pins. After the device has been powered, signals may be applied to the shutdown pins (see MICRO POWER SHUTDOWN) and input pins. 2 / (2π2RL) (1) Since the LM4926 has two operational amplifiers in one package, the maximum internal power dissipation point is twice that of the number which results from Equation 1. Even with large internal power dissipation, the LM4926 does not require heat sinking over a large range of ambient temperatures. From Equation 1, assuming a 3V power supply and a 16Ω load, the maximum power dissipation point is 28mW per amplifier. Thus the maximum package dissipation point is 56mW. The maximum power dissipation point obtained must not be greater than the power dissipation that results from Equation 2: ELIMINATING THE OUTPUT COUPLING CAPACITOR The LM4926 features a low noise inverting charge pump that generates an internal negative supply voltage. This allows the outputs of the LM4926 to be biased about GND instead of a nominal DC voltage, like traditional headphone amplifiers. Because there is no DC component, the large DC blocking capacitors (typically 220µF) are not necessary. The coupling capacitors are replaced by two, small ceramic charge pump capacitors, saving board space and cost. Eliminating the output coupling capacitors also improves low frequency response. In traditional headphone amplifiers, the headphone impedance and the output capacitor form a high pass filter that not only blocks the DC component of the output, but also attenuates low frequencies, impacting the bass response. Because the LM4926 does not require the output coupling capacitors, the low frequency response of the device is not degraded by external components. In addition to eliminating the output coupling capacitors, the ground referenced output nearly doubles the available dynamic range of the LM4926 when compared to a traditional headphone amplifier operating from the same supply voltage. PDMAX = (TJMAX - TA) / (θJA) (2) For the micro SMD package, θJA = 105˚C/W. TJMAX = 150˚C for the LM4926. 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 a 16Ω 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. 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. OUTPUT TRANSIENT (’CLICK AND POPS’) ELIMINATED The LM4926 contains advanced circuitry that virtually eliminates output transients (’clicks and pops’). This circuitry prevents all traces of transients when the supply voltage is first applied or when the part resumes operation after coming out of shutdown mode. POWER SUPPLY BYPASSING As with any power amplifier, proper supply bypassing is critical for low noise performance and high power supply rejection. Applications that employ a 3V power supply typically use a 4.7µF capacitor in parallel with a 0.1µF ceramic filter capacitor to stabilize the power supply’s output, reduce noise on the supply line, and improve the supply’s transient response. Keep the length of leads and traces that connect capacitors between the LM4926’s power supply pin and ground as short as possible. AMPLIFIER CONFIGURATION EXPLANATION As shown in Figure 2, the LM4926 has two internal operational amplifiers. The two amplifiers have internally configured gain, the closed loop gain is set by selecting the ratio of Rf to Ri. Consequently, the gain for each channel of the IC is AUTOMATIC STANDBY MODE The LM4926 features Automatic Standby Mode circuitry (patent pending). In the absence of an input signal, after approximately 12 seconds, the LM4926 goes into low current standby mode. The LM4926 recovers into full power operating mode immediately after a signal, which is greater than the input threshold voltage, is applied to either the left or right input pins. The input threshold voltage is not a static value, as the supply voltage increases, the input threshold voltage decreases. This feature reduces power supply current consumption in battery operated applications. Please see also the graph entitled Representation of Automatic Standby Mode Behavior in the Typical Performance Characteristics section. AV = -(Rf / Ri) = 1.5 V/V where RF = 30kΩ and Ri = 20kΩ. Since this is an output ground-referenced amplifier, by driving the headphone through ROUT (Pin C2) and LOUT (Pin D2), the LM4926 does not require output coupling capacitors. The typical single-ended amplifier configuration requires large, expensive output capacitors. 17 www.national.com LM4926 POWER DISSIPATION Application Information LM4926 Application Information negative supply. Higher ESR capacitors result in reduced output power from the audio amplifiers. Charge pump load regulation and output impedance are affected by the value of the flying capacitor (C1). A larger valued C1 (up to 3.3uF) improves load regulation and minimizes charge pump output resistance. Beyond 3.3uF, the switch-on resistance dominates the output impedance for capacitor values above 2.2uF. (Continued) To ensure correct operation of Automatic Standby Mode, proper layout techniques should be implemented. Separating PGND and SGND can help reduce noise entering the LM4926 in noisy environments. Auto Standby mode works best when output impedance of the audio source driving LM4926 is equal or less than 50 Ohms. While Automatic Standby Mode reduces power consumption very effectively during silent periods, maximum power saving is achieved by putting the device into shutdown when it is not in use. The output ripple is affected by the value and ESR of the output capacitor (C2). Larger capacitors reduce output ripple on the negative power supply. Lower ESR capacitors minimize the output ripple and reduce the output impedance of the charge pump. MICRO POWER SHUTDOWN The voltage applied to the SD_LC (shutdown left channel) pin and the SD_RC (shutdown right channel) pin controls the LM4926’s shutdown function. When active, the LM4926’s micropower shutdown feature turns off the amplifiers’ bias circuitry, reducing the supply current. The trigger point is 0.3*CPVDD for a logic-low level, and 0.7*CPVDD for logichigh level. The low 0.01µA (typ) shutdown current is achieved by applying a voltage that is as near as ground a possible to the SD_LC/SD_RC pins. 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 SD_LC/SD_RC pins and VDD. Connect the switch between the SD_LC/SD_RC pins and ground. Select normal amplifier operation by opening the switch. Closing the switch connects the SD_LC/SD_RC pins to ground, activating micropower shutdown. The switch and resistor guarantee that the SD_LC/SD_RC pins 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 SD_LC/SD_RC pins. Driving the SD_LC/SD_RC pins with active circuitry eliminates the pull-up resistor. The LM4926 charge pump design is optimized for 2.2uF, low ESR, ceramic, flying, and output capacitors. Input Capacitor Value Selection Amplifying the lowest audio frequencies requires high value input coupling capacitors (Ci in Figure 1). A high value capacitor can be expensive and may compromise space efficiency in portable designs. In many cases, however, the speakers used in portable systems, whether internal or external, have little ability to reproduce signals below 150Hz. Applications using speakers with this limited frequency response reap little improvement by using high value input and output capacitors. Besides affecting system cost and size, Ci has an effect on the LM4926’s click and pop performance. The magnitude of the pop is directly proportional to the input capacitor’s size. Thus, pops can be minimized by selecting an input capacitor value that is no higher than necessary to meet the desired −3dB frequency. As shown in Figure 1, the internal input resistor, Ri and the input capacitor, Ci, produce a -3dB high pass filter cutoff frequency that is found using Equation (3). Conventional headphone amplifiers require output capacitors; Equation (3) can be used, along with the value of RL, to determine towards the value of output capacitor needed to produce a –3dB high pass filter cutoff frequency. SELECTING PROPER EXTERNAL COMPONENTS Optimizing the LM4926’s performance requires properly selecting external components. Though the LM4926 operates well when using external components with wide tolerances, best performance is achieved by optimizing component values. fi-3dB = 1 / 2πRiCi Also, careful consideration must be taken in selecting a certain type of capacitor to be used in the system. Different types of capacitors (tantalum, electrolytic, ceramic) have unique performance characteristics and may affect overall system performance. (See the section entitled Charge Pump Capacitor Selection.) Charge Pump Capacitor Selection Use low ESR (equivalent series resistance) ( < 100mΩ) ceramic capacitors with an X7R dielectric for best performance. Low ESR capacitors keep the charge pump output impedance to a minimum, extending the headroom on the www.national.com (3) 18 LM4926 Application Information (Continued) LM4926 micro SMD DEMO BOARD ARTWORK Top Overlay Top Layer 20116116 20116117 Mid Layer 1 Mid Layer 2 201161H1 201161H0 19 www.national.com LM4926 Application Information (Continued) Bottom Layer 20116115 www.national.com 20 LM4926 Revision History Rev Date Description 1.0 6/22/05 Initial WEB release. 1.1 6/24/05 Added Mid Layer 1 and Mid Layer 2 boards, then re-released D/S to the WEB (per Nisha P.) 21 www.national.com LM4926 Ground-Referenced, Ultra Low Noise, Fixed Gain, 80mW Stereo Headphone Amplifier Physical Dimensions inches (millimeters) unless otherwise noted 14 – Bump micro SMD Order Number LM4926TL NS Package Number TLE1411A X1 = 1.970 ± 0.03mm, X2 = 1.970 ± 0.03mm, X3 = 0.600 ± 0.075mm, 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. 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