LM4668LD

LM4668 10W High-Efficiency Mono BTL Audio Power Amplifier October 2004 LM4668 10W High-Efficiency Mono BTL Audio Power Amplifier General Description The LM4668 is a high efficiency switching audio power amplifier primarily designed for demanding applications in flat panel monitors and TV’s. It is capable of delivering 6W to an 8Ω mono BTL load with less than 1% distortion (THD+N) from a 12VDC power supply. Boomer audio power amplifiers were designed specifically to provide high quality output power with a minimal amount of external components. The LM4668 features a micro-power, active-low shutdown mode, an internal thermal shutdown protection mechanism, and short circuit protection. The LM4668 contains advanced transient (“pop and click”) suppression circuitry that eliminates noises that would otherwise occur during turn-on and turn-off transitions. Key Specifications j Power Output BTL (VDD = 14V, fIN = 1kHz, THD+N = 10%, RL = 8Ω) j Quiescent Power Supply Current j Efficiency (VDD = 12V, fIN = 1kHz, 10W (typ) 30mA (typ) 79% (typ) 0.15mA (typ) 30dB (typ) RL = 8Ω, POUT = 6W) j Shutdown Current j Fixed Gain Features n Soft-start circuitry eliminates noise during turn-on transition n Low current shutdown mode n Low quiescent current n 6W BTL output, RL = 8Ω n Short circuit protection n Fixed, internally set gain of 30dB Applications n Flat Panel Monitors n Flat Panel TVs n Computer Sound Cards Connection Diagrams LD Package MH Package 20089102 Top View Order Number LM4668LD See NS Package Number LDC14A 200891G3 Top View Order Number LM4668MH See NS Package Number MXA20A Boomer ® is a registered trademark of National Semiconductor Corporation. © 2004 National Semiconductor Corporation DS200891 www.national.com LM4668 Typical Application 20089101 FIGURE 1. Typical Audio Amplifier Application Circuit (* Zetex ZHCS506) www.national.com 2 LM4668 Absolute Maximum Ratings (Notes 1, 2) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Supply Voltage Storage Temperature Input Voltage Power Dissipation (Note 3) ESD Susceptibility (Note 4) ESD Susceptibility (Note 5) 16V −65˚C to +150˚C −0.3V to VDD +0.3V Internally limited 2000V 200V Junction Temperature (LD and MH) Thermal Resistance θJC θJA 150˚C 2˚C/W 40˚C/W Operating Ratings Temperature Range TMIN ≤ TA ≤ TMAX Supply Voltage (Note 10) −40˚C ≤ TA ≤ 85˚C 9V ≤ VDD ≤ 14.0V (Note 1) The following specifications apply for the circuit shown in Figure 1 operating with VDD = 12V, RL = 8Ω, and fIN = 1kHz, unless otherwise specified. Limits apply for TA = 25˚C. Symbol Parameter Conditions LM4668 Typical (Note 6) IDD ISD AV Quiescent Power Supply Current Shutdown Current Amplifier Gain VIN = 0V, IO = 0A, RL = 8Ω VSHUTDOWN = GND (Note 9) BTL output voltage with respect to input voltage, VIN = 100mVp-p THD+N = 1% (max) THD+N = 10%, VDD = 14V POUT = 6W, post filter, -3dB relative to the output amplitude at 1kHz, See Figure 1 POUT = 6W, including output filter A-Weighted Filter, VIN = 0V A-Weighted Filter, POUT = 6W AV = 30dB VRIPPLE = 20mVp-p, CBYPASS_1 = 10µF, input referred f = 50Hz f = 60Hz f = 100Hz f = 120Hz f = 1kHz CBYPASS = 10µF 30 0.15 30 6 10 0.2 20 20000 79 220 90 32 28 5 Limit (Notes 7, 8) 65 mA (max) mA dB (max) dB (min) W (min) W % Hz Hz % µV dB Units (Limits) Electrical Characteristics for the LM4668 PO THD+N fBW Output Power Total Harmonic Distortion + Noise POUT = 1WRMS Frequency Response Bandwith η éN SNR PSRR Efficiency Output Noise Signal-to-Noise Ratio Power Supply Rejection Ratio 79 82 85 84 75 600 170 4 1.5 dB tWU TSD VSDIH VSDIL Wake-Up time Thermal Shutdown Temperature Shutdown Voltage Input High Shutdown Voltage Input Low ms ˚C (min) ˚C (max) V (min) V (max) Note 1: All voltages are measured with respect to the GND 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 which guarantee specific performance limits. This assumes that the device is within the Operating Ratings. Specifications are not guaranteed for parameters where no limit is given, however, the typical value is a good indication of device performance. Note 3: The maximum power dissipation must be de-rated at elevated temperatures and is dictated by TJMAX, θJA, and the ambient temperature TA. The maximum allowable power dissipation is PDMAX = (TJMAX − TA)/θJA or the number given in Absolute Maximum Ratings, whichever is lower. For the LM4668 typical application (shown in Figure 1) with VDD = 12V, RL = 8Ω stereo operation, the total power dissipation is 900mW. θJA = 40˚C/W 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). 3 www.national.com LM4668 Electrical Characteristics for the LM4668 (Note 1) (Continued) Note 8: Datasheets min/max specification limits are guaranteed by design, test, or statistical analysis. Note 9: Shutdown current is measured in a normal room environment. The SHUTDOWN pin should be driven as close as possible to GND for minimum shutdown current. Note 10: Please refer to “Under Voltage Protection” on page 8 under “General Features.” Typical Performance Characteristics THD+N vs Frequency VDD = 9V, RL = 8Ω, PO = 1W THD+N vs Frequency VDD = 12V, RL = 8Ω, PO = 1W 20089106 20089107 THD+N vs Frequency VDD = 14V, RL = 8Ω, PO = 1W THD+N vs Output Power RL = 8Ω, VDD = 9V, f = 1kHz 20089108 20089109 THD+N vs Output Power RL = 8Ω, VDD = 12V, f = 1kHz THD+N vs Output Power RL = 8Ω, VDD = 14V, f = 1kHz 20089110 20089111 www.national.com 4 LM4668 Typical Performance Characteristics Amplifier Output Power vs Power Supply Voltage RL = 8Ω, f = 1kHz (Continued) Amplifier Output Magnitude vs Frequency RL = 8Ω, VDD = 12V 20089112 20089113 Power Rejection Ratio vs Frequency VDD = 9V, RL = 8Ω, Input Referred Power Rejection Ratio vs Frequency VDD = 12V, RL = 8Ω, Input Referred 20089114 20089115 Power Rejection Ratio vs Frequency VDD = 14V, RL = 8Ω, Input Referred Amplifier Power Dissipation vs Amplifier Load Dissipation VDD = 14V, RL = 8Ω, f = 1kHz 20089116 20089117 5 www.national.com LM4668 Typical Performance Characteristics Amplifier Power Dissipation vs Load Power Dissipation VDD = 12V, RL = 8Ω, f = 1kHz (Continued) Amplifier Power Dissipation vs Total Load Power Dissipation VDD = 9V, RL = 8Ω, f = 1kHz 20089118 20089119 Output Power vs Load Resistance VDD = 14V, f = 1kHz Output Power vs Load Resistance VDD = 12V, f = 1kHz 20089120 20089121 Output Power vs Load Resistance VDD = 9V, f = 1kHz Power Supply Current vs Power Supply Voltage VIN = 0V, RL = 8Ω 20089122 20089123 www.national.com 6 LM4668 Typical Performance Characteristics Power Dissipation vs Ambient Temperature (Continued) 20089124 7 www.national.com LM4668 General Features SYSTEM FUNCTIONAL INFORMATION Modulation Technique Unlike typical Class D amplifiers that use single-ended comparators to generate a pulse-width modulated switching waveform and RC timing circuits to set the switching frequency, the LM4668 uses a balanced differential floating modulator. Oscillation is a result of injecting complimentary currents onto the respective plates of a floating, on-die capacitor. The value of the floating capacitor and value of the components in the modulator’s feedback network and sets the nominal switching frequency at 450kHz. Modulation results from imbalances in the injected currents. The amount of current imbalance is directly proportional to the applied input signal’s magnitude and frequency. Using a balanced, floating modulator produces a Class D amplifier that is immune to common mode noise sources such as substrate noise. This noise occurs because of the high frequency, high current switching in the amplifier’s output stage. The LM4668 is immune to this type of noise because the modulator, the components that set its switching frequency, and even the load all float with respect to ground. The balanced modulator’s pulse width modulated output drives the gates of the LM4668’s H-bridge configured output power MOSFETs. The pulse-train present at the power MOSFETs’ output is applied to an LC low pass filter that removes the 450kHz energy component. The filter’s output signal, which is applied to the driven load, is an amplified replica of the audio input signal. Shutdown Function The LM4668’s active-low shutdown function allows the user to place the amplifier in a shutdown mode while the system power supply remains active. Activating shutdown deactivates the output switching waveform and minimizes the quiescent current. Applying logic 0 (GND) to pin 8 enables the shutdown function. Applying logic 1 (4V ≤ VLOGIC ≤ VDD) to pin 8 disables the shutdown function and restores full amplifier operation. Under Voltage Protection The under voltage protection disables the output driver section of the LM4668 while the supply voltage is below 8V. This condition may occur as power is first applied or during low line conditions, changes in load resistance, or when power supply sag occurs. The under voltage protection ensures that all of the LM4668’s power MOSFETs are off. This action eliminates shoot-through current and minimizes output transients during turn-on and turn-off. The under voltage protection gives the digital logic time to stabilize into known states, further minimizing turn output transients. Turn-On Time The LM4668 has an internal timer that determines the amplifier’s turn-on time. After power is first applied or the part returns from shutdown, the nominal turn-on time is 600ms. This delay allows all externally applied capacitors to charge to a final value of VDD/2. Further, during turn-on, the outputs are muted. This minimizes output transients that may occur while the part settles into is quiescent operating mode. Output Stage Fault Detection And Protection The output stage MOSFETs are protected against output conditions that could otherwise compromise their operational status. An onboard fault detection circuit continuously monitors the signal on each output MOSFET’s gate and compares it against the respective drain voltage. When a condition is detected that violates a MOSFET’s Safe Operating Area (SOA), the drive signal is disconnected from the output MOSFETs’ gates. The fault detect circuit maintains this protective condition for approximately 600ms, at which time the drive signal is reconnected. If the fault condition is no longer present, normal operation resumes. If the fault condition remains, however, the drive signal is again disconnected. Thermal Protection The LM4668 has thermal shutdown circuitry that monitors the die temperature. Once the LM4668 die temperature reaches 170˚C, the LM4668 disables the output switching waveform and remains disabled until the die temperature falls below 140˚C (typ). Over-Modulation Protection The LM4668’s over-modulation protection is a result of the preamplifier’s (AMP1 and AMP2, Figure 1) inability to produce signal magnitudes that equal the power supply voltages. Since the preamplifier’s output magnitude will always be less than the supply voltage, the duty cycle of the amplifier’s switching output will never reach zero. Peak modulation is limited to a nominal 95%. Application Hints SUPPLY BYPASSING Correct power supply bypassing has two important goals. The first is to reduce noise on the power supply lines and minimize deleterious effects that the noise may cause to the amplifier’s operation. The second is to help stabilize an unregulated power supply and to improve the supply’s transient response under heavy current demands. These two goals require different capacitor value ranges. Therefore, various types and values are recommended for supply bypassing. For noise de-coupling, generally small ceramic capacitors (0.01µF to 0.1µF) are recommended. Larger value (1µF to 10µF) tantalum capacitors are needed for the transient current demands. These two capacitors in parallel will do an adequate job of removing most noise from the supply rails and providing the necessary transient current. These capacitors should be placed as close as possible to each IC’s supply pin(s) using leads as short as possible. The LM4668 has two VDD pins: a power VDD (PVDD) and a signal VDD (SVDD). The parallel combination of the low value ceramic (0.1µF) and high value tantalum (10µF) should be used to bypass the PVDD pin. A small value (0.1µF) ceramic or tantalum can be used to bypass the SVDD pin. AMPLIFIER OUTPUT FILTERING The LM4668 requires a lowpass filter connected between the amplifier’s bridge output and the load. The second-order LC output filter shown in Figure 1 creates the lowpass response that is necessary to attenuate signal energy at the amplifier’s switching frequency. It also serves to suppress EMI. Together, the output filter’s 0.27µF capacitors and the recommended minimum inductor value of 27µH produce a www.national.com 8 LM4668 Application Hints (Continued) THD+N MEASUREMENTS AND OUT OF AUDIO BAND NOISE THD+N (Total Harmonic Distortion plus Noise) is a very important parameter by which all audio amplifiers are measured. Often it is shown as a graph where either the output power or frequency is changed over the operating range. A very important variable in the measurement of THD+N is the bandwidth-limiting filter at the input of the test equipment. Class D amplifiers, by design, switch their output power devices at a much higher frequency than the accepted audio range (20Hz - 20kHz). Alternately switching the output voltage between VDD and GND allows the LM4668 to operate at much higher efficiency than that achieved by traditional Class AB amplifiers. Switching the outputs at high frequency also increases the out-of-band noise. Under normal circumstances the output lowpass filter significantly reduces this out-of-band noise. If the low pass filter is not optimized for a given switching frequency, there can be significant increase in out-of-band noise. THD+N measurements can be significantly affected by out-of-band noise, resulting in a higher than expected THD+N measurement. To achieve a more accurate measurement of THD, the test equipment’s input bandwidth of the must be limited. Some common upper filter points are 22kHz, 30kHz, and 80kHz. The input filter limits the noise component of the THD+N measurement to a smaller bandwidth resulting in a more real-world THD+N value. RECOMMENDED PRINTED CIRCUIT BOARD LAYOUT Figures 2 through 4 show the recommended two-layer PC board layout that is optimized for the 14-pin MH-packaged LM4668 and associated external components. Figures 5 through 7 show the recommended two-layer PC board layout that is optimized for the 14-pin LD-packaged LM4668 and associated external components. These circuits are designed for use with an external 12V supply and 8Ω speakers (or load resistors). This circuit board is easy to use. Apply 12V and ground to the board’s VDD and GND terminals, respectively. Connect speakers (or load resistors) between the board’s -OUT and +OUT terminals. Apply the input signal to the input pin labeled -IN. nominal cutoff frequency of 47kHz. This cutoff frequency ensures that the attenuation is much less than 3dB at 20kHz. The output filter cutoff frequency and topology are also optimized for operational efficiency. A higher cutoff frequency compromises efficiency, whereas a lower cutoff frequency compromises the high frequencies within the audio frequency range. The filter’s topology also minimizes high frequency peaking, which can also decrease the amplifier’s efficiency. The output filter inductors must have a current rating that exceeds the amplifier’s output current when driving the load to maximum dissipation. Assuming a load dissipation of 10W in an 8Ω load with the amplifier operating on a 14V supply, the RMS current is 1.1A. In this case, the inductors’ current rating should be at least 1.2ARMS or 1.6APEAK. If a different output filter cutoff frequency (fC) is desired, the following brief discussion covers the selection of the capacitor and inductor values. In the following equations, RL is the load resistance and CL is three times the final value of the three common-mode filter capacitor found between the two output filter inductors (each inductor is L) as shown in Figure 1. When calculating values for L and CL, RL should be 8Ω, since the LM4668 is specified for 8Ω loads. The filter’s two inductors are equal to L = RL / 2πfC and each of the three capacitors are equal to C = L / 1.5R2 (2) (1) SCHOTTKY DIODE AMPLIFIER OUTPUT OVERDRIVE PROTECTION The Schottky diodes shown in Figure 1 provide protection against an over-voltage condition that may be caused by inductor-induced transients. These diodes are necessary when the nominal supply voltage exceeds 12V, the load impedance falls below 6Ω or the ambient temperature in the operating environment rises above 50˚C. 9 www.national.com LM4668 Demonstration Board Layout 20089103 FIGURE 2. Recommended MH PCB Layout Top Silkscreen 20089104 FIGURE 3. Recommended MH PCB Layout Top Layer www.national.com 10 LM4668 Demonstration Board Layout (Continued) 20089105 FIGURE 4. Recommended MH PCB Layout Bottom Layer 20089125 FIGURE 5. Recommended LD PCB Layout Top Silkscreen Layer 11 www.national.com LM4668 Demonstration Board Layout (Continued) 20089126 FIGURE 6. Recommended LD PCB Layout Top Layer 20089127 FIGURE 7. Recommended LD PCB Layout Bottom Layer www.national.com 12 LM4668 Physical Dimensions unless otherwise noted inches (millimeters) LD Package Order Number LM4668LD NS Package Number LDC14A 13 www.national.com LM4668 10W High-Efficiency Mono BTL Audio Power Amplifier Physical Dimensions inches (millimeters) unless otherwise noted (Continued) MH Package Order Number LM4668MH NS Package Number MXA20A National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications. For the most current product information visit us at www.national.com. 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