LM4671 Filterless High Efficiency 2.5W Switching Audio Amplifier General Description Key Specifications The LM4671 is a single supply, high efficiency 2.5W switching audio amplifier. A low noise, filterless PWM architecture eliminates the output filter, reducing external component count, board area consumption, system cost, and simplifying design. The LM4671 is designed to meet the demands of mobile phones and other portable communication devices. Operating on a single 5V supply, it is capable of driving a 4Ω speaker load at a continuous average output of 2.1W with less than 1% THD+N. Its flexible power supply requirements allow operation from 2.4V to 5.5V. The LM4671 has high efficiency with speaker loads compared to a typical Class AB amplifier. With a 3V supply driving an 8Ω speaker, the IC’s efficiency for a 100mW power level is 80%, reaching 88% at 400mW output power. The LM4671 features a low-power consumption shutdown mode. Shutdown may be enabled by driving the Shutdown pin to a logic low (GND). The gain of the LM4671 is externally configurable which allows independent gain control from multiple sources by summing the signals. j Efficiency at 3.6V, 100mW into 8Ω speaker 80% (typ) j Efficiency at 3.6V, 400mW into 8Ω speaker 88% (typ) j Efficiency at 5V, 1W into 8Ω speakerr j Quiescent current, 3.6V supply 86% (typ) 2.8mA (typ) j Total shutdown power supply current 0.01µA (typ) j Single supply range 2.4V to 5.5V Features n n n n n n n No output filter required for inductive loads Externally configurable gain Very fast turn on time: 1ms (typ) Minimum external components "Click and pop" suppression circuitry Micro-power shutdown mode Available in space-saving microSMD package Applications n Mobile phones n PDAs n Portable electronic devices Typical Application 201073J3 FIGURE 1. Typical Audio Amplifier Application Circuit Boomer ® is a registered trademark of National Semiconductor Corporation. © 2005 National Semiconductor Corporation DS201073 www.national.com LM4671 Filterless High Efficiency 2.5W Switching Audio Amplifier March 2005 LM4671 Connection Diagrams 9 Bump micro SMD Package micro SMD Marking 201073C6 Top View X — Date Code T — Die Traceability G — Boomer Family E7 — LM4671ITL 20107336 Top View Order Number LM4671ITL, LM4671ITLX See NS Package Number TLA09AAA www.national.com 2 Thermal Resistance If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Soldering Information Supply Voltage (Note 1) θJA (micro SMD) See AN-1112 "microSMD Wafers Level Chip Scale Package." 6.0V Storage Temperature 220˚C/W −65˚C to +150˚C Voltage at Any Input Pin VDD + 0.3V ≥ V ≥ GND - 0.3V Power Dissipation (Note 3) ESD Susceptibility, all other pins (Note 4) 2.0kV ESD Susceptibility (Note 5) 200V Junction Temperature (TJMAX) Operating Ratings (Note 1) (Note 2) Internally Limited Temperature Range TMIN ≤ TA ≤ TMAX −40˚C ≤ TA ≤ 85˚C 2.4V ≤ VDD ≤ 5.5V Supply Voltage 150˚C Electrical Characteristics (Notes 1, 2) The following specifications apply for AV = 2V/V (RI = 150kΩ), RL = 15µH + 8Ω + 15µH unless otherwise specified. Limits apply for TA = 25˚C. LM4671 Symbol Parameter Conditions Typical Limit (Note 6) (Notes 7, 8) Units (Limits) VI = 0V, AV = 2V/V, VDD = 2.4V to 5.0V 5 mV (max) PSRRGSM GSM Power Supply Rejection Ratio VDD = 2.4V to 5.0V |VOS| Differential Output Offset Voltage 61 dB (min) 68 dB (min) 17 100 5 CMRRGSM GSM Common Mode Rejection Ratio VDD = 2.4V to 5.0V VIC = VDD/2 to 0.5V, VIC = VDD/2 to VDD – 0.8V |IIH| Logic High Input Current VDD = 5.0V, VI = 5.5V |IIL| Logic Low Input Current VDD = 5.0V, VI = –0.3V 0.9 VIN = 0V, No Load, VDD = 5.0V 6.4 IDD Quiescent Power Supply Current µA (max) µA (max) mA (max) VIN = 0V, No Load, VDD = 3.6V 3.8 6.2 mA VIN = 0V, No Load, VDD =2.4V 2.0 3.0 mA (max) VSHUTDOWN = 0V VDD = 2.4V to 5.0V 0.01 1 µA (max) ISD Shutdown Current VSDIH Shutdown voltage input high 1.2 1.4 V (min) VSDIL Shutdown voltage input low 1.1 0.4 V (max) ROSD Output Impedance 270kΩ/RI 330kΩ/RI V/V (min) V/V (max) AV Gain RSD Resistance from Shutdown Pin to GND PO Output Power VSHUTDOWN = 0.4V 100 300kΩ/RI kΩ 300 kΩ RL = 15µH + 4Ω + 15µH THD = 10% (max) f = 1kHz, 22kHz BW VDD = 5V VDD = 3.6V VDD = 2.5V 2.5 1.3 520 W W mW RL = 15µH + 4Ω + 15µH THD = 1% (max) f = 1kHz, 22kHz BW VDD = 5V VDD = 3.6V VDD = 2.5V 2.21 1.06 420 W W mW 3 www.national.com LM4671 Absolute Maximum Ratings (Notes 1, 2) LM4671 Electrical Characteristics (Notes 1, 2) The following specifications apply for AV = 2V/V (RI = 150kΩ), RL = 15µH + 8Ω + 15µH unless otherwise specified. Limits apply for TA = 25˚C. (Continued) LM4671 Symbol Parameter Conditions Typical Limit (Note 6) (Notes 7, 8) Units (Limits) RL = 15µH + 8Ω + 15µH THD = 10% (max) f = 1kHz, 22kHz BW PO THD+N PSRR SNR eOUT Output Power Total Harmonic Distortion + Noise Power Supply Rejection Ratio VDD = 5V 17 W VDD = 3.6V 870 mW VDD = 2.5V 425 W VDD = 5V 1.19 W VDD = 3.6V 700 VDD = 2.5V 350 W VDD = 5V, PO = 0.1WRMS, f = 1kHz 0.09 % VDD = 3.6V, PO = 0.1WRMS, f = 1kHz 0.04 % VDD = 3.6V, PO = 0.1WRMS, f = 5kHz 0.12 % VDD = 3.6V, PO = 0.1WRMS, f = 10kHz 0.05 % VDD = 3.6V, 5V VRipple = 200mVPP Sine, fRipple = 217Hz Inputs to AC GND, CI = 2µF 61.8 dB VDD = 3.6V, 5V VRipple = 200mVPP Sine, fRipple = 1kHz Inputs to AC GND, CI = 2µF 59.8 dB VDD = 3.6V, 5V VRipple = 200mVPP Sine, fRipple = 10kHz Inputs to AC GND, CI = 2µF 48.65 dB VDD = 3.6V, 5V VRipple = 200mVPP Sine, fRipple = 217Hz fIN = 1kHz, PO = 10mWRMS 65.71 dB RL = 15µH + 8Ω + 15µH THD = 1% (max) f = 1kHz, 22kHz BW 600 mW Signal to Noise Ratio VDD = 5V, PO = 1WRMS 93 dB 58 µVRMS Output Noise VDD = 3.6V, f = 20Hz – 20kHz Inputs to AC GND, CI = 2µF No Weighting VDD = 3.6V, Inputs to AC GND CI = 2µF, A Weighted 38 µVRMS 68.3 dB CMRR Common Mode Rejection Ratio VDD = 3.6V, VRipple = 1VPP Sine fRipple = 217Hz TWU Wake-up Time VDD = 3.6V TSD Shutdown Time www.national.com 4 17 µs 140 µs 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 LM4671, TJMAX = 150˚C. The typical θJA is 220˚C/W for the microSMD package. Note 4: Human body model, 100pF discharged through a 1.5kΩ resistor. Note 5: Machine Model, 220pF–240pF discharged through all pins. Note 6: Typical specifications are specified at 25˚C and represent the parametric norm. Note 7: Tested 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: Shutdown current is measured in a normal room environment. Exposure to direct sunlight will increase ISD by a maximum of 2µA. The Shutdown pin should be driven as close as possible to GND for minimal shutdown current and to VDD for the best THD performance in PLAY mode. See the Application Information section under SHUTDOWN FUNCTION for more information. Note 10: The performance graphs were taken using the Audio Precision AUX-0025 Switching Amplifier measurement Filter in series with the LC filter on the demo board. External Components Description (Figure 1) Components Functional Description 1. 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. 2. CI Input AC coupling capacitor which blocks the DC voltage at the amplifier’s input terminals. 5 www.national.com LM4671 Note 1: All voltages are measured with respect to the ground pin, unless otherwise specified. LM4671 Typical Performance Characteristics THD+N vs Frequency VDD = 3.6V, RL = 15µH+4Ω+15µH, PO = 750mW, 22kHz BW THD+N vs Frequency VDD = 2.4V, RL = 15µH+4Ω+15µH, PO = 375mW, 22kHz BW 20107303 20107304 THD+N vs Frequency VDD = 2.4V, RL = 15µH+8Ω+15µH, PO = 200mW, 22kHz BW THD+N vs Frequency VDD = 5V, RL = 15µH+4Ω+15µH, PO = 1.5mW, 22kHz BW 20107305 www.national.com 20107306 6 LM4671 Typical Performance Characteristics (Continued) THD+N vs Frequency VDD = 5V, RL = 15µH+8Ω+15µH, PO = 1W, 22kHz BW THD+N vs Frequency VDD = 3.6V, RL = 15µH+8Ω+15µH, PO = 500mW, 22kHz BW 20107307 20107308 THD+N vs Output Power VDD = 5V, RL = 15µH+8Ω+15µH, f = 1kHz, 22kHz BW THD+N vs Output Power VDD = 5V, RL = 15µH+4Ω+15µH, f = 1kHz, 22kHz BW 20107310 20107309 7 www.national.com LM4671 Typical Performance Characteristics (Continued) PSRR vs Frequency VDD = 3.6V, RL = 15µH+8Ω+15µH, Vripple = 200mVp-p, 22kHz BW CMRR vs Frequency VDD = 3.6V, RL = 15µH+8Ω+15µH, Vripple = 1Vp-p, 22kHz BW 20107311 20107312 Efficiency vs Output Power RL = 15µH+8Ω+15µH, f = 1kHz, 22kHz BW Efficiency vs Output Power RL = 15µH+4Ω+15µH, f = 1kHz, 22kHz BW 20107314 20107313 www.national.com 8 LM4671 Typical Performance Characteristics (Continued) Power Dissipation vs Output Power RL = 15µH+4Ω+15µH, f = 1kHz, 22kHz BW Power Dissipation vs Output Power RL = 15µH+8Ω+15µH, f = 1kHz, 22kHz BW 20107316 20107315 Output Power vs Supply Voltage RL = 15µH+8Ω+15µH, VDD = 3.6V Output Power vs Supply Voltage RL = 15µH+4Ω+15µH, VDD = 3.6V 20107318 20107317 9 www.national.com LM4671 Typical Performance Characteristics (Continued) Gain vs Supply Voltage Rin = 150kΩ Supply Current vs Supply Voltage RL = 15µH+8Ω+15µH 20107319 20107320 Shutdown Current vs Supply Voltage RL = 15µH+8Ω+15µH 20107321 Application Information POWER DISSIPATION AND EFFICIENCY In general terms, efficiency is considered to be the ratio of useful work output divided by the total energy required to produce it with the difference being the power dissipated, typically, in the IC. The key here is “useful” work. For audio systems, the energy delivered in the audible bands is considered useful including the distortion products of the input signal. Sub-sonic (DC) and super-sonic components ( > 22kHz) are not useful. The difference between the power flowing from the power supply and the audio band power being transduced is dissipated in the LM4671 and in the transducer load. The amount of power dissipation in the LM4671 is very low. This is because the ON resistance of the switches used to form the output waveforms is typically less than 0.25Ω. This leaves only the transducer load as a potential "sink" for the small excess of input power over audio GENERAL AMPLIFIER FUNCTION The LM4671 features a filterless modulation scheme. The differential outputs of the device switch at 300kHz from VDD to GND. When there is no input signal applied, the two outputs (VO1 and VO2) switch with a 50% duty cycle, with both outputs in phase. Because the outputs of the LM4671 are differential, the two signals cancel each other. This results in no net voltage across the speaker, thus there is no load current during an idle state, conserving power. With an input signal applied, the duty cycle (pulse width) of the LM4671 outputs changes. For increasing output voltages, the duty cycle of VO1 increases, while the duty cycle of VO2 decreases. For decreasing output voltages, the converse occurs, the duty cycle of VO2 increases while the duty cycle of VO1 decreases. The difference between the two pulse widths yields the differential output voltage. www.national.com 10 Ferrite chip inductors placed close to the LM4671 may be needed to reduce EMI radiation. The value of the ferrite chip is very application specific. (Continued) band output power. The LM4671 dissipates only a fraction of the excess power requiring no additional PCB area or copper plane to act as a heat sink. POWER SUPPLY BYPASSING As with any power amplifier, proper supply bypassing is critical for low noise performance and high power supply rejection ratio (PSRR). The capacitor (CS) location should be as close as possible to the LM4671. Typical applications employ a voltage regulator with a 10µF and a 0.1µF bypass capacitors that increase supply stability. These capacitors do not eliminate the need for bypassing on the supply pin of the LM4671. A 1µF tantalum capacitor is recommended. DIFFERENTIAL AMPLIFIER EXPLANATION As logic supply voltages continue to shrink, designers are increasingly turning to differential analog signal handling to preserve signal to noise ratios with restricted voltage swing. The LM4671 is a fully differential amplifier that features differential input and output stages. A differential amplifier amplifies the difference between the two input signals. Traditional audio power amplifiers have typically offered only single-ended inputs resulting in a 6dB reduction in signal to noise ratio relative to differential inputs. The LM4671 also offers the possibility of DC input coupling which eliminates the two external AC coupling, DC blocking capacitors. The LM4671 can be used, however, as a single ended input amplifier while still retaining it’s fully differential benefits. In fact, completely unrelated signals may be placed on the input pins. The LM4671 simply amplifies the difference between the signals. A major benefit of a differential amplifier is the improved common mode rejection ratio (CMRR) over single input amplifiers. The common-mode rejection characteristic of the differential amplifier reduces sensitivity to ground offset related noise injection, especially important in high noise applications. SHUTDOWN FUNCTION In order to reduce power consumption while not in use, the LM4671 contains shutdown circuitry that reduces current draw to less than 0.01µA. The trigger point for shutdown is shown as a typical value in the Electrical Characteristics Tables and in the Shutdown Hysteresis Voltage graphs found in the Typical Performance Characteristics section. It is best to switch between ground and supply for minimum current usage while in the shutdown state. While the LM4671 may be disabled with shutdown voltages in between ground and supply, the idle current will be greater than the typical 0.01µA value. Increased THD may also be observed with voltages less than VDD on the Shutdown pin when in PLAY mode. The LM4671 has an internal resistor connected between GND and Shutdown pins. The purpose of this resistor is to eliminate any unwanted state changes when the Shutdown pin is floating. The LM4671 will enter the shutdown state when the Shutdown pin is left floating or if not floating, when the shutdown voltage has crossed the threshold. To minimize the supply current while in the shutdown state, the Shutdown pin should be driven to GND or left floating. If the Shutdown pin is not driven to GND, the amount of additional resistor current due to the internal shutdown resistor can be found by Equation (1) below. PCB LAYOUT CONSIDERATIONS As output power increases, interconnect resistance (PCB traces and wires) between the amplifier, load and power supply create a voltage drop. The voltage loss on the traces between the LM4671 and the load results is lower output power and decreased efficiency. Higher trace resistance between the supply and the LM4671 has the same effect as a poorly regulated supply, increase ripple on the supply line also reducing the peak output power. The effects of residual trace resistance increases as output current increases due to higher output power, decreased load impedance or both. To maintain the highest output voltage swing and corresponding peak output power, the PCB traces that connect the output pins to the load and the supply pins to the power supply should be as wide as possible to minimize trace resistance. The use of power and ground planes will give the best THD+N performance. While reducing trace resistance, the use of power planes also creates parasite capacitors that help to filter the power supply line. The inductive nature of the transducer load can also result in overshoot on one or both edges, clamped by the parasitic diodes to GND and VDD in each case. From an EMI standpoint, this is an aggressive waveform that can radiate or conduct to other components in the system and cause interference. It is essential to keep the power and output traces short and well shielded if possible. Use of ground planes, beads, and micro-strip layout techniques are all useful in preventing unwanted interference. As the distance from the LM4671 and the speaker increase, the amount of EMI radiation will increase since the output wires or traces acting as antenna become more efficient with length. What is acceptable EMI is highly application specific. (VSD - GND) / 60kΩ (1) With only a 0.5V difference, an additional 8.3µA of current will be drawn while in the shutdown state. PROPER SELECTION OF EXTERNAL COMPONENTS The gain of the LM4671 is set by the external resistors, Ri in Figure 1, The Gain is given by Equation (2) below. Best THD+N performance is achieved with a gain of 2V/V (6dB). AV = 2 * 150 kΩ / Ri (V/V) (2) It is recommended that resistors with 1% tolerance or better be used to set the gain of the LM4671. The Ri resistors should be placed close to the input pins of the LM4671. Keeping the input traces close to each other and of the same length in a high noise environment will aid in noise rejection due to the good CMRR of the LM4671. Noise coupled onto input traces which are physically close to each other will be common mode and easily rejected by the LM4671. Input capacitors may be needed for some applications or when the source is single-ended (see Figures 3, 5). Input capacitors are needed to block any DC voltage at the source so that the DC voltage seen between the input terminals of 11 www.national.com LM4671 Application Information LM4671 Application Information mum design flexibility using the Ri resistors for each input and Equation (2). Input capacitors can be used with one or more sources as well to have different frequency responses depending on the source or if a DC voltage needs to be blocked from a source. (Continued) the LM4671 is 0V. Input capacitors create a high-pass filter with the input resistors, Ri. The –3dB point of the high-pass filter is found using Equation (3) below. fC = 1 / (2πRi Ci ) (Hz) SINGLE-ENDED CIRCUIT CONFIGURATIONS (3) The LM4671 can also be used with single-ended sources but input capacitors will be needed to block any DC at the input terminals. Figure 5 shows the typical single-ended application configuration. The equations for Gain, Equation (2), and frequency response, Equation (3), hold for the single-ended configuration as shown in Figure 5. The input capacitors may also be used to remove low audio frequencies. Small speakers cannot reproduce low bass frequencies so filtering may be desired . When the LM4671 is using a single-ended source, power supply noise on the ground is seen as an input signal by the +IN input pin that is capacitor coupled to ground (See Figures 5 – 7). Setting the high-pass filter point above the power supply noise frequencies, 217Hz in a GSM phone, for example, will filter out this noise so it is not amplified and heard on the output. Capacitors with a tolerance of 10% or better are recommended for impedance matching. When using more than one single-ended source as shown in Figure 6, the impedance seen from each input terminal should be equal. To find the correct values for Ci3 and Ri3 connected to the +IN input pin the equivalent impedance of all the single-ended sources are calculated. The singleended sources are in parallel to each other. The equivalent capacitor and resistor, Ci3 and Ri3, are found by calculating the parallel combination of all Civalues and then all Ri values. Equations (4) and (5) below are for any number of single-ended sources. DIFFERENTIAL CIRCUIT CONFIGURATIONS The LM4671 can be used in many different circuit configurations. The simplest and best performing is the DC coupled, differential input configuration shown in Figure 2. Equation (2) above is used to determine the value of the Ri resistors for a desired gain. Input capacitors can be used in a differential configuration as shown in Figure 3. Equation (3) above is used to determine the value of the Ci capacitors for a desired frequency response due to the high-pass filter created by Ci and Ri. Equation (2) above is used to determine the value of the Ri resistors for a desired gain The LM4671 can be used to amplify more than one audio source. Figure 4 shows a dual differential input configuration. The gain for each input can be independently set for maxi- Ci3 = Ci1 + Ci2 + Cin ... (µF) (4) Ri3 = 1 / (1/Ri1 + 1/Ri2 + 1/Rin ...) (Ω) (5) The LM4671 may also use a combination of single-ended and differential sources. A typical application with one singleended source and one differential source is shown in Figure 7. Using the principle of superposition, the external component values can be determined with the above equations corresponding to the configuration. 201073I7 FIGURE 2. Differential input configuration www.national.com 12 LM4671 Application Information (Continued) 201073I8 FIGURE 3. Differential input configuration with input capacitors 201073I9 FIGURE 4. Dual differential input configuration 13 www.national.com LM4671 Application Information (Continued) 201073J0 FIGURE 5. Single-ended input configuration 201073J1 FIGURE 6. Dual single-ended input configuration www.national.com 14 LM4671 Application Information (Continued) 201073J2 FIGURE 7. Dual input with a single-ended input and a differential input 15 www.national.com LM4671 Application Information (Continued) REFERENCE DESIGN BOARD SCHEMATIC 201073J4 FIGURE 8. In addition to the minimal parts required for the application circuit, a measurement filter is provided on the evaluation circuit board so that conventional audio measurements can be conveniently made without additional equipment. This is a balanced input, grounded differential output low pass filter with a 3dB frequency of approximately 35kHz and an on board termination resistor of 300Ω (see schematic). Note that the capacitive load elements are returned to ground. This is not optimal for common mode rejection purposes, but due to the independent pulse format at each output there is a significant amount of high frequency common mode component on the outputs. The grounded capacitive filter elements attenuate this component at the board to reduce the high frequency CMRR requirement placed on the analysis instruments. The commonly used Audio Precision analyzer is differential, but its ability to accurately reject high frequency signals is questionable necessitating the on board measurement filter. When in doubt or when the signal needs to be single-ended, use an audio signal transformer to convert the differential output to a single ended output. Depending on the audio transformer’s characteristics, there may be some attenuation of the audio signal which needs to be taken into account for correct measurement of performance. Measurements made at the output of the measurement filter suffer attenuation relative to the primary, unfiltered outputs even at audio frequencies. This is due to the resistance of the inductors interacting with the termination resistor (300Ω) and is typically about -0.25dB (3%). In other words, the voltage levels (and corresponding power levels) indicated through the measurement filter are slightly lower than those that actually occur at the load placed on the unfiltered outputs. This small loss in the filter for measurement gives a lower output power reading than what is really occurring on the unfiltered outputs and its load. Even with the grounded filter the audio signal is still differential, necessitating a differential input on any analysis instrument connected to it. Most lab instruments that feature BNC connectors on their inputs are NOT differential responding because the ring of the BNC is usually grounded. www.national.com 16 LM4671 Application Information (Continued) LM4671 micro SMD BOARD ARTWORK Composite View Silk Screen 201073J6 201073J9 Top Layer Internal Layer 1, GND 201073K0 201073J7 Internal Layer 2, GND Bottom Layer 201073J8 201073J5 17 www.national.com LM4671 Filterless High Efficiency 2.5W Switching Audio Amplifier Physical Dimensions inches (millimeters) unless otherwise noted 9 Bump micro SMD Order Number LM4671ITL, LM4671ITLX NS Package Number TLA09AAA X1 = 1.514 X2 = 1.514 X3 = 0.600 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. 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