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LM4809 Dual 105mW Headphone Amplifier with Active-Low Shutdown Mode
February 2001
LM4809 Dual 105mW Headphone Amplifier with Active-Low Shutdown Mode
General Description
The LM4809 is a dual audio power amplifier capable of delivering 105mW per channel of continuous average power into a 16 load with 0.1% (THD+N) from a 5V power supply. Boomer audio power amplifiers were designed specifically to provide high quality output power with a minimal amount of external components. Since the LM4809 does not require bootstrap capacitors or snubber networks, it is optimally suited for low-power portable systems. The unity-gain stable LM4809 can be configured by external gain-setting resistors. The LM4809 features an externally controlled, active-low, micropower consumption shutdown mode, as well as an internal thermal shutdown protection mechanism.
Key Specifications
n THD+N at 1kHz at 105mW continuous average power into 16 0.1% (typ) n THD+N at 1kHz at 70mW continuous average power into 32 0.1% (typ) n Shutdown Current 0.4A (typ)
Features
n n n n n n Active-low shutdown mode 'Click and Pop' reduction circuitry Low shutdown current MSOP surface mount packaging No bootstrap capacitors required Unity-gain stable
Applications
n n n n Headphone Amplifier Personal Computers Microphone Preamplifier PDA's
Typical Application
Connection Diagram
MSOP Package
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Top View Order Number LM4809MM See NS Package Number MUA08A
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*Refer to the Application Information Section for information concerning proper selection of the input and output coupling capacitors.
FIGURE 1. Typical Audio Amplifier Application Circuit
Boomer (R) is a registered trademark of National Semiconductor Corporation.
(c) 2001 National Semiconductor Corporation
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LM4809
Absolute Maximum Ratings (Note 2)
If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Supply Voltage Storage Temperature ESD Susceptibility (Note 5) ESD Machine model (Note 6) Junction Temperature (TJ) Soldering Information (Note 1) Small Outline Package Vapor Phase (60 sec.) 6.0V -65C to +150C 3.5kV 250V 150C
Infrared (15 sec.) Thermal Resistance JA (MSOP) JC (MSOP)
220C 210C/W 56C/W
Operating Ratings
Temperature Range TMIN TA TMAX Supply Voltage (VCC) -40C T A 85C 2.0V VCC 5.5V
Note 1: See AN-450 "Surface Mounting and their Effects on Product Reliability" for other methods of soldering surface mount devices.
215C
Electrical Characteristics (Notes 2, 3)
The following specifications apply for VDD = 5V unless otherwise specified, limits apply to TA = 25C. Symbol Parameter Conditions LM4809 Typ (Note 5) VDD IDD ISD VOS PO Supply Voltage Supply Current Shutdown Current Output Offset Voltage Output Power VIN = 0V, IO = 0A VIN = 0V VIN = 0V THD+N = 0.1%, f = 1kHz RL = 16 RL = 32 THD+N Crosstalk PSRR Total Harmonic Distortion Channel Separation Power Supply Rejection Ratio PO = 50mW, RL = 32 f = 20Hz to 20kHz RL = 32; PO = 70mW CB = 1.0F; VRIPPLE = 200mVPP, f = 1kHz; Input terminated into 50 105 70 0.3 70 70 65 mW mW (min) % dB dB 1.4 0.4 4.0 Limit (Note 7) 2.0 5.5 3 2 50 V (min) V (max) mA (max) A(max) mV(max) Units (Limits)
Electrical Characteristics (Notes 2, 3)
The following specifications apply for VDD = 3.3V unless otherwise specified, limits apply to TA = 25C. Symbol Parameter Conditions LM4809 Typ (Note 5) IDD ISD VOS PO Supply Current Shutdown Current Output Offset Voltage Output Power VIN = 0V, IO = 0A VIN = 0V VIN = 0V THD+N = 0.1%, f = 1kHz RL = 16 RL = 32 THD+N Crosstalk PSRR Total Harmonic Distortion Channel Separation Power Supply Rejection Ratio PO = 25mW, RL = 32 f = 20Hz to 20kHz RL = 32; PO = 25mW CB = 1.0F; VRIPPLE = 200mVPP, f = 1kHz; Input terminated into 50 40 28 0.4 70 70 mW mW % db dB 1.1 0.4 4.0 Limit (Note 7) mA A mV Units (Limits)
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LM4809
Electrical Characteristics (Notes 2, 3)
The following specifications apply for VDD = 2.6V unless otherwise specified, limits apply to TA = 25C. Symbol Parameter Conditions LM4809 Typ (Note 5) IDD ISD VOS PO Supply Current Shutdown Current Output Offset Voltage Output Power VIN = 0V, IO = 0A VIN = 0V VIN = 0V THD+N = 0.1%, f = 1kHz RL = 16 RL = 32 THD+N Crosstalk PSRR Total Harmonic Distortion Channel Separation Power Supply Rejection Ratio PO = 15mW, RL = 32 f = 20Hz to 20kHz RL = 32; PO = 15mW CB = 1.0F; VRIPPLE = 200mVPP, f = 1kHz; Input terminated into 50 20 16 0.6 70 70 mW mW % db dB 0.9 0.2 4.0 Limit (Note 7) mA A mV Units (Limits)
Note 2: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Note 3: Operating Ratings indicate conditions for which the device is functional, but do not guarantee specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics. The guaranteed specifications apply only for the test conditions listed. Some performance characteristics may degrade when the device is not operated under the listed test conditions. Note 4: Human body model, 100pF discharged through a 1.5k resistor. Note 5: Typical specifications are specified at +25OC and represent the most likely parametric norm. Note 6: Tested limits are guaranteed to National's AOQL (Average Outgoing Quality Level). Note 7: Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis. Note 8: Machine Model ESD test is covered by specification EIAJ IC-121-1981. A 200pF cap is charged to the specified voltage, then discharged directly into the IC with no external series resistor (resistance of discharge path must be under 50Ohms).
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LM4809
External Components Description
Components 1. Ri 2. Ci 3. Rf 4. CS 5. CB 6. CO
(Figure 1) Functional Description
The inverting input resistance, along with Rf, set the closed-loop gain. Ri, along with Ci, form a high pass filter with fc = 1/(2RiCi). The input coupling capacitor blocks DC voltage at the amplifier's input terminals. Ci, along with Ri, create a highpass filter with fC = 1/(2RiCi). Refer to the section, Selecting Proper External Components, for an explanation of determining the value of Ci. The feedback resistance, along with Ri, set closed-loop gain. This is the supply bypass capacitor. It provides power supply filtering. Refer to the Application Information section for proper placement and selection of the supply bypass capacitor. This is the BYPASS pin capacitor. It provides half-supply filtering. Refer to the section, Selecting Proper External Components, for information concerning proper placement and selection of CB. This is the output coupling capacitor. It blocks the DC voltage at the amplifier's output and forms a high pass filter with RL at fO = 1/(2RLCO)
Typical Performance Characteristics
THD+N vs Frequency THD+N vs Frequency THD+N vs Frequency
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THD+N vs Frequency
THD+N vs Frequency
THD+N vs Frequency
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LM4809
Typical Performance Characteristics
THD+N vs Frequency
(Continued) THD+N vs Frequency
THD+N vs Frequency
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THD+N vs Frequency
THD+N vs Output Power
THD+N vs Output Power
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THD+N vs Output Power
THD+N vs Output Power
THD+N vs Output Power
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LM4809
Typical Performance Characteristics
THD+N vs Output Power
(Continued) THD+N vs Output Power
THD+N vs Output Power
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THD+N vs Output Power
Output Power vs Load Resistance
Output Power vs Load Resistance
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Output Power vs Load Resistance
Output Power vs Supply Voltage
Output Power vs Power Supply
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LM4809
Typical Performance Characteristics
Output Power vs Power Supply
(Continued)
Dropout Voltage vs Supply Voltage
Power Dissipation vs Output Power
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Power Dissipation vs Output Power
Power Dissipation vs Output Power
Channel Separation
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Noise Floor
Power Supply Rejection Ratio
Open Loop Frequency Response
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LM4809
Typical Performance Characteristics
Open Loop Frequency Response
(Continued)
Open Loop Frequency Response
Supply Current vs Supply Voltage
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Application Information
MICRO-POWER SHUTDOWN The voltage applied to the SHUTDOWN pin controls the LM4809's shutdown function. Activate micro-power shutdown by applying a logic low voltage to the SHUTDOWN pin. The logic threshold is typically VDD/2. When active, the LM4809's micro-power shutdown feature turns off the amplifier's bias circuitry, reducing the supply current. The low 0.4A typical shutdown current is achieved by applying a voltage that is as near as GND as possible to the SHUTDOWN pin. A voltage that is above GND may increase the shutdown current. There are a few ways to control the micro-power shutdown. These include using a single-pole, single-throw switch, a microprocessor, or a microcontroller. When using a switch, connect an external 100k pull-down resistor between the SHUTDOWN pin and GND. Connect the switch between the SHUTDOWN pin and VDD. Select normal amplifier operation by closing the switch. Opening the switch connects the SHUTDOWN pin to GND through the pull-down 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 pull-down resistor. POWER DISSIPATION Power dissipation is a major concern when using any power amplifier and must be thoroughly understood to ensure a successful design. Equation 1 states the maximum power dissipation point for a single-ended amplifier operating at a given supply voltage and driving a specified output load. PDMAX = (VDD)
2
per amplifier. Thus the maximum package dissipation point is 80mW. The maximum power dissipation point obtained must not be greater than the power dissipation that results from Equation 2: PDMAX = (TJMAX - TA) / JA (2)
For package MUA08A, JA = 210C/W. TJMAX = 150C for the LM4809. Depending on the ambient temperature, TA, of the system surroundings, Equation 2 can be used to find the maximum internal power dissipation supported by the IC packaging. If the result of Equation 1 is greater than that of Equation 2, then either the supply voltage must be decreased, the load impedance increased or TA reduced. For the typical application of a 5V power supply, with a 32 load, the maximum ambient temperature possible without violating the maximum junction temperature is approximately 133.2C provided that device operation is around the maximum power dissipation point. Power dissipation is a function of output power and thus, if typical operation is not around the maximum power dissipation point, the ambient temperature may be increased accordingly. Refer to the Typical Performance Characteristics curves for power dissipation information for lower output powers. 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 10F in parallel with a 0.1F filter capacitors 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 1.0F tantalum bypass capacitance connected between the LM4809's supply pins and ground. Keep the length of leads and traces that connect capacitors between the LM4809's power supply pin and ground as short as possible. Connecting a 4.7F capacitor, CB, between the BYPASS pin and ground improves the internal bias voltage's stability and improves the amplifier's PSRR. The PSRR improvements increase as the bypass pin capacitor value increases. Too large, however, increases the amplifier's turn-on time. The selection of bypass capacitor values, especially CB, depends on desired PSRR requirements, click and pop performance (as explained in the section, Selecting Proper External Components), system cost, and size constraints.
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/ (22RL)
(1)
Since the LM4809 has two operational amplifiers in one package, the maximum internal power dissipation point is twice that of the number which results from Equation 1. Even with the large internal power dissipation, the LM4809 does not require heat sinking over a large range of ambient temperature. From Equation 1, assuming a 5V power supply and a 32 load, the maximum power dissipation point is 40mW
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LM4809
Application Information
(Continued)
SELECTING PROPER EXTERNAL COMPONENTS Optimizing the LM4809's performance requires properly selecting external components. Though the LM4809 operates well when using external components with wide tolerances, best performance is achieved by optimizing component values. The LM4809 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 demands 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. Input and Output Capacitor Value Selection Amplifying the lowest audio frequencies requires high value input and output coupling capacitors (CI and CO in Figure 1). A high value capacitor can be expensive and may compromise space efficiency in portable designs. In many cases, however, the speakers used in portable systems, whether internal or external, have little ability to reproduce signals below 150Hz. Applications using speakers with this limited frequency response reap little improvement by using high value input and output capacitors. Besides affecting system cost and size, Ci has an effect on the LM4809's click and pop performance. The magnitude of the pop is directly proportional to the input capacitor's size. Thus, pops can be minimized by selecting an input capacitor value that is no higher than necessary to meet the desired -3dB frequency. Please refer to the Optimizing Click and Pop Reduction Performance section for a more detailed discussion on click and pop performance. As shown in Figure 1, the input resistor, RI and the input capacitor, CI, produce a -3dB high pass filter cutoff frequency that is found using Equation (3). In addition, the output load RL, and the output capacitor CO, produce a -3db high pass filter cutoff frequency defined by Equation (4). fI-3db =1/2RICI fO-3db =1/2RLCO (3) (4)
tion. As discussed above, choosing Ci no larger than necessary for the desired bandwith helps minimize clicks and pops. OPTIMIZING CLICK AND POP REDUCTION PERFORMANCE The LM4809 contains circuitry that minimizes turn-on and shutdown transients or "clicks and pop". For this discussion, turn-on refers to either applying the power supply voltage or when the shutdown mode is deactivated. During turn-on, the LM4809's internal amplifiers are configured as unity gain buffers. An internal current source charges up the capacitor on the BYPASS pin in a controlled, linear manner. 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. During device turn-on, a transient (pop) is created from a voltage difference between the input and output of the amplifier as the voltage on the BYPASS pin reaches 1/2 VDD. For this discussion, the input of the amplifier refers to the node between RI and CI. Ideally, the input and output track the voltage applied to the BYPASS pin. During turn-on, the buffer-configured amplifier output charges the input capacitor, CI, through the input resistor, RI. This input resistor delays the charging time of CI thereby causing the voltage difference between the input and output that results in a transient (pop). Higher value capacitors need more time to reach a quiescent DC voltage (usually 1/2 VDD) when charged with a fixed current. Decreasing the value of CI and RI will minimize turn-on pops at the expense of the desired -3dB frequency. 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. Here are some typical turn-on times for various values of CB: CB 0.1F 0.22F 0.33F 0.47F 0.68F 1.0F 2.2F 3.3F 4.7F 10F TON 80ms 170ms 270ms 370ms 490ms 920ms 1.8sec 2.8sec 3.4sec 7.7sec
Also, careful consideration must be taken in selecting a certain type of capacitor to be used in the system. Different types of capacitors (tantalum, electrolytic, ceramic) have unique performance characteristics and may affect overall system performance. Bypass Capacitor Value Selection Besides minimizing the input capacitor size, careful consideration should be paid to value of CB, the capacitor connected to the BYPASS pin. Since CB determines how fast the LM4809 settles to quiescent operation, its value is critical when minimizing turn-on pops. The slower the LM4809's outputs ramp to their quiescent DC voltage (nominally 1/2 VDD), the smaller the turn-on pop. Choosing CB equal to 4.7F along with a small value of Ci (in the range of 0.1F to 0.47F), produces a click-less and pop-less shutdown func-
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 is coupled to the load by CO. This capacitor usually has a high value. CO discharges through internal 20k resistors. Depending on the size of CO, the discharge time constant can be relatively large. To reduce transients in single-ended mode, an external 1k-5k resistor can be placed in parallel with the internal 20k resistor. The tradeoff for using this resistor is increased quiescent current.
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LM4809
Application Information
AUDIO POWER AMPLIFIER DESIGN
(Continued)
Design a Dual 70mW/32 Audio Amplifier Given: Power Output Load Impedance Input Level Input Impedance 70 mW 32 1 Vrms (max) 20k
band magnitude variation limit, the low frequency response must extend to at lease one-fifth the lower bandwidth limit and the high frequency response must extend to at least five times the upper bandwidth limit. The gain variation for both response limits is 0.17dB, well within the 0.25dB desired limit. The results are an fL = 100Hz/5 = 20Hz and a fH = 20kHz*5 = 100kHz (10) (9)
Bandwidth 100 Hz-20 kHz 0.50dB 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 (5), 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 (5). For a single-ended application, the result is Equation (6).
As stated in the External Components section, both Ri in conjunction with Ci, and Co with RL, create first order highpass filters. Thus to obtain the desired low frequency response of 100Hz within 0.5dB, both poles must be taken into consideration. The combination of two single order filters at the same frequency forms a second order response. This results in a signal which is down 0.34dB at five times away from the single order filter -3dB point. Thus, a frequency of 20Hz is used in the following equations to ensure that the response is better than 0.5dB down at 100Hz. Ci 1 / (2 * 20k * 20Hz) = 0.397F; use 0.39F. (11)
(5) VDD (2VOPEAK + (VODTOP + VODBOT)) (6)
Co 1 / (2 * 32 * 20Hz) = 249F; use 330F. (12) The high frequency pole is determined by the product of the desired high frequency pole, fH, and the closed-loop gain, AV. With a closed-loop gain of 1.5 and fH = 100kHz, the resulting GBWP = 150kHz which is much smaller than the LM4809's GBWP of 900kHz. This figure displays that if a designer has a need to design an amplifier with a higher gain, the LM4809 can still be used without running into bandwidth limitations.
The Output Power vs Supply Voltage graph for a 32 load indicates a minimum supply voltage of 4.8V. This is easily met by the commonly used 5V supply voltage. The additional voltage creates the benefit of headroom, allowing the LM4809 to produce peak output power in excess of 70mW without clipping or other audible distortion. The choice of supply voltage must also not create a situation that violates maximum power dissipation as explained above in the Power Dissipation section. Remember that the maximum power dissipation point from Equation (1) must be multiplied by two since there are two independent amplifiers inside the package. Once the power dissipation equations have been addressed, the required gain can be determined from Equation (7).
(7) Thus, a minimum gain of 1.497 allows the LM4809 to reach full output swing and maintain low noise and THD+N perfromance. For this example, let AV =1.5. The amplifiers overall gain is set using the input (Ri ) and feedback (Rf ) resistors. With the desired input impedance set at 20k, the feedback resistor is found using Equation (8). AV = Rf/Ri (8)
The value of Rf is 30k. The last step in this design is setting the amplifier's -3db frequency bandwidth. To achieve the desired 0.25dB pass
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LM4809
Demonstration Board Schematic
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FIGURE 2. LM4809 Demonstration Board Schematic
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LM4809
Demonstration Board Layout
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FIGURE 3. Recommended PC Board Layout Component-Side Silkscreen
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FIGURE 4. Recommended PC Board Layout Component-Side Layout
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FIGURE 5. Recommended PC Board Layout Bottom-Side Layout
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LM4809
Physical Dimensions
inches (millimeters) unless otherwise noted
Order Number LM4809MM NS Package Number MUA08A
Order Number LM4809M NS Package Number M08A
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LM4809 Dual 105mW Headphone Amplifier with Active-Low Shutdown Mode
Notes
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.
National Semiconductor Corporation Americas Tel: 1-800-272-9959 Fax: 1-800-737-7018 Email: support@nsc.com www.national.com National Semiconductor Europe Fax: +49 (0) 180-530 85 86 Email: europe.support@nsc.com Deutsch Tel: +49 (0) 69 9508 6208 English Tel: +44 (0) 870 24 0 2171 Francais Tel: +33 (0) 1 41 91 8790
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.
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National Semiconductor Japan Ltd. Tel: 81-3-5639-7560 Fax: 81-3-5639-7507
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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