LM4755 Stereo 11W Audio Power Amplifier with Mute General Description The LM4755 is a stereo audio amplifier capable of delivering 11W per channel of continuous average output power to a 4Ω load or 7W per channel into 8Ω using a single 24V supply at 10% THD+N. The internal mute circuit and pre-set gain resistors provide for a very economical design solution. Output power specifications at both 20V and 24V supplies and low external component count offer high value to consumer electronic manufacturers for stereo TV and compact stereo applications. The LM4755 is specifically designed for single supply operation. Key Specifications n Output power at 10% THD with 1kHz into 4Ω at VCC = 24V: 11W (typ) n Output power at 10% THD with 1kHz into 8Ω at VCC = 24V: 7W (typ) n Closed loop gain: 34dB (typ) n PO at 10% THD+N @ 1kHz into 4Ω single-ended TO-263 package at VCC=12V: 2.5W (typ) n PO at 10% THD+N @ 1kHz into 8Ω bridged TO-263 package at VCC=12V: 5W (typ) Features n n n n n n n n Drives 4Ω and 8Ω loads Integrated mute function Internal Gain Resistors Minimal external components needed Single supply operation Internal current limiting and thermal protection Compact 9-lead TO-220 package Wide supply range 9V - 40V Applications n Stereos TVs n Compact stereos n Mini component stereos Typical Application 10005901 FIGURE 1. Typical Audio Amplifier Application Circuit © 2004 National Semiconductor Corporation DS100059 www.national.com LM4755 Stereo 11W Audio Power Amplifier with Mute May 2002 LM4755 Connection Diagrams Plastic Package 10005902 Package Description Top View Order Number LM4755T Package Number TA09A 10005936 Top View Order Number LM4755TS Package Number TS9A www.national.com 2 T Package (10 seconds) Storage Temperature If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Supply Voltage Temperature Range ± 0.7V Output Current TMIN ≤ TA ≤ TMAX Internally Limited Power Dissipation (Note 3) ESD Susceptability (Note 4) Junction Temperature −40˚C to 150˚C Operating Ratings 40V Input Voltage 250˚C −40˚C ≤ TA ≤ +85˚C Supply Voltage 62.5W 2 kV 150˚C 9V to 32V θJC 2˚C/W θJA 76˚C/W Soldering Information Electrical Characteristics The following specifications apply to each channel with VCC = 24V, TA = 25˚C unless otherwise specified. LM4755 Symbol ITOTAL PO Parameter Total Quiescent Power Supply Current Output Power (Continuous Average per Channel) Conditions Mute Off Typical (Note 5) Limit Units (Limits) 10 15 mA(max) 7 mA(min) Mute On 7 mA f = 1 kHz, THD+N = 10%, RL = 8Ω 7 W f = 1 kHz, THD+N = 10%, RL = 4Ω 11 VS = 20V, RL = 8Ω 4 W VS = 20V, RL = 4Ω 7 W f = 1 kHz, THD+N = 10%, RL = 4Ω VS = 12V, TO-263 Pkg. 2.5 W 10 W(min) THD Total Harmonic Distortion f = 1 kHz, PO = 1 W/ch, RL = 8Ω 0.08 % VOSW Output Swing PO = 10W, RL = 8Ω 15 V PO = 10W, RL = 4Ω 14 V See Apps. Circuit 55 dB 50 dB XTALK Channel Separation f = 1 kHz, VO = 4 Vrms PSRR Power Supply Rejection Ratio See Apps. Circuit VODV Differential DC Output Offset Voltage SR Slew Rate RIN Input Impedance PBW Power Bandwidth AVCL Closed Loop Gain (Internally Set) RL = 8Ω f = 120 Hz, VO = 1 mVrms VIN = 0V 0.09 0.4 2 3 dB BW at PO = 2.5W, RL = 8Ω kΩ 65 kHz eIN Noise IO Output Short Circuit Limit VIN = 0.5V, RL = 2Ω Mute Pin VIL Mute Low Input Voltage Not in Mute Mode VIH Mute High Input Voltage In Mute Mode 2.0 AM Mute Attenuation VMUTE = 5.0V 80 3 V/µs 83 34 IHF-A Weighting Filter, RL = 8Ω Output Referred V(max) 33 dB(min) 35 dB(max) 0.2 mVrms 2 A(min) 0.8 V(max) 2.5 V(min) dB www.national.com LM4755 Absolute Maximum Ratings (Note 2) LM4755 Electrical Characteristics (Continued) Note 1: All voltages are measured with respect to the GND pin (5), unless otherwse 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 which guarantee specific performance limits. This assumes that the device is within the Operating Ratings. Specifications are not guaranteed for parameters where no limit is given, however, the typical value is a good indication of device performance. Note 3: For operating at case temperatures above 25˚C, the device must be derated based on a 150˚C maximum junction temperature and a thermal resistance of θJC = 2˚C/W (junction to case). Refer to the section Determining the Maximum Power Dissipation in the Application Information section for more information. Note 4: Human body model, 100 pF discharged through a 1.5 kΩ resistor. Note 5: Typicals are measured at 25˚C and represent the parametric norm. Note 6: Limits are guaranteed that all parts are tested in production to meet the stated values. Note 7: The TO-263 Package is not recommended for VS > 16V due to impractical heatsinking limitations. www.national.com 4 Equivalent Schematic 10005903 LM4755 5 www.national.com LM4755 Test Circuit 10005904 FIGURE 2. Test Circuit www.national.com 6 LM4755 System Application Circuit 10005905 FIGURE 3. Circuit for External Components Description External Components Description Components Function Description 1, 2 CS Provides power supply filtering and bypassing. 3, 4 RSN Works with CSN to stabilize the output stage from high frequency oscillations. 5, 6 CSN Works with RSN to stabilize the output stage from high frequency oscillations. 7 Cb Provides filtering for the internally generated half-supply bias generator. 8, 9 Ci Input AC coupling capacitor which blocks DC voltage at the amplifier’s input terminals. Also creates a high pass filter with fc=1/(2 • π • Rin • Cin). 10, 11 Co Output AC coupling capacitor which blocks DC voltage at the amplifier’s output terminal. Creates a high pass filter with fc=1/(2 • π • Rout • Cout). 12, 13 Ri Voltage control - limits the voltage level allowed to the amplifier’s input terminals. 14 Rm Works with Cm to provide mute function timing. 15 Cm Works with Rm to provide mute function timing. 7 www.national.com LM4755 Typical Performance Characteristics(Note 5) THD+N vs Output Power THD+N vs Output Power 10005912 10005913 THD+N vs Output Power THD+N vs Output Power 10005914 10005906 THD+N vs Output Power THD+N vs Output Power 10005907 www.national.com 10005908 8 THD+N vs Output Power 5) LM4755 Typical Performance Characteristics(Note (Continued) THD+N vs Output Power 10005915 10005916 THD+N vs Output Power THD+N vs Output Power 10005917 10005909 THD+N vs Output Power THD+N vs Output Power 10005910 10005911 9 www.national.com LM4755 Typical Performance Characteristics(Note THD+N vs Output Power 5) (Continued) THD+N vs Output Power 10005938 10005939 THD+N vs Output Power THD+N vs Output Power 10005940 10005941 THD+N vs Output Power THD+N vs Output Power 10005942 www.national.com 10005943 10 THD+N vs Output Power 5) LM4755 Typical Performance Characteristics(Note (Continued) THD+N vs Output Power 10005944 10005945 THD+N vs Output Power THD+N vs Output Power 10005946 10005947 THD+N vs Output Power THD+N vs Output Power 10005948 10005949 11 www.national.com LM4755 Typical Performance Characteristics(Note Output Power vs Supply Voltage 5) (Continued) Output Power vs Supply Voltage 10005918 10005919 Frequency Response THD+N vs Frequency 10005921 10005920 THD+N vs Frequency Frequency Response 10005923 10005922 www.national.com 12 Channel Separation 5) LM4755 Typical Performance Characteristics(Note (Continued) PSRR vs Frequency 10005924 10005925 Supply Current vs Supply Voltage Power Derating Curve 10005926 10005927 Power Dissipation vs Output Power Power Dissipation vs Output Power 10005928 10005929 13 www.national.com LM4755 Typical Performance Characteristics(Note Power Dissipation vs Output Power (Continued) Power Dissipation vs Output Power 10005960 10005961 mentioned earlier in the External Components section these capacitors create high-pass filters with their corresponding input/output impedances. The Typical Application Circuit shown in Figure 1 shows input and output capacitors of 0.1 µF and 1,000 µF respectively. At the input, with an 83 kΩ typical input resistance, the result is a high pass 3 dB point occurring at 19 Hz. There is another high pass filter at 39.8 Hz created with the output load resistance of 4Ω. Careful selection of these components is necessary to ensure that the desired frequency response is obtained. The Frequency Response curves in the Typical Performance Characteristics section show how different output coupling capacitors affect the low frequency roll-off. Application Information The LM4755 contains circuitry to pull down the bias line internally, effectively shutting down the input stage. An external R-C should be used to adjust the timing of the pulldown. If the bias line is pulled down too quickly, currents induced in the internal bias resistors will cause a momentary DC voltage to appear across the inputs of each amplifier’s internal differential pair, resulting in an output DC shift towards Vsupply. An R-C timing circuit should be used to limit the pull-down time such that output “pops” and signal feedthroughs will be minimized. The pull-down timing is a function of a number of factors, including the internal mute circuitry, the voltage used to activate the mute, the bias capacitor, the half-supply voltage, and internal resistances used in the half-supply generator. Table 1 shows a list of recommended values for the external R-C. OPERATING IN BRIDGE-MODE Though designed for use as a single-ended amplifier, the LM4755 can be used to drive a load differentially (bridgemode). Due to the low pin count of the package, only the non-inverting inputs are available. An inverted signal must be provided to one of the inputs. This can easily be done with the use of an inexpensive op-amp configured as a standard inverting amplifier. An LF353 is a good low-cost choice. Care must be taken, however, for a bridge-mode amplifier must theoretically dissipate four times the power of a single-ended type. The load seen by each amplifier is effectively half that of the actual load being used, thus an amplifier designed to drive a 4Ω load in single-ended mode should drive an 8Ω load when operating in bridge-mode. TABLE 1. Recommended Values for Mute Circuit VMUTE VCC Rm Cm 5V 12V 18 kΩ 10 µF 5V 15V 18 kΩ 10 µF 5V 20V 12 kΩ 10 µF 5V 24V 12 kΩ 10 µF 5V 28V 8.2 kΩ 10 µF 5V 30V 8.2 kΩ 10 µF CAPACITOR SELECTION AND FREQUENCY RESPONSE With the LM4755, as in all single supply amplifiers, AC coupling capacitors are used to isolate the DC voltage present at the inputs (pins 3, 7) and outputs (pins 1, 8). As www.national.com 5) 14 LM4755 Application Information (Continued) 10005930 FIGURE 4. Bridge-Mode Application 10005931 10005937 FIGURE 5. THD+N vs POUT for Bridge-Mode Application 15 www.national.com LM4755 Application Information (Continued) PREVENTING OSCILLATIONS With the integration of the feedback and bias resistors onchip, the LM4755 fits into a very compact package. However, due to the close proximity of the non-inverting input pins to the corresponding output pins, the inputs should be AC terminated at all times. If the inputs are left floating, the amplifier will have a positive feedback path through high impedance coupling, resulting in a high frequency oscillation. In most applications, this termination is typically provided by the previous stage’s source impedance. If the application will require an external signal, the inputs should be terminated to ground with a resistance of 50 kΩ or less on the AC side of the input coupling capacitors. UNDERVOLTAGE SHUTDOWN If the power supply voltage drops below the minimum operating supply voltage, the internal under-voltage detection circuitry pulls down the half-supply bias line, shutting down the preamp section of the LM4755. Due to the wide operating supply range of the LM4755, the threshold is set to just under 9V. There may be certain applications where a higher threshold voltage is desired. One example is a design requiring a high operating supply voltage, with large supply and bias capacitors, and there is little or no other circuitry connected to the main power supply rail. In this circuit, when the power is disconnected, the supply and bias capacitors will discharge at a slower rate, possibly resulting in audible output distortion as the decaying voltage begins to clip the output signal. An external circuit may be used to sense for the desired threshold, and pull the bias line (pin 6) to ground to disable the input preamp. Figure 6 shows an example of such a circuit. When the voltage across the zener diode drops below its threshold, current flow into the base of Q1 is interrupted. Q2 then turns on, discharging the bias capacitor. This discharge rate is governed by several factors, including the bias capacitor value, the bias voltage, and the resistor at the emitter of Q2. An equation for approximating the value of the emitter discharge resistor, R, is given below: R = (0.7v) / (Cb • (VCC/2) / 0.1s) Note that this is only a linearized approximation based on a discharge time of 0.1s. The circuit should be evaluated and adjusted for each application. As mentioned earlier in the Built-in Mute Circuit section, when using an external circuit to pull down the bias line, the rate of discharge will have an effect on the turn-off induced distortions. Please refer to the Built-in Mute Circuit section for more information. 10005932 FIGURE 6. External Undervoltage Pull-Down THERMAL CONSIDERATIONS Heat Sinking Proper heatsinking is necessary to ensure that the amplifier will function correctly under all operating conditions. A heatsink that is too small will cause the die to heat excessively and will result in a degraded output signal as the thermal protection circuitry begins to operate. The choice of a heatsink for a given application is dictated by several factors: the maximum power the IC needs to dissipate, the worst-case ambient temperature of the circuit, the junction-to-case thermal resistance, and the maximum junction temperature of the IC. The heat flow approximation equation used in determining the correct heatsink maximum thermal resistance is given below: TJ–TA = PDMAX • (θJC + θCS + θSA) where: PDMAX = maximum power dissipation of the IC TJ(˚C) = junction temperature of the IC TA(˚C) = ambient temperature θJC(˚C/W) = junction-to-case thermal resistance of the IC θCS(˚C/W) = case-to-heatsink thermal resistance (typically 0.2 to 0.5 ˚C/W) θSA(˚C/W) = thermal resistance of heatsink When determining the proper heatsink, the above equation should be re-written as: θSA ≤ [(TJ–TA) / PDMAX] - θJC–θCS TO-263 HEATSINKING Surface mount applications will be limited by the thermal dissipation properties of printed circuit board area. The TO263 package is not recommended for surface mount applications with VS > 16V due to limited printed circuit board area. There are TO-263 package enhancements, such as clip-on heatsinks and heatsinks with adhesives, that can be used to improve performance. Standard FR-4 single-sided copper clad will have an approximate Thermal resistance (θSA) ranging from: www.national.com 16 1.5 x 1.5 in. sq. 20–27˚C/W 2 x 2 in. sq. 16–23˚C/W PDMAX from PD vs PO Graph: (Continued) PDMAX ≈ 3.7W Calculating PDMAX: PDMAX = VCC2/(π2RL) = (12V)2/π2(4Ω)) = 3.65W (TA=28˚C, Sine wave testing, 1 oz. Copper) Calculating Heatsink Thermal Resistance: θSA < TJ − TA / PDMAX − θJC − θCS θSA < 120˚C/3.7W − 2.0˚C/W − 0.2˚C/W = 30.2˚C/W The above values for θSA vary widely due to dimensional proportions (i.e. variations in width and length will vary θSA). For audio applications, where peak power levels are short in duration, this part will perform satisfactory with less heatsinking/copper clad area. As with any high power design proper bench testing should be undertaken to assure the design can dissipate the required power. Proper bench testing requires attention to worst case ambient temperature and air flow. At high power dissipation levels the part will show a tendency to increase saturation voltages, thus limiting the undistorted power levels. Therefore the recommendation is to use 1.5 x 1.5 square inch of single-sided copper clad. Example 2: (Stereo Single-Ended Output) Given: VS=12V θJC=2˚C/W DETERMINING MAXIMUM POWER DISSIPATION For a single-ended class AB power amplifier, the theoretical maximum power dissipation point is a function of the supply voltage, VS, and the load resistance, RL and is given by the following equation: PDMAX from PD vs PO Graph: PDMAX ≈ 3.7W Calculating PDMAX: PDMAX = VCC2/(π2RL)= (12V) 2/(π2(4Ω)) = 3.65W Calculating Heatsink Thermal Resistance: θSA < [(TJ − TA) / PDMAX] − θJC − θCS θSA < 100˚C/3.7W − 2.0˚C/W − 0.2˚C/W = 24.8˚C/W Therefore the recommendation is to use 2.0 x 2.0 square inch of single-sided copper clad. Example 3: (Bridged Output) Given: TA=50˚C TJ=150˚C RL=8Ω VS=12V θJC=2˚C/W (single channel) PDMAX (W) = [VS2 / (2 • π2 • RL)] The above equation is for a single channel class-AB power amplifier. For dual amplifiers such as the LM4755, the equation for calculating the total maximum power dissipated is: (dual channel) PDMAX (W) = 2 • [VS2 / (2 • π2 • RL)] or VS2 / (π2 • RL) (Bridged Outputs) PDMAX (W) = 4[VS2 / (2π2 • RL)] Calculating PDMAX: PDMAX = 4[VCC2/(2π2RL)] = 4(12V)2/(2π2(8Ω)) = 3.65W Calculating Heatsink Thermal Resistance: θSA < [(TJ − TA) / PDMAX] − θJC − θCS θSA < 100˚C / 3.7W − 2.0˚C/W − 0.2˚C/W = 24.8˚C/W Therefore the recommendation is to use 2.0 x 2.0 square inch of single-sided copper clad. HEATSINK DESIGN EXAMPLE Determine the system parameters: VS = 24V Operating Supply Voltage RL = 4Ω Minimum Load Impedance TA = 55˚C Worst Case Ambient Temperature Device parameters from the datasheet: TJ = 150˚C Maximum Junction Temperature θJC = 2˚C/W Junction-to-Case Thermal Resistance TA=50˚C TJ=150˚C RL=4Ω LAYOUT AND GROUND RETURNS Proper PC board layout is essential for good circuit performance. When laying out a PC board for an audio power amplifier, particular attention must be paid to the routing of the output signal ground returns relative to the input signal and bias capacitor grounds. To prevent any ground loops, the ground returns for the output signals should be routed separately and brought together at the supply ground. The input signal grounds and the bias capacitor ground line should also be routed separately. The 0.1 µF high frequency supply bypass capacitor should be placed as close as possible to the IC. Calculations: 2 • PDMAX = 2 • [VS2 / 2 • π2 • RL)] = (24V)2 / (2 • π2 • 4Ω) = 14.6W θSA ≤ [(TJ-TA) / PDMAX] - θJC–θCS = [ (150˚C - 55˚C) / 14.6W] - 2˚C/W–0.2˚C/W = 4.3˚C/W Conclusion: Choose a heatsink with θSA ≤ 4.3˚C/W. TO-263 HEATSINK DESIGN EXAMPLES Example 1: (Stereo Single-Ended Output) Given: TA=30˚C TJ=150˚C RL=4Ω VS=12V θJC=2˚C/W 17 www.national.com LM4755 Application Information LM4755 Application Information (Continued) PC BOARD LAYOUT-COMPOSITE 10005933 www.national.com 18 LM4755 Application Information (Continued) PC BOARD LAYOUT-SILK SCREEN 10005934 19 www.national.com LM4755 Application Information (Continued) PC BOARD LAYOUT-SOLDER SIDE 10005935 www.national.com 20 LM4755 Physical Dimensions inches (millimeters) unless otherwise noted Order Number LM4755T NS Package Number TA9A 21 www.national.com LM4755 Physical Dimensions inches (millimeters) unless otherwise noted (Continued) Order Number LM4755TS NS Package Number TS9A www.national.com 22 LM4755 Stereo 11W Audio Power Amplifier with Mute Notes 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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