LM4875 750 mW Audio Power Amplifier with DC Volume Control and Headphone Switch General Description Applications The LM4875 is a mono bridged audio power amplifier with DC voltage volume control. The LM4875 is capable of delivering 750mW of continuous average power into an 8Ω load with less than 1% THD when powered by a 5V power supply. Switching between bridged speaker mode and headphone (single ended) mode is accomplished using the headphone sense pin. To conserve power in portable applications, the LM4875’s micropower shutdown mode (IQ = 0.7µA, typ) is activated when less than 300mV is applied to the DC Vol/SD pin. Boomer audio power amplifiers are designed specifically to provide high power audio output while maintaining high fidelity. They require few external components and operate on low supply voltages. n GSM phones and accessories, DECT, office phones n Hand held radio n Other portable audio devices Key Specifications j PO at 1.0% THD+N into 8Ω 1W (typ) j Shutdown current 0.7µA(typ) j Supply voltage range 2.7V to 5.5V Features n n n n n Typical Application 750mW (typ) j PO at 10% THD+N into 8Ω Precision DC voltage volume control Headphone amplifier mode “Click and pop” suppression Shutdown control when volume control pin is low Thermal shutdown protection Connection Diagram Small Outline Package (SO) Mini Small Outline Package (MSOP) 10104202 Top View Order Number LM4875M, LM4875MM See NS Package Number M08A, MUA08A 10104201 FIGURE 1. Typical Audio Amplifier Application Circuit BOOMER™ is a trademark of National Semiconductor Corporation. © 2004 National Semiconductor Corporation DS101042 www.national.com LM4875 750 mW Audio Power Amplifier with DC Volume Control and Headphone Switch January 2002 LM4875 Absolute Maximum Ratings (Note 2) Thermal Resistanc If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. θJC (SOP) 35˚C/W θJA (SOP) 150˚C/W θJC (MSOP) 56˚C/W Supply Voltage θJA (MSOP) 190˚C/W 6.0V Storage Temperature Input Voltage −65˚C to +150˚C −0.3V to VDD +0.3V Operating Ratings Power Dissipation (Note 3) Internally Limited Temperature Range ESD Susceptibility (Note 4) 2000V TMIN ≤ TA ≤ TMAX ESD Susceptibility (Note 5) 200V Junction Temperature 150˚C 215˚C Infrared (15 sec.) 220˚C 2.7V ≤ VDD ≤ 5.5V Supply Voltage See AN-450 “Surface Mounting and their Effects on Product Reliability” for other methods of soldering surface mount devices. Soldering Information Vapor Phase (60 sec.) −40˚C ≤ TA ≤ +85˚C Electrical Characteristics (Notes 1, 2) The following specifications apply for VDD = 5V, unless otherwise specified. Limits apply for TA = 25˚C. LM4875 Symbol Parameter Conditions Min (Note 7) Typical (Note 6) Max (Note 7) Units 5.5 V VDD Supply Voltage IDD Quiescent Power Supply Current VIN = 0V, IO = 0A, HP Sense = 0V 4 7 mA VIN = 0V, IO - 0A, HP Sense = 5V 3.5 6 mA ISD Shutdown Current VPIN4 ≤ 0.3V 0.7 VOS Output Offset Voltage VIN = 0V 2.7 5 THD = 1% (max), HP Sense < 0.8V, f = 1kHz, RL = 8Ω PO Output Power 500 µA 50 mV 750 mW THD = 10% (max), HP Sense < 0.8V, f = 1kHz, RL = 8Ω 1.0 W THD + N = 1%, HP Sense > 4V, f = 1kHz, RL = 32Ω 80 mW THD = 10%, HP Sense > 4V, f = 1kHz, RL = 32Ω 110 mW THD+N Total Harmonic Distortion + Noise PO = 300 mWrms, f = 20Hz–20kHz, RL = 8Ω 0.6 % PSRR Power Supply Rejection Ratio VRIPPLE = 200mVrms, RL = 8Ω, CB = 1.0 µF, f = 1kHz 50 dB GainRANGE Single-Ended Gain Range VIH HP Sense High Input Voltage VIL HP Sense Low Input Voltage Gain with VPIN4 ≥ 4.0V, (80% of VDD) 18.8 20 dB Gain with VPIN4 ≤ 0.9V, (20% of VDD) −70 −72 dB 4 V 0.8 V Note 1: All voltages are measured with respect to the ground 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 operates within the Operating Ratings. Specifications are not guaranteed for parameters where no limit is given. The typical value, however, 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 the Absolute Maximum Ratings, whichever is lower. For the LM4875M, TJMAX = 150˚C. 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: The quiescent power supply current depends on the offset voltage when a practical load is connected to the amplifier. www.national.com 2 LM4875 External Components Description (Figure 1) Components Functional Description 1. Ci Input coupling capacitor blocks DC voltage at the amplifier’s input terminals. It also creates a highpass filter with the internal Ri that produces an fc = 1/(2πRiCi) (10kΩ ≤ Ri ≤ 100kΩ). Refer to the Application Information section, Selecting External Components, for an explanation of determining the value of Ci. 2. CS The supply bypass capacitor. Refer to the Power Supply Bypassing section for information about properly placing, and selecting the value of, this capacitor. 3. CB The capacitor, CB, filters the half-supply voltage present on the BYPASS pin. Refer to the Application Information section, Selecting External Components, for information concerning proper placement and selecting CB’s value. Typical Performance Characteristics THD+N vs Frequency THD+N vs Frequency 10104205 10104206 THD+N vs Output Power THD+N vs Output Power 10104207 10104208 3 www.national.com LM4875 Typical Performance Characteristics (Continued) THD+N vs Output Power THD+N vs Output Power 10104211 10104210 Power Dissipation vs Load Resistance Power Dissipation vs Output Power 10104213 10104212 Power Derating Curve Clipping Voltage vs RL 10104214 www.national.com 10104215 4 LM4875 Typical Performance Characteristics (Continued) Frequency Response vs Input Capacitor Size Noise Floor 10104217 10104216 Power Supply Rejection Ratio Attenuation Level vs DC-Vol Amplitude 10104218 10104219 THD+N vs Frequency THD+N vs Frequency 10104220 10104221 5 www.national.com LM4875 Typical Performance Characteristics (Continued) THD+N vs Frequency THD+N vs Output Power 10104222 10104223 THD+N vs Output Power THD+N vs Output Power 10104224 10104228 Output Power vs Load Resistance Clipping Voltage vs Supply Voltage 10104230 10104229 www.national.com 6 LM4875 Typical Performance Characteristics (Continued) Output Power vs Supply Voltage Output Power vs Supply Voltage 10104232 10104231 Supply Current vs Supply Voltage 10104233 mum 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 load. This results from biasing VO1 and VO2 at half-supply. This eliminates the coupling capacitor that single supply, single-ended 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. The current flow created by the halfsupply bias voltage increases internal IC power dissipation and may permanently damage loads such as speakers. Application Information BRIDGE CONFIGURATION EXPLANATION As shown in Figure 1, the LM4875 consists of two operational amplifiers internally. An external DC voltage sets the closed-loop gain of the first amplifier, whereas two internal 20kΩ resistors set the second amplifier’s gain at -1. The LM4875 can be used to drive a speaker connected between the two amplifier outputs or a monaural headphone connected between VO1 and GND. Figure 1 shows that the output of Amp1 serves as the input to Amp2. This results in both amplifiers producing signals that are identical in magnitude, but 180˚ out of phase. Taking advantage of this phase difference, a load placed between VO1 and VO2 is driven differentially (commonly referred to as “bridge mode“ ). This mode is different from single-ended driven loads that are connected between a single amplifier’s output and ground. Bridge mode has a distinct advantage over the single-ended configuration: its differential drive to the load doubles the output swing for a specified supply voltage. This results in four times the output power when compared to a singleended amplifier under the same conditions. This increase in attainable output assumes that the amplifier is not current limited or the output signal is not clipped. To ensure mini- POWER DISSIPATION Power dissipation is a major concern when designing a successful bridged or single-ended amplifier. Equation (1) states the maximum power dissipation point for a singleended amplifier operating at a given supply voltage and driving a specified output load. (1) PDMAX = (VDD)2/(2π2RL) Single-Ended However, a direct consequence of the increased power delivered to the load by a bridge amplifier is an increase in internal power dissipation point for a bridge amplifier operating at the same given conditions. (2) PDMAX = 4*(VDD)2/(2π2RL) Bridge Mode The LM4875 has two operational amplifiers in one package and the maximum internal power dissipation is 4 times that 7 www.national.com LM4875 Application Information actual DC voltage that produces the volume. Therefore, the accuracy of the volume control is not critical, as long as volume changes monotonically and step size is small enough to reach a desired volume that is not too loud or too soft. Since gain accuracy is not critical, there may be a volume variation from part-to-part even with the same applied DC control voltage. The gain of a given LM4875 can be set with a fixed external voltage, but another LM4875 may require a different control voltage to achieve the same gain. Figure 2 is a curve showing the volume variation of seven typical LM4875s as the voltage applied to the DC Vol/SD pin is varied. For gains between -20dB and +16dB, the typical part-to-part variation is typically ± 1dB for a given control voltage. (Continued) of a single-ended amplifier. However, even with this substantial increase in power dissipation, the LM4875 does not require heatsinking. From Equation (2), assuming a 5V power supply and an 8Ω load, the maximum power dissipation point is 633 mW. The maximum power dissipation point obtained from Equation (2) must not be greater than the power dissipation that results from Equation (3): (3) PDMAX = (TJMAX–TA)/θJA For the SO package, θJA = 150˚C/W. The MSOP package has a 190˚C/W θJA. TJMAX = 150˚C for the LM4875. For a given ambient temperature TA, Equation (3) can be used to find the maximum internal power dissipation supported by the IC packaging. If the result of Equation (2) is greater than that of Equation (3), then either decrease the supply voltage, increase the load impedance, or reduce the ambient temperature. For a typical application using the SO packaged LM4875, a 5V power supply, and an 8Ω load, the maximum ambient temperature that does not violate the maximum junction temperature is approximately 55˚C. The maximum ambient temperature for the MSOP package with the same conditions is approximately 30˚C. These results further 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 decreases. Refer to the Typical Performance Characteristics curves for power dissipation information at lower output power levels. 10104248 POWER SUPPLY BYPASSING As with any power amplifier, proper supply bypassing is critical for low noise performance and high power supply rejection. The capacitors connected to the bypass and power supply pins should be placed as close to the LM4875 as possible. The capacitor connected between the bypass pin and ground improves the internal bias voltage’s stability, producing improved PSRR. The improvements to PSRR increase as the bypass pin capacitor value increases. Typical applications employ a 5V regulator with 10µF and a 0.1µF filter capacitors that aid in supply stability. Their presence, however does not eliminate the need for bypassing the supply nodes of the LM4875. The selection of bypass capacitor values, especially CB, depends on desired PSRR requirements, click and pop performance (as explained in the section, Proper Selection of External Components), system cost, and size constraints. FIGURE 2. Typical part-to-part gain variation as a function of DC-Vol control voltage MUTE AND SHUTDOWN FUNCTION The LM4875’s mute and shutdown functions are controlled through the DC Vol/SD pin. Mute is activated by applying a voltage in the range of 500mV to 1V. A typical attenuation of 75dB is achieved is while mute is active. The LM4875’s micropower shutdown mode turns off the amplifier’s bias circuitry. The micropower shutdown mode is activated by applying less than 300mVDC to the DC Vol/SD pin. When shutdown is active, they supply current is reduced to 0.7µA (typ). A degree of uncertainty exists when the voltage applied to the DC Vol/SD pin is in the range of 300mV to 500mV. The LM4875 can be in mute, still fully powered, or in micropower shutdown and fully muted. In mute mode, the LM4875 draws the typical quiescent supply current. The DC Vol/SD pin should be tied to GND for best shutdown mode performance. As the DC Vol/SD is increased above 0.5V the amplifier will follow the attenuation curve in Typical Performance Characteristics. DC VOLTAGE VOLUME CONTROL The LM4875’s internal volume control is controlled by the DC voltage applied its DC Vol/SD pin (pin 4). The volume control’s input range is from GND to VDD. A graph showing a typical volume response versus input control voltage is shown in the Typical Performance Characteristicssection. The DC Vol/SD pin also functions as the control pin for the LM4875’s micropower shutdown feature. See the Mute and Shutdown Function section for more information. Like all volume controls, the Lm4875’s internal volume control is set while listening to an amplified signal that is applied to an external speaker. The actual voltage applied to the DC Vol/SD pin is a result of the volume a listener desires. As such, the volume control is designed for use in a feedback system that includes human ears and preferences. This feedback system operates quite well without the need for accurate gain. The user simply sets the volume to the desired level as determined by their ear, without regard to the www.national.com HP-Sense FUNCTION Applying a voltage between 4V and VCC to the LM4875’s HP-Sense headphone control pin turns off Amp2 and 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 LM4875’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 (pin 3) at approximately 50mV. This 50mV enables the LM4875 and places it in bridged mode operation. 8 As the volume changes from minimum to maximum, RIN decrease from 110kΩ to 10kΩ. Equation (4) reveals that the -3dB frequency will increase as the volume increases. The nominal value of Ci for lowest desired frequency response should be calculated with RIN = 10kΩ . As an example when using a speaker with a low frequency limit of 150Hz, Ci, using Equation (4) is 0.1µF. The 0.22µF Ci shown in Figure 1 is optimized for a speaker whose response extends down to 75Hz. (Continued) Bypass Capacitor Value Selection Besides minimizing the input capacitor size, careful consideration should be paid to value of the bypass capacitor CB. Since CB determines how fast the LM4875 turns on, its value is the most critical when minimizing turn-on pops. The slower the LM4875’s outputs ramp to their quiescent DC voltage (nominally VDD/2), the smaller the turn-on pop. Choosing CB equal to 1.0µF, along with a small value of Ci (in the range of 0.1µF to 0.39µF), produces a clickless and popless shutdown function. Choosing Ci as small as possible helps minimize clicks and pops. 10104234 FIGURE 3. Headphone Circuit While the LM4875 operates in bridged mode, the DC potential across the load is essentially 0V. Since the HP-Sense threshold is set at 4V, 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 VO1 and allows R1 to pull the HP Sense pin up to VCC. This enables the headphone function, turns off Amp2, and mutes the bridged speaker. The amplifier then drives the headphones, whose impedance is in parallel with resistor R2. Resistor R2 has negligible effect on output drive capability since the typical impedance of headphones is 32Ω. The output coupling capacitor blocks the amplifier’s half supply DC voltage, protecting the 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 Sense pin, a bridgeconnected speaker is muted and Amp1 drives the headphones. CLICK AND POP CIRCUITRY The LM4875 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 LM4875’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 and the gain is set by the external voltage applied to the DC Vol/SD pin. 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. Shown below are some typical turn-on times for various values of CB: SELECTING EXTERNAL COMPONENTS Optimizing the LM4875’s performance requires properly selecting external components. Though the LM4875 operates well when using external components having wide tolerances, the best performance is achieved by optimizing component values. Input Capacitor Value Selection Amplification of the lowest audio frequencies requires high value input coupling capacitors. These 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 150Hz. In application 5 using speakers with this limited frequency response, a large input capacitor will offer little improvement in system performance. Figure 1 shows that the nominal input impedance (RIN) is 10kΩ at maximum volume and 110kΩ at minimum volume. Together, the input capacitor, Ci, and RIN, produce a -3dB high pass filter cutoff frequency that is found using Equation (4). CB TON 0.01µF 3ms 0.1µF 30ms 0.22µF 65ms 0.47µF 135ms 1.0µF 280ms In order eliminate "clicks and pops", all capacitors must be discharged before turn-on. Rapidly switching VDD may not allow the capacitors to fully discharge, which may cause "clicks and pops". In a single-ended configuration, the output coupling capacitor, COUT, is of particular concern. This capacitor discharges through an internal 20kΩ resistor. Depending on the size of COUT, the time constant can be relatively large. To reduce transients in single-ended mode, (4) 9 www.national.com LM4875 Application Information LM4875 Application Information (Continued) an external 1kΩ - 5kΩ resistor can be placed in parallel with the internal 20kΩ resistor. The tradeoff for using this resistor is increased quiescent current. RECOMMENDED PRINTED CIRCUIT BOARD LAYOUT Figure 4 through Figure 6 show the recommended two-layer PC board layout that is optimized for the SO-8 packaged LM4875 and associated external components. Figure 7 through Figure 9 show the recommended two-layer PC board layout for the MSOP packaged LM4875. Both layouts are designed for use with an external 5V supply, 8Ω speakers, and 8Ω - 32Ω headphones. The schematic for both recommended PC board layouts is Figure 1. Both circuit boards are easy to use. Apply a 5V supply voltage and ground to the board’s VDD and GND pads, respectively. Connect a speaker with an 8Ω minimum impedance between the board’s -OUT and +OUT pads. For headphone use, the layout has provisions for a headphone jack, J1. When a jack is connected as shown, inserting a headphone plug automatically switches off the external speaker. 10104239 FIGURE 5. Recommended SO PC board layout: component side layout 10104238 FIGURE 4. Recommended SO PC board layout: component side silkscreen 10104240 FIGURE 6. Recommended SO PC board layout: bottom side layout www.national.com 10 LM4875 Application Information (Continued) 10104245 FIGURE 9. Recommended MSOP PC board layout: bottom side layout 10104241 FIGURE 7. Recommended MSOP PC board layout: component side silkscreen 10104242 FIGURE 8. Recommended MSOP PC board layout: component side layout 11 www.national.com LM4875 Physical Dimensions inches (millimeters) unless otherwise noted Order Number LM4875M NS Package Number M08A 8-Lead (0.118" Wide) Molded Mini Small Outline Package Order Number LM4875MM NS Package Number MUAO8A www.national.com 12 National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications. For the most current product information visit us at www.national.com. LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. 2. A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. BANNED SUBSTANCE COMPLIANCE National Semiconductor certifies that the products and packing materials meet the provisions of the Customer Products Stewardship Specification (CSP-9-111C2) and the Banned Substances and Materials of Interest Specification (CSP-9-111S2) and contain no ‘‘Banned Substances’’ as defined in CSP-9-111S2. National Semiconductor Americas Customer Support Center Email: [email protected] Tel: 1-800-272-9959 www.national.com National Semiconductor Europe Customer Support Center Fax: +49 (0) 180-530 85 86 Email: [email protected] Deutsch Tel: +49 (0) 69 9508 6208 English Tel: +44 (0) 870 24 0 2171 Français Tel: +33 (0) 1 41 91 8790 National Semiconductor Asia Pacific Customer Support Center Email: [email protected] National Semiconductor Japan Customer Support Center Fax: 81-3-5639-7507 Email: [email protected] Tel: 81-3-5639-7560 LM4875 750 mW Audio Power Amplifier with DC Volume Control and Headphone Switch Notes