LM4938 Stereo 2W Audio Power Amplifiers with DC Volume Control and Selectable Gain General Description Key Specifications The LM4938 is a monolithic integrated circuit that provides DC volume control, and stereo bridged audio power amplifiers capable of producing 2W into 4Ω (Note 1) with less than 1.0% THD or 2.2W into 3Ω (Note 2) with less than 1.0% THD. Boomer ® audio integrated circuits were designed specifically to provide high quality audio while requiring a minimum amount of external components. The LM4938 incorporates a DC volume control, stereo bridged audio power amplifiers and a selectable gain or bass boost, making it optimally suited for multimedia monitors, portable radios, desktop, and portable computer applications. The LM4938 features an externally controlled, low-power consumption shutdown mode, and both a power amplifier and headphone mute for maximum system flexibility and performance. n PO at 1% THD+N n into 3Ω n into 4Ω n into 8Ω n Single-ended mode - THD+N at 92mW into 32Ω n Shutdown current Note 1: When properly mounted to the circuit board, LM4938MH will deliver 2W into 4Ω. See Application Information section Exposed-DAP package PCB Mounting Considerations for more information. Note 2: An LM4938MH that has been properly mounted to the circuit board and forced-air cooled will deliver 2.2W into 3Ω. 2.2W (typ) 2.0W (typ) 1.3W (typ) 1.0%(typ) 0.5µA (typ) Features n Improved click and pop circuitry virtually eliminates noise during turn on/off transitions n DC Volume Control Interface n System Beep Detect n Stereo switchable bridged/single-ended power amplifiers n Selectable internal/external gain and bass boost n Thermal shutdown protection circuitry n Unity gain stable Applications n n n n Flat Panel Displays Portable and Desktop Computers Multimedia Monitors Portable Radios, PDAs, and Portable TVs Block Diagram 20095601 FIGURE 1. LM4938 Block Diagram Boomer ® is a registered trademark of NationalSemiconductor Corporation. © 2005 National Semiconductor Corporation DS200956 www.national.com LM4938 Stereo 2W Audio Power Amplifiers with DC Volume Control and Selectable Gain February 2005 LM4938 Connection Diagram TSSOP Package 20095602 Top View Order Number LM4938MH See NS Package Number MXA28A for Exposed-DAP TSSOP www.national.com 2 See AN-450 “Surface Mounting and their Effects on Product Reliability” for other methods of soldering surface mount devices. If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Supply Voltage θJC (typ) - MXA28A 2˚C/W 6.0V θJA (typ) - MXA28A (exposed DAP) (Note 3) 41˚C/W -65˚C to +150˚C θJA (typ) - MXA28A (exposed DAP) (Note 4) 54˚C/W −0.3V to VDD +0.3V θJA (typ) - MXA28A (exposed DAP) (Note 5) 59˚C/W Power Dissipation (Note 11) Internally limited θJA (typ) - MXA28A (exposed DAP) (Note 6) 93˚C/W ESD Susceptibility (Note 12) 2000V ESD Susceptibility (Note 13) 200V Storage Temperature Input Voltage Junction Temperature Operating Ratings 150˚C Soldering Information Small Outline Package Vapor Phase (60 sec.) 215˚C Infrared (15 sec.) 220˚C Temperature Range TMIN ≤ TA ≤TMAX −20˚C ≤TA ≤ 85˚C Supply Voltage 2.7V≤ VDD ≤ 5.5V Electrical Characteristics for Entire IC (Notes 7, 10) The following specifications apply for VDD = 5V unless otherwise noted. Limits apply for TA = 25˚C. LM4938 Symbol VDD Parameter Conditions Typical (Note 14) Limit (Note 15) Supply Voltage Units (Limits) 2.7 V (min) 5.5 V (max) mA (max) IDD Quiescent Power Supply Current VIN = 0V, IO = 0A 11 30 ISD Shutdown Current Vshutdown = VDD 0.5 2.0 VIH Headphone Sense High Input Voltage 4 V (min) VIL Headphone Sense Low Input Voltage 0.8 V (max) µA (max) Electrical Characteristics for Volume Attenuators (Notes 7, 10) The following specifications apply for VDD = 5V. Limits apply for TA = 25˚C. LM4938 Symbol Parameter Conditions Gain accuracy with VDCVol = 5V, No Load CRANGE AM Attenuator Range Mute Attenuation Typical (Note 14) Limit (Note 15) ± 0.5 ± 0.75 Gain accuracy with VDCVol < 0.5V, No Load ±2 Attenuation with VDCVol = 0V (BM & SE) 89 Vmute = 5V, Bridged Mode (BM) 89 Units (Limits) dB (max) dB (max) 75 Vmute = 5V, Single-Ended Mode (SE) dB (min) 78 dB (min) 78 dB (min) Electrical Characteristics for Bridged Mode Operation (Notes 7, 10) The following specifications apply for VDD = 5V, unless otherwise noted. Limits apply for TA = 25˚C. LM4938 Symbol VOS Parameter Output Offset Voltage Conditions VIN = 0V, No Load 3 Typical (Note 14) Limit (Note 15) 5 ± 50 Units (Limits) mV (max) www.national.com LM4938 Absolute Maximum Ratings (Note 10) LM4938 Electrical Characteristics for Bridged Mode Operation (Notes 7, 10) (Continued) The following specifications apply for VDD = 5V, unless otherwise noted. Limits apply for TA = 25˚C. LM4938 Symbol PO THD+N PSRR Parameter Output Power Total Harmonic Distortion + Noise Power Supply Rejection Ratio Conditions Typical (Note 14) Limit (Note 15) Units (Limits) THD + N = 1.0%; f = 1kHz RL = 3Ω (Note 8) 2.2 W THD + N = 1.0%; f = 1kHz RL = 4Ω (Note 9) 2 W THD = 1% (max); f = 1kHz RL = 8Ω 1.3 THD+N = 10%; f = 1 kHz; RL = 8Ω 1.5 W PO = 0.4W, f = 1kHz, RL = 8Ω, AVD = 2 0.05 % CB = 1.0 µF, f = 120 Hz, VRIPPLE = 200 mVrms; RL = 8Ω, Floating 78 dB CB = 1.0 µF, f = 120 Hz, VRIPPLE = 200 mVrms; RL = 8Ω, Terminated 60 dB 1.0 W (min) SNR Signal to Noise Ratio VDD = 5V, POUT = 1.2W, RL = 8Ω, A-Wtd Filter, 1kHz 100 dB Xtalk Channel Separation f = 1kHz, CB = 1.0µF, 1W 76 dB Electrical Characteristics for Single-Ended Mode Operation (Notes 7, 10) The following specifications apply for VDD = 5V. Limits apply for TA = 25˚C. LM4938 Symbol Parameter Conditions PO Output Power THD = 1.0%; f = 1kHz; RL = 32Ω THD+N Total Harmonic Distortion + Noise VOUT = 1VRMS, f = 1kHz, RL = 10kΩ, AVD = 1 PSRR Power Supply Rejection Ratio Limit (Note 15) Units (Limits) 92 mW 0.065 % CB = 1.0 µF, f = 120 Hz, VRIPPLE = 200 mVrms, Floating 63 dB CB = 1.0 µF, f = 120 Hz, VRIPPLE = 200 mVrms, Terminated 59 dB 100 dB 73 dB SNR Signal to Noise Ratio POUT = 75mW, R A-Wtd Filter Xtalk Channel Separation f = 1kHz, CB = 1.0 µF www.national.com Typical (Note 14) 4 L = 32Ω, LM4938 Electrical Characteristics for Single-Ended Mode Operation (Notes 7, 10) (Continued) Note 3: The θJA given is for an MXA28A package whose exposed-DAP is soldered to an exposed 2in 2 piece of 1 ounce printed circuit board copper. Note 4: The θJA given is for an MXA28A package whose exposed-DAP is soldered to a 2in2 piece of 1 ounce printed circuit board copper on a bottom side layer through 21 8mil vias. Note 5: The θJA given is for an MXA28A package whose exposed-DAP is soldered to an exposed 1in 2 piece of 1 ounce printed circuit board copper. Note 6: The θJA given is for an MXA28A package whose exposed-DAP is not soldered to any copper. Note 7: All voltages are measured with respect to the ground pins, unless otherwise specified. All specifications are tested using the typical application as shown in Figure 1. Note 8: When driving 3Ω loads from a 5V supply the LM4938MH must be mounted to the circuit board and forced-air cooled. Note 9: When driving 4Ω loads from a 5V supply the LM4938MH must be mounted to the circuit board. Note 10: 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 11: The maximum power dissipation must be derated at elevated temperatures and is dictated by TJMAX, θ JA, and the ambient temperature TA. The maximum allowable power dissipation is PDMAX = (TJMAX − TA )/θJA. For the LM4938, TJMAX = 150˚C, and the typical junction-to-ambient thermal resistance for each package can be found in the Absolute Maximum Ratings section above. Note 12: Human body model, 100pF discharged through a 1.5kΩ resistor. Note 13: Machine Model, 200pF – 220pF discharged through all pins. Note 14: Typicals are measured at 25˚C and represent the parametric norm. Note 15: Limits are guaranteed to National’s AOQL ( Average Outgoing Quality Level). Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis. 5 www.national.com LM4938 Typical Application 20095603 FIGURE 2. Typical Application Circuit Truth Table for Logic Inputs (Note 16) Gain Sel Mode Headphone Sense Mute Shutdown Output Stage Set To DC Volume Output Stage Configuration 0 0 0 0 0 0 0 1 0 0 Internal Gain Fixed BTL Internal Gain Fixed 0 1 0 0 SE 0 Internal Gain Adjustable BTL 0 1 1 0 0 Internal Gain Adjustable SE 1 0 1 0 0 0 0 External Gain Fixed BTL 1 0 0 External Gain Fixed 1 1 SE 0 0 0 External Gain Adjustable BTL 1 1 1 0 0 External Gain Adjustable SE X X X 1 0 Muted X Muted X X X X 1 Shutdown X X Note 16: If system beep is detected on the Beep In pin, the system beep will be passed through the bridged amplifier regardless of the logic of the Mute and HP sense pins. www.national.com 6 LM4938 Typical Performance Characteristics THD+N vs Output Power VDD = 3V, RL = 8Ω THD+N vs Output Power VDD = 3V, RL = 4Ω 200956A2 200956A3 THD+N vs Output Power VDD = 5V, RL = 3Ω THD+N vs Output Power VDD = 3V, RL = 32Ω, SE 200956A4 200956A5 THD+N vs Output Power VDD = 5V, RL = 8Ω, BTL THD+N vs Output Power VDD = 5V, RL = 4Ω 200956A6 200956A7 7 www.national.com LM4938 Typical Performance Characteristics (Continued) THD+N vs Output Power VDD = 5V, RL = 32Ω, SE THD+N vs Frequency VDD = 5V, RL = 8Ω, PO = 1W, BTL 200956A8 20095695 THD+N vs Frequency VDD = 3V, RL = 8Ω, PO = 160mW THD+N vs Frequency VDD = 3V, RL = 4Ω, PO = 170mW 20095696 20095697 THD+N vs Frequency VDD = 5V, RL = 3Ω, PO = 600mW THD+N vs Frequency VDD = 3V, RL = 32Ω, PO = 20mW, SE 20095698 www.national.com 20095699 8 LM4938 Typical Performance Characteristics (Continued) THD+N vs Frequency VDD = 5V, RL = 4Ω, PO = 600mW THD+N vs Frequency VDD = 5V, RL = 32Ω, PO = 70mW, SE 200956A0 200956A1 Frequency Response VDD = 3V, RL = 8Ω, PO = 570mW Frequency Response VDD = 3V, RL = 4Ω, PO = 1.8W 20095636 20095637 Frequency Response VDD = 5V, RL = 3Ω, PO = 1.8W Frequency Response VDD = 3V, RL = 32Ω, PO = 30mW, SE 20095638 20095643 9 www.national.com LM4938 Typical Performance Characteristics (Continued) Frequency Response VDD = 5V, RL = 4Ω, PO = 1.5W Frequency Response VDD = 5V, RL = 8Ω, PO = 1W 20095644 20095645 PSRR vs Frequency VDD = 3V, RL = 8Ω, Terminated Frequency Response VDD = 5V, RL = 32Ω, PO = 30mW, SE 20095647 20095646 PSRR vs Frequency VDD = 3V, RL = 32Ω, Terminated PSRR vs Frequency VDD = 3V, RL = 8Ω, Unterminated 20095648 www.national.com 20095650 10 LM4938 Typical Performance Characteristics (Continued) PSRR vs Frequency VDD = 3V, RL = 32Ω, Unterminated PSRR vs Frequency VDD = 5V, RL = 8Ω, Terminated 20095655 200956B0 PSRR vs Frequency VDD = 5V, RL = 32Ω, Terminated, SE PSRR vs Frequency VDD = 5V, RL = 8Ω, Unterminated 20095667 20095668 Crosstalk vs Frequency VDD = 3V, RL = 8Ω, PO = 570mW PSRR vs Frequency VDD = 5V, RL = 32Ω, Unterminated, SE 20095669 20095605 11 www.national.com LM4938 Typical Performance Characteristics (Continued) Crosstalk vs Frequency VDD = 3V, RL = 32Ω, PO = 30mW, SE Crosstalk vs Frequency VDD = 5V, RL = 8Ω, PO = 1W 20095610 20095612 Crosstalk vs Frequency VDD = 5V, RL = 32Ω, PO = 30mW, SE Volume Control Characteristics 200956A9 20095613 Power Derating Curve (Note 17) Dropout Voltage 20095664 www.national.com 20095653 12 LM4938 Typical Performance Characteristics (Continued) External Gain/ Bass Boost Characteristics Power Dissipation vs Output Power 20095661 20095665 Power Dissipation vs Output Power Power Dissipation vs Output Power 20095651 20095652 13 www.national.com LM4938 Typical Performance Characteristics (Continued) Output Power vs Supply Voltage 20095654 Note 17: These curves show the thermal dissipation ability of the LM4938MH at different ambient temperatures given these conditions: 500LFPM + 2in2: The part is soldered to a 2in2, 1 oz. copper plane with 500 linear feet per minute of forced-air flow across it. 2in2on bottom: The part is soldered to a 2in2, 1oz. copper plane that is on the bottom side of the PC board through 21 8 mil vias. 2in2: The part is soldered to a 2in2, 1oz. copper plane. 1in2: The part is soldered to a 1in2, 1oz. copper plane. Not Attached: The part is not soldered down and is not forced-air cooled. www.national.com 14 EXPOSED-DAP PACKAGE PCB MOUNTING CONSIDERATIONS Poor power supply regulation adversely affects maximum output power. A poorly regulated supply’s output voltage decreases with increasing load current. Reduced supply voltage causes decreased headroom, output signal clipping, and reduced output power. Even with tightly regulated supplies, trace resistance creates the same effects as poor supply regulation. Therefore, making the power supply traces as wide as possible helps maintain full output voltage swing. The LM4938’s exposed-DAP (die attach paddle) package (MH) provides a low thermal resistance between the die and the PCB to which the part is mounted and soldered. This allows rapid heat transfer from the die to the surrounding PCB copper traces, ground plane and, finally, surrounding air. The result is a low voltage audio power amplifier that produces 2.0W at ≤ 1% THD with a 4Ω load. This high power is achieved through careful consideration of necessary thermal design. Failing to optimize thermal design may compromise the LM4938’s high power performance and activate unwanted, though necessary, thermal shutdown protection. The MH package must have its exposed DAP soldered to a grounded copper pad on the PCB. The DAP’s PCB copper pad is connected to a large grounded plane of continuous unbroken copper. This plane forms a thermal mass heat sink and radiation area. Place the heat sink area on either outside plane in the case of a two-sided PCB, or on an inner layer of a board with more than two layers. Connect the DAP copper pad to the inner layer or backside copper heat sink area with 32(4x8) (MH) vias. The via diameter should be 0.012in–0.013in with a 1.27mm pitch. Ensure efficient thermal conductivity by plating-through and solder-filling the vias. Best thermal performance is achieved with the largest practical copper heat sink area. If the heatsink and amplifier share the same PCB layer, a nominal 2.5in2 (min) area is necessary for 5V operation with a 4Ω load. Heatsink areas not placed on the same PCB layer as the LM4938 MH package should be 5in2 (min) for the same supply voltage and load resistance. The last two area recommendations apply for 25˚C ambient temperature. Increase the area to compensate for ambient temperatures above 25˚C. In systems using cooling fans, the LM4938MH can take advantage of forced air cooling. With an air flow rate of 450 linear-feet per minute and a 2.5in2 exposed copper or 5.0in2 inner layer copper plane heatsink, the LM4938MH can continuously drive a 3Ω load to full power. In all circumstances and conditions, the junction temperature must be held below 150˚C to prevent activating the LM4938’s thermal shutdown protection. The LM4938’s power de-rating curve in the Typical Performance Characteristics shows the maximum power dissipation versus temperature. Example PCB layouts for the exposed-DAP TSSOP are shown in the Demonstration Board Layout section. Further detailed and specific information concerning PCB layout, fabrication, and mounting a package is available in National Semiconductor’s AN1187. BRIDGE CONFIGURATION EXPLANATION As shown in Figure 2, the LM4938 output stage consists of two pairs of operational amplifiers, forming a two-channel (channel A and channel B) stereo amplifier. (Though the following discusses channel A, it applies equally to channel B.) Figure 2 shows that the first amplifier’s negative (-) output serves as the second amplifier’s input. This results in both amplifiers producing signals identical in magnitude, but 180˚ out of phase. Taking advantage of this phase difference, a load is placed between −OUTA and +OUTA and driven differentially (commonly referred to as “bridge mode”). This results in a differential gain of AVD = 2 * (Rf/R i) (1) Bridge mode amplifiers are different from single-ended amplifiers that drive loads connected between a single amplifier’s output and ground. For a given supply voltage, bridge mode has a distinct advantage over the single-ended configuration: its differential output doubles the voltage swing across the load. This produces four times the output power when compared to a single-ended amplifier under the same conditions. This increase in attainable output power assumes that the amplifier is not current limited or that the output signal is not clipped. To ensure minimum output signal clipping when choosing an amplifier’s closed-loop gain, refer to the Audio Power Amplifier Design section. Another advantage of the differential bridge output is no net DC voltage across the load. This is accomplished by biasing channel A’s and channel B’s outputs at half-supply. This eliminates the coupling capacitor that single supply, singleended amplifiers require. Eliminating an output coupling capacitor in a single-ended configuration forces a single-supply amplifier’s half-supply bias voltage across the load. This increases internal IC power dissipation and may permanently damage loads such as speakers. POWER DISSIPATION Power dissipation is a major concern when successful single-ended or bridged amplifier. states the maximum power dissipation point ended amplifier operating at a given supply driving a specified output load. PCB LAYOUT AND SUPPLY REGULATION CONSIDERATIONS FOR DRIVING 3Ω AND 4Ω LOADS Power dissipated by a load is a function of the voltage swing across the load and the load’s impedance. As load impedance decreases, load dissipation becomes increasingly dependent on the interconnect (PCB trace and wire) resistance between the amplifier output pins and the load’s connections. Residual trace resistance causes a voltage drop, which results in power dissipated in the trace and not in the load as desired. For example, 0.1Ω trace resistance reduces the output power dissipated by a 4Ω load from 2.1W to 2.0W. This problem of decreased load dissipation is exacerbated as load impedance decreases. Therefore, to maintain the PDMAX = (VDD)2/(2π2RL) designing a Equation (2) for a singlevoltage and Single-Ended (2) However, a direct consequence of the increased power delivered to the load by a bridge amplifier is higher internal power dissipation for the same conditions. 15 www.national.com LM4938 highest load dissipation and widest output voltage swing, PCB traces that connect the output pins to a load must be as wide as possible. Application Information LM4938 Application Information impedance.) Refer to the Typical Performance Characteristics curves for power dissipation information at lower output power levels. (Continued) The LM4938 has two operational amplifiers per channel. The maximum internal power dissipation per channel operating in the bridge mode is four times that of a single-ended amplifier. From Equation (3), assuming a 5V power supply and a 4Ω load, the maximum single channel power dissipation is 1.27W or 2.54W for stereo operation. PDMAX = 4 * (VDD)2/(2π2RL) Bridge 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 5V regulator typically use a 10 µF in parallel with a 0.1 µF filter capacitor to stabilize the regulator’s output, reduce noise on the supply line, and improve the supply’s transient response. However, their presence does not eliminate the need for a local 1.0µF tantalum bypass capacitance connected between the LM4938’s supply pins and ground. Do not substitute a ceramic capacitor for the tantalum. Doing so may cause oscillation. Keep the length of leads and traces that connect capacitors between the LM4938’s power supply pin and ground as short as possible. Connecting a 1µF capacitor, CB, between the BYPASS pin and ground improves the internal bias voltage’s stability and the amplifier’s PSRR. The PSRR improvements increase as the BYPASS pin capacitor value increases. Too large a capacitor, however, increases turn-on time and can compromise the amplifier’s click and pop performance. The selection of bypass capacitor values, especially CB, depends on desired PSRR requirements, click and pop performance (as explained in the following section, Selecting Proper External Components), system cost, and size constraints. (3) The LM4938’s power dissipation is twice that given by Equation (2) or Equation (3) when operating in the single-ended mode or bridge mode, respectively. Twice the maximum power dissipation point given by Equation (3) must not exceed the power dissipation given by Equation (4): PDMAX' = (TJMAX − TA)/θJA (4) The LM4938’s TJMAX = 150˚C. In the MH package soldered to a DAP pad that expands to a copper area of 2in2 on a PCB, the LM4938MH’s θJA is 41˚C/W. At any given ambient temperature TA, use Equation (4) to find the maximum internal power dissipation supported by the IC packaging. Rearranging Equation (4) and substituting PDMAX for PDMAX' results in Equation (5). This equation gives the maximum ambient temperature that still allows maximum stereo power dissipation without violating the LM4938’s maximum junction temperature. TA = TJMAX – 2*PDMAX θJA SELECTING PROPER EXTERNAL COMPONENTS Optimizing the LM4938’s performance requires properly selecting external components. Though the LM4938 operates well when using external components with wide tolerances, best performance is achieved by optimizing component values. The LM4938 is unity-gain stable, giving a designer maximum design flexibility. The gain should be set to no more than a given application requires. This allows the amplifier to achieve minimum THD+N and maximum signal-to-noise ratio. These parameters are compromised as the closed-loop gain increases. However, low gain circuits demand input signals with greater voltage swings to achieve maximum output power. Fortunately, many signal sources such as audio CODECs have outputs of 1VRMS (2.83VP-P). Please refer to the Audio Power Amplifier Design section for more information on selecting the proper gain. (5) For a typical application with a 5V power supply and an 4Ω load, the maximum ambient temperature that allows maximum stereo power dissipation without exceeding the maximum junction temperature is approximately 45˚C for the MH package. TJMAX = PDMAX θJA + TA (6) Equation (6) gives the maximum junction temperature TJMAX. If the result violates the LM4938’s 150˚C TJMAX, reduce the maximum junction temperature by reducing the power supply voltage or increasing the load resistance. Further allowance should be made for increased ambient temperatures. The above examples assume that a device is a surface mount part operating around the maximum power dissipation point. Since internal power dissipation is a function of output power, higher ambient temperatures are allowed as output power or duty cycle decreases. If the result of Equation (2) is greater than that of Equation (3), then decrease the supply voltage, increase the load impedance, or reduce the ambient temperature. If these measures are insufficient, a heat sink can be added to reduce θJA. The heat sink can be created using additional copper area around the package, with connections to the ground pin(s), supply pin and amplifier output pins. External, solder attached SMT heatsinks such as the Thermalloy 7106D can also improve power dissipation. When adding a heat sink, the θJA is the sum of θJC, θCS, and θSA. (θJC is the junction-to-case thermal impedance, θCS is the case-to-sink thermal impedance, and θSA is the sink-to-ambient thermal www.national.com INPUT CAPACITOR VALUE SELECTION Amplifying the lowest audio frequencies requires a high value input coupling capacitor (0.33µF in Figure 2), but high value capacitors 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 150 Hz. Applications using speakers with this limited frequency response reap little improvement by using a large input capacitor. Besides effecting system cost and size, the input coupling capacitor has an affect on the LM4938’s click and pop performance. When the supply voltage is first applied, a transient (pop) is created as the charge on the input capacitor changes from zero to a quiescent state. The magnitude of the pop is directly proportional to the input capacitor’s size. Higher value capacitors need more time to reach a quiescent DC voltage (usually VDD/2) when charged with a fixed current. The amplifier’s output charges the input capacitor 16 amplifiers can drive loads of > 1kΩ (such as powered speakers) with a rail-to-rail signal. Since the output signal present on the RIGHT DOCK and LEFT DOCK pins is biased to VDD/2, coupling capacitors should be connected in series with the load when using these outputs. Typical values for the output coupling capacitors are 0.33µF to 1.0µF. If polarized coupling capacitors are used, connect their "+" terminals to the respective output pin, see Figure 2. Since the DOCK outputs precede the internal volume control, the signal amplitude will be equal to the input signal’s magnitude and cannot be adjusted. However, the input amplifier’s closed-loop gain can be adjusted using external resistors. These 20k resistors (RFR, RFL) are shown in Figure 2 and they set each input amplifier’s gain to -1. Use Equation 7 to determine the input and feedback resistor values for a desired gain. (8) - AVR = RFR/RIR and - AVL = RFL/RIL (Continued) through the feedback resistor, Rf. Thus, pops can be minimized by selecting an input capacitor value that is no higher than necessary to meet the desired −6dB frequency. As shown in Figure 2, the input resistor (RIR, RIL = 20k) ( and the input capacitor (CIR, CIL = 0.33µF) produce a −6dB high pass filter cutoff frequency that is found using Equation (7). (7) As an example when using a speaker with a low frequency limit of 150Hz, the input coupling capacitor, using Equation (7), is 0.053µF. The 0.33µF input coupling capacitor shown in Figure 2 allows the LM4938 to drive a high efficiency, full range speaker whose response extends below 30Hz. Adjusting the input amplifier’s gain sets the minimum gain for that channel. Although the single ended output of the Bridge Output Amplifiers can be used to drive line level outputs, it is recommended that the R & L Dock Outputs simpler signal path be used for better performance. OPTIMIZING CLICK AND POP REDUCTION PERFORMANCE The LM4938 contains circuitry that minimizes turn-on and shutdown transients or “clicks and pops”. For this discussion, turn-on refers to either applying the power supply voltage or when the shutdown mode is deactivated. While the power supply is ramping to its final value, the LM4938’s internal amplifiers are configured as unity gain buffers. An internal current source changes the voltage of the BYPASS pin 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 1/2 VDD . As soon as the voltage on the BYPASS pin is stable, the device becomes fully operational. Although the BYPASS pin current cannot be modified, changing the size of CB alters the device’s turn-on time and the magnitude of “clicks and pops”. Increasing the value of CB reduces the magnitude of turn-on pops. However, this presents a tradeoff: as the size of CB increases, the turn-on time increases. There is a linear relationship between the size of CB and the turn-on time. BEEP DETECT FUNCTION Computers and notebooks produce a system “beep“ signal that drives a small speaker. The speaker’s auditory output signifies that the system requires user attention or input. To accommodate this system alert signal, the LM4938’s beep input pin is a mono input that accepts the beep signal. Internal level detection circuitry at this input monitors the beep signal’s magnitude. When a signal level greater than VDD/2 is detected on the BEEP IN pin, the bridge output amplifiers are enabled. The beep signal is amplified and applied to the load connected to the output amplifiers. A valid beep signal will be applied to the load even when MUTE is active. Use the input resistors connected between the BEEP IN pin and the stereo input pins to accommodate different beep signal amplitudes. These resistors (RBEEP) are shown as 200kΩ devices in Figure 2. Use higher value resistors to reduce the gain applied to the beep signal. The resistors must be used to pass the beep signal to the stereo inputs. The BEEP IN pin is used only to detect the beep signal’s magnitude: it does not pass the signal to the output amplifiers. The LM4938’s shutdown mode must be deactivated before a system alert signal is applied to BEEP IN pin. If the “Beep” feature is not needed, remove the two Beep Resistors (200k) and Beep input capacitor (.33µf). Then, tie the Beep input pin (#11) to ground. Note that the Beep Circuit is designed to operate with only a square wave input from a control source. DOCKING STATION INTERFACE Applications such as notebook computers can take advantage of a docking station to connect to external devices such as monitors or audio/visual equipment that sends or receives line level signals. The LM4938 has two outputs, Right Dock and Left Dock, which connect to outputs of the internal input amplifiers that drive the volume control inputs. These input 17 www.national.com LM4938 Application Information LM4938 Application Information (Continued) MICRO-POWER SHUTDOWN The voltage applied to the SHUTDOWN pin controls the LM4938’s shutdown function. Activate micro-power shutdown by applying VDD to the SHUTDOWN pin. When active, the LM4938’s micro-power shutdown feature turns off the amplifier’s bias circuitry, reducing the supply current. The logic threshold is typically VDD/2. The low 0.5 µA typical shutdown current is achieved by applying a voltage that is as near as VDD as possible to the SHUTDOWN pin. A voltage that is less than VDD 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 10kΩ pull-up resistor between the SHUTDOWN pin and VDD. Connect the switch between the SHUTDOWN pin and ground. Select normal amplifier operation by closing the switch. Opening the switch connects the SHUTDOWN pin to VDD through the pull-up resistor, 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 a microcontroller, use a digital output to apply the control voltage to the SHUTDOWN pin. Driving the SHUTDOWN pin with active circuitry eliminates the need for a pull up resistor. 20095604 FIGURE 3. Headphone Sensing Circuit MODE FUNCTION The LM4938’s MODE function has 2 states controlled by the voltage applied to the MODE pin. Mode 0, selected by applying 0V to the MODE pin, forces the LM4938 to effectively function as a "line-out," unity-gain amplifier. Mode 1, which uses the internal DC controlled volume control is selected by applying VDD to the MODE pin. This mode sets the amplifier’s gain according to the DC voltage applied to the DC VOL CONTROL pin. Unanticipated gain behavior can be prevented by connecting the MODE pin to VDD or ground. Note: Do not let the mode pin float. HP SENSE FUNCTION ( Head Phone In ) Applying a voltage between 4V and VDD to the LM4938’s HP-IN headphone control pin turns off the amps that drive the Left out "+" and Right out "+" pins. This action mutes a bridged-connected load. Quiescent current consumption is reduced when the IC is in this single-ended mode. Figure 3 shows the implementation of the LM4938’s headphone control function. With no headphones connected to the headphone jack, the R1-R2 voltage divider sets the voltage applied to the HP SENSE pin at approximately 50mV. This 50mV puts the LM4938 into bridged mode operation. The output coupling capacitor blocks the amplifier’s half supply DC voltage, protecting the headphones. The HP-IN threshold is set at 4V. While the LM4938 operates in bridged mode, the DC potential across the load is essentially 0V. Therefore, even in an ideal situation, the output swing cannot cause a false single-ended trigger. Connecting headphones to the headphone jack disconnects the headphone jack contact pin from R2 and allows R1 to pull the HP Sense pin up to VDD through R4. This enables the headphone function, turns off both of the "+" output amplifiers, and mutes the bridged speaker. The remaining single-ended amplifiers then drive the headphones, whose impedance is in parallel with resistors R2 and R3. These resistors have negligible effect on the LM4938’s output drive capability since the typical impedance of headphones is 32Ω. Figure 3 also shows the suggested headphone jack electrical connections. The jack is designed to mate with a threewire plug. The plug’s tip and ring should each carry one of the two stereo output signals, whereas the sleeve should carry the ground return. A headphone jack with one control pin contact is sufficient to drive the HP-IN pin when connecting headphones. A microprocessor or a switch can replace the headphone jack contact pin. When a microprocessor or switch applies a voltage greater than 4V to the HP-IN pin, a bridge-connected speaker is muted and the single ended output amplifiers 1A and 2A will drive a pair of headphones. MUTE FUNCTION The LM4938 mutes the amplifier and DOCK outputs when VDD is applied to the MUTE pin. Even while muted, the LM4938 will amplify a system alert (beep) signal whose magnitude satisfies the BEEP DETECT circuitry. Applying 0V to the MUTE pin returns the LM4938 to normal, unmuted operation. Prevent unanticipated mute behavior by connecting the MUTE pin to VDD or ground. Do not let the mute pain float. www.national.com 18 The result is increased bridge-amplifier gain at low frequencies. The combination of RLFE and CLFE form a -6dB corner frequency at (Continued) GAIN SELECT FUNCTION (Bass Boost) The LM4938 features selectable gain, using either internal or external feedback resistors. Either set of feedback resistors set the gain of the output amplifiers. The voltage applied to the GAIN SELECT pin controls which gain is selected. Applying VDD to the GAIN SELECT pin selects the external gain mode. Applying 0V to the GAIN SELECT pin selects the internally set unity gain. fC = 1/(2πRLFEC LFE) (9) The bridged-amplifier low frequency differential gain is: AVD = 2(RF + RLFE) / R In some cases a designer may want to improve the low frequency response of the bridged amplifier or incorporate a bass boost feature. This bass boost can be useful in systems where speakers are housed in small enclosures. A resistor, RLFE, and a capacitor, CLFE, in parallel, can be placed in series with the feedback resistor of the bridged amplifier as seen in Figure 4. i (10) Using the component values shown in Figure 1 (RF = 20kΩ, RLFE = 20kΩ, and CLFE = 0.068µF), a first-order, -6dB pole is created at 120Hz. Assuming R i = 20kΩ, the low frequency differential gain is 4. The input (Ci) and output (CO) capacitor values must be selected for a low frequency response that covers the range of frequencies affected by the desired bass-boost operation. DC VOLUME CONTROL The LM4938 has an internal stereo volume control whose setting is a function of the DC voltage applied to the DC VOL CONTROL pin. The LM4938 volume control consists of 31 steps that are individually selected by a variable DC voltage level on the volume control pin. The range of the steps, controlled by the DC voltage, are from 0dB - 89dB. Each gain step corresponds to a specific input voltage range, as shown in table 2. To minimize the effect of noise on the volume control pin, which can affect the selected gain level, hysteresis has been implemented. The amount of hysteresis corresponds to half of the step width, as shown in Volume Control Characterization Graph (DS200133-40). For highest accuracy, the voltage shown in the ’recommended voltage’ column of the table is used to select a desired gain. This recommended voltage is exactly halfway between the two nearest transitions to the next highest or next lowest gain levels. 20095611 The gain levels are 1dB/step from 0dB to -6dB, 2dB/step from -6dB to -36dB, 3dB/step from -36dB to -47dB, 4dB/step from -47db to -51dB, 5dB/step from -51dB to -66dB, and 12dB to the last step at -89dB. FIGURE 4. Low Frequency Enhancement At low, frequencies CLFE is a virtual open circuit and at high frequencies, its nearly zero ohm impedance shorts RLFE. 19 www.national.com LM4938 Application Information LM4938 Application Information (Continued) VOLUME CONTROL TABLE ( Table 2 ) Gain (dB) Voltage Range (% of Vdd) Low Voltage Range (Vdd = 3) Recommended Low High Recommended Low High Recommended 0 77.5% 100.00% 100.000% 3.875 5.000 5.000 2.325 3.000 3.000 -1 75.0% 78.5% 76.875% 3.750 3.938 3.844 2.250 2.363 2.306 -2 72.5% 76.25% 74.375% 3.625 3.813 3.719 2.175 2.288 2.231 -3 70.0% 73.75% 71.875% 3.500 3.688 3.594 2.100 2.213 2.156 -4 67.5% 71.25% 69.375% 3.375 3.563 3.469 2.025 2.138 2.081 -5 65.0% 68.75% 66.875% 3.250 3.438 3.344 1.950 2.063 2.006 -6 62.5% 66.25% 64.375% 3.125 3.313 3.219 1.875 1.988 1.931 -8 60.0% 63.75% 61.875% 3.000 3.188 3.094 1.800 1.913 1.856 -10 57.5% 61.25% 59.375% 2.875 3.063 2.969 1.725 1.838 1.781 -12 55.0% 58.75% 56.875% 2.750 2.938 2.844 1.650 1.763 1.706 -14 52.5% 56.25% 54.375% 2.625 2.813 2.719 1.575 1.688 1.631 -16 50.0% 53.75% 51.875% 2.500 2.688 2.594 1.500 1.613 1.556 -18 47.5% 51.25% 49.375% 2.375 2.563 2.469 1.425 1.538 1.481 -20 45.0% 48.75% 46.875% 2.250 2.438 2.344 1.350 1.463 1.406 -22 42.5% 46.25% 44.375% 2.125 2.313 2.219 1.275 1.388 1.331 -24 40.0% 43.75% 41.875% 2.000 2.188 2.094 1.200 1.313 1.256 -26 37.5% 41.25% 39.375% 1.875 2.063 1.969 1.125 1.238 1.181 -28 35.0% 38.75% 36.875% 1.750 1.938 1.844 1.050 1.163 1.106 -30 32.5% 36.25% 34.375% 1.625 1.813 1.719 0.975 1.088 1.031 -32 30.0% 33.75% 31.875% 1.500 1.688 1.594 0.900 1.013 0.956 -34 27.5% 31.25% 29.375% 1.375 1.563 1.469 0.825 0.937 0.881 -36 25.0% 28.75% 26.875% 1.250 1.438 1.344 0.750 0.862 0.806 -39 22.5% 26.25% 24.375% 1.125 1.313 1.219 0.675 0.787 0.731 -42 20.0% 23.75% 21.875% 1.000 1.188 1.094 0.600 0.712 0.656 -45 17.5% 21.25% 19.375% 0.875 1.063 0.969 0.525 0.637 0.581 -47 15.0% 18.75% 16.875% 0.750 0.937 0.844 0.450 0.562 0.506 -51 12.5% 16.25% 14.375% 0.625 0.812 0.719 0.375 0.487 0.431 -56.5 10.0% 13.75% 11.875% 0.500 0.687 0.594 0.300 0.412 0.356 -62.5 7.5% 11.25% 9.375% 0.375 0.562 0.469 0.225 0.337 0.281 -68.5 5.0% 8.75% 6.875% 0.250 0.437 0.344 0.150 0.262 0.206 -89 0.0% 6.25% 0.000% 0.000 0.312 0.000 0.000 0.187 0.000 www.national.com High Voltage Range (Vdd = 5) 20 (Continued) The last step in this design example is setting the amplifier’s −6dB 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 AUDIO POWER AMPLIFIER DESIGN Audio Amplifier Design: Driving 1W into an 8Ω Load The following are the desired operational parameters: Power Output: 1 WRMS Load Impedance: 8Ω Input Level: 1 VRMS Input Impedance: Bandwidth: 20 kΩ 100 Hz−20 kHz ± 0.25 dB fL = 100Hz/5 = 20Hz (14) fH = 20kHz x 5 = 100kHz (15) and an The design begins by specifying the minimum supply voltage necessary to obtain the specified output power. One way to find the minimum supply voltage is to use the Output Power vs Supply Voltage curve in the Typical Performance Characteristics section. Another way, using Equation (10), is to calculate the peak output voltage necessary to achieve the desired output power for a given load impedance. To account for the amplifier’s dropout voltage, two additional voltages, based on the Dropout Voltage vs Supply Voltage in the Typical Performance Characteristics curves, must be added to the result obtained by Equation (10). The result is Equation (11). As mentioned in the Selecting Proper External Components section, Ri (Right & Left) and Ci (Right & Left) create a highpass filter that sets the amplifier’s lower bandpass frequency limit. Find the input coupling capacitor’s value using Equation (14). Ci ≥ 1/(2πRifL) (16) 1/(2π*20kΩ*20Hz) = 0.397µF (17) The result is (11) VDD ≥ (VOUTPEAK+ (VODTOP + VODBOT)) Use a 0.39µF capacitor, the closest standard value. (12) The product of the desired high frequency cutoff (100kHz in this example) and the differential gain AVD, determines the upper passband response limit. With AVD = 3 and fH = 100kHz, the closed-loop gain bandwidth product (GBWP) is 300kHz. This is less than the LM4938’s 3.5MHz GBWP. With this margin, the amplifier can be used in designs that require more differential gain while avoiding performance,restricting bandwidth limitations. The Output Power vs Supply Voltage graph for an 8Ω load indicates a minimum supply voltage of 4.6V. This is easily met by the commonly used 5V supply voltage. The additional voltage creates the benefit of headroom, allowing the LM4938 to produce peak output power in excess of 1W without clipping or other audible distortion. The choice of supply voltage must also not create a situation that violates of maximum power dissipation as explained above in the Power Dissipation section. After satisfying the LM4938’s power dissipation requirements, the minimum differential gain needed to achieve 1W dissipation in an 8Ω load is found using Equation (12). Recommended Printed Circuit Board Layout The following figures show the recommended PC board layouts for the LM4938MH. This circuit is designed for use with an external 5V supply and 4Ω speakers. This circuit board is easy to use. Apply 5V and ground to the board’s VDD and GND pads, respectively. Connect 4Ω speakers between the board’s −OUTA and +OUTA and OUTB and +OUTB pads. (13) Thus, a minimum overall gain of 2.83 allows the LM4938’s to reach full output swing and maintain low noise and THD+N performance. 21 www.national.com LM4938 Application Information LM4938 Recommended Printed Circuit Board Layout (Continued) 20095682 FIGURE 5. Top Layer Silkscreen 20095683 FIGURE 6. Top Layer TSSOP www.national.com 22 LM4938 Recommended Printed Circuit Board Layout (Continued) 20095685 FIGURE 7. Inner Layer (2) 20095686 FIGURE 8. Inner Layer (3) 23 www.national.com LM4938 Recommended Printed Circuit Board Layout (Continued) 20095684 FIGURE 9. Bottom Layer TSSOP www.national.com 24 LM4938 Analog Audio LM4938 TSSOP Eval Board Assembly Part Number: 980011373-100 Revision: A Bill of Material Item Part Number Part Description Qty 1 551011373-001 LM4938 Eval Board PCB etch 001 1 10 482911373-001 LM4938 TSSOP 1 20 151911368-001 Cer Cap 0.068µF 50V 10% 1206 2 25 152911368-001 Tant Cap 0.1µF 10V 10% 3 Size = A 3216 CS, CS, CV 26 152911368-002 Tant Cap 0.33µF 10V 10% Size = A 3216 3 CIN 27 152911368-003 Tant Cap 1µF 16V 10% Size = A 3216 3 CB, CO1, CO2 28 152911368-004 Tant Cap 10µF 10V 10% Size = C 6032 1 CS1 29 152911368-005 Tant Cap 220µF 16V 10% 2 Size = D 7343 CoutL, R 30 472911368-001 Res 1.5K Ohm 1/8W 1% 1206 2 RL 31 472911368-002 Res 20K Ohm 1/8W 1% 1206 10 RIN(4), RF(2), RDOCK(2), RBS(2) 32 472911368-003 Res 100K Ohm 1/8W 1% 1206 2 RPU, RS 33 472911368-004 Res 200K Ohm 1/16W 1% 0603 2 RBEEP 40 131911368-001 Stereo Headphone Jack W/ Switch 1 Mouser # 161-3500 41 131911368-002 Slide Switch 4 mute, mode, Gain, Mouser # SD 10SP003 42 131911368-003 Potentiometer 1 Volume Control Mouser # 317-2090-100K 43 131911368-004 RCA Jack 3 Right-In, Beep-In, Left-In Mouser # 16PJ097 44 131911368-005 Banana Jack, Black 3 Mouser # ME164-6219 45 131911368-006 Banana Jack, Red 3 Mouser # ME164-6218 25 Ref Designator Remark CBS www.national.com LM4938 Stereo 2W Audio Power Amplifiers with DC Volume Control and Selectable Gain Physical Dimensions inches (millimeters) unless otherwise noted Exposed-DAP TSSOP Package Order Number LM4938MH NS Package Number MXA28A for Exposed-DAP TSSOP 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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