LM4816

LM4816 1W Stereo Audio Amplifier + Adjustable Output Limiter October 2003 LM4816 1W Stereo Audio Amplifier + Adjustable Output Limiter General Description The LM4816 combines a bridged-connected (BTL) stereo audio power amplifier with an adjustable output voltage magnitude limiter. The audio amplifier delivers 1.0W to an 8Ω load with less than 1.0% THD+N while operating on a 5V power supply. With VLIM set to 1.0V, the amplifier outputs are clamped to 6Vp-p, ± 800mV. The LM4816 features an external controlled micropower shutdown mode and thermal shutdown protection. It also utilizes circuitry that reduces “clicks and pops” during device turn-on and return from shutdown. Boomer audio power amplifiers are designed specifically to use few external components and provide high quality output power in a surface mount package. Key Specifications j POUT (BTL): VDD = 5V, THD = 1%, RL = 8Ω j Power supply range j Limiter adjustment range j Shutdown current 1.0W (typ) 3.0V to 5.5V GND to VDD/2 0.06µA (typ) Features n n n n n n Stereo BTL amplifier Adjustable output voltage magnitude limiter “Click and pop” suppression circuitry Unity-gain stable audio amplifiers Thermal shutdown protection circuitry TSSOP (MT) package Applications n n n n Notebook computers Multimedia monitors Desktop computers Portable televisions Typical Application 20033201 Boomer ® is a registered trademark of National Semiconductor Corporation. © 2003 National Semiconductor Corporation DS200332 www.national.com LM4816 Connection Diagram 20033229 Top View Order Number LM4816MT See NS Package Number MTC20 for TSSOP www.national.com 2 LM4816 Absolute Maximum Ratings (Note 1) Vapor Phase (60 sec.) Infrared (15 sec.) 215˚C 220˚C 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 2) ESD Susceptibility(Note 3) ESD Susceptibility (Note 4) Junction Temperature Solder Information Small Outline Package 6.0V −65˚C to +150˚C −0.3V to VDD +0.3V Internally limited 2000V 200V 150˚C See AN-450 “Surface Mounting and their Effects on Product Reliablilty” for other methods of soldering surface mount devices. Thermal Resistance θJC (typ) — MTC20 θJA (typ) — MTC20 20˚C/W 80˚C/W Operating Ratings Temperature Range TMIN ≤ TA ≤ TMAX Supply Voltage −40˚C ≤ TA ≤ 85˚C 3.0V ≤ VDD ≤ 5.5V Electrical Characteristics (Notes 1, 5) The following specifications apply for VDD= 5V unless otherwise specified. Limits apply for TA= 25˚C. Symbol Parameter Conditions Typical (Note 6) VDD Supply Voltage Quiescent Power Supply Current Shutdown Current Shutdown Logic High Input Threshold Voltage Shutdown Logic Low Input Threshold Voltage Output Offset Voltage VIN = 0V THD+N = 1%, f = 1kHz, RL = 8Ω THD+N = 10%, f = 1kHz, RL = 8Ω 20Hz ≤ f ≤ 20kHz, AVD = 2 RL = 8Ω, PO = 400mW VLIM = 1.0V, RL = ∞, VIN = 4VP-P VO P-P = (VOUT+ - VOUT-) VDD = 5V, VRIPPLE = 200VRMS RL = 8Ω, CB = 1.0µF Inputs Floating Inputs terminated with 10Ω f = 1kHz, CB = 1.0µF VDD = 5V, PO = 1.0W, RL = 8Ω 5 1.0 1.5 LM4816 Limit (Note 7) 3.0 5.5 IDD ISD VIH VIL VOS VIN = 0V, IO = 0A (Note 8) VDD applied to the SHUTDOWN pin 9.0 0.06 15 5 2 3.0 1.8 50 0.9 V (min) V (max) mA (max) mA (min) µA (min) V (min) V (max) mV (max) W (min) W Units (Limits) PO Output Power (Note 9) THD+N Total Harmonic Distortion + Noise Limiter Clamp Voltage Power Supply Rejection ratio 0.03 % VLIM PSRR 6.0 5.2 6.8 VP-P (min) VP-P (max) 67 43 90 98 dB dB dB dB XTALK SNR Channel Separation Signal to Noise Ratio Note 1: 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 2: The maximum power dissipation is dictated by TJMAX, θJA, and the ambient temperature TA and must be derated at elevated temperatures. The maximum allowable power dissipation is PDMAX = (TJMAX − TA)/θJA. For the LM4816, TJMAX = 150˚C. For the θJAs for different packages, please see the Application Information section or the Absolute Maximum Ratings section. 3 www.national.com LM4816 Note 3: Human body model, 100pF discharged through a 1.5kΩ resistor. Note 4: Machine model, 220pF–240pF discharged through all pins. Note 5: All voltages are measured with respect to the ground (GND) pins unless otherwise specified. Note 6: Typicals are measured at 25˚C and represent the parametric norm. Note 7: Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis. Note 8: The quiescent power supply current depends on the offset voltage when a practical load is connected to the amplifier. Note 9: Output power is measured at the device terminals. Typical Performance Characteristics THD+N vs Frequency THD+N vs Output Power 20033267 20033299 VDD = 5V, RL = 8Ω, POUT = 150mW THD+N vs Frequency VDD = 5V, RL = 8Ω, fIN = 1kHz THD+N vs Output Power 20033265 20033268 VDD = 3V, RL = 8Ω, POUT = 150mW VDD = 3V, RL = 8Ω, fIN = 1kHz www.national.com 4 LM4816 Typical Performance Characteristics THD+N vs Frequency (Continued) THD+N vs Output Power 20033266 20033295 VDD = 5.5V, RL = 8Ω, POUT = 150mW THD+N vs Output Power VDD = 5.5V, RL = 8Ω, fIN = 1kHz PSRR vs Frequency 20033242 20033264 VLIM VDD = 5V, RL = 8Ω, fIN = 1kHz, at (from left to right at 7% THD+N): = 2V, 1.9V, 1.8V, 1.7V, 1.6V, 1.5V, 1.0V, 0.5V, 0V PSRR vs Frequency VRIPPLE VDD = 5V, RL = 8Ω, RSOURCE = 10Ω, = 200mVP-P, at (from top to bottom at 500Hz): CBYPASS = 0.1µF, CBYPASS = 1.0µF PSRR vs Frequency VRIPPLE VDD = 5V, RL = 8Ω, RSOURCE = ∞, = 200mVP-P, at (from top to bottom at 500Hz): CBYPASS = 0.1µF, CBYPASS = 1.0µF 20033243 20033240 VRIPPLE VDD = 3V, RL = 8Ω, RSOURCE = 10Ω, = 200mVP-P, at (from top to bottom at 500Hz): CBYPASS = 0.1µF, CBYPASS = 1.0µF 5 www.national.com LM4816 Typical Performance Characteristics PSRR vs Frequency (Continued) Cross Talk VRIPPLE VDD = 3V, RL = 8Ω, RSOURCE = ∞, = 200mVP-P, at (from top to bottom at 500Hz): CBYPASS = 0.1µF, CBYPASS = 1.0µF 20033241 20033239 VDD = 5V, RL = 8Ω, POUT = 150mW, at (from top to bottom at 2kHz): -N A driven, VOUTB measured; -N B driven, VOUTA measured Cross Talk Cross Talk 20033237 20033238 VDD = 3V, RL = 8Ω, POUT = 150mW, at (from top to bottom at 2kHz): -N A driven, VOUTB measured; -N B driven, VOUTA measured Supply Current vs Supply Voltage VDD = 5.5V, RL = 8Ω, POUT = 150mW, at (from top to bottom at 2kHz): -N A driven, VOUTB measured; -N B driven, VOUTA measured Output Power vs Supply Voltage 20033253 RL = 8Ω, VIN = 0V RSOURCE = 50Ω 20033230 RL = 8Ω, fIN = 1kHz, at (from top to bottom at 4.6V): THD+N = 10%, THD+N = 1% www.national.com 6 LM4816 Typical Performance Characteristics Output Power vs Load Resistance (Continued) Dropout Voltage vs Supply Voltage 20033255 20033257 VDD = 5V, fIN = 1kHz, at (from top to bottom at 32Ω): THD+N = 10%, THD+N = 1% Power Dissipation vs Output Power RL = 8Ω, fIN = 1kHz, at (from top to bottom at 4.5V): positive signal swing, negative signal swing Power Derating Curve 20033252 20033256 VDD = 5V, fIN = 1kHz, at (from top to bottom at 0.20W): RL = 8Ω, 16Ω, 32Ω Open Loop Frequency Response 20033222 7 www.national.com LM4816 External Components Description (Refer to Figure 1.) Components 1. 2. Ri Ci 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/(2πRiCi). The input coupling capacitor blocks DC voltage at the amplifier’s input terminals. Ci, along with Ri, create a highpass filter with fc = 1/(2πRiCi). Refer to the section, SELECTING PROPER EXTERNAL COMPONENTS, for an explanation of determining the value of Ci. The feedback resistance, along with Ri, set the closed-loop gain. The supply bypass capacitor. Refer to the POWER SUPPLY BYPASSING section for information about properly placing, and selecting the value of, this capacitor. The capacitor, CB, filters the half-supply voltage present on the BYPASS pin. Refer to the SELECTING PROPER EXTERNAL COMPONENTS section for information concerning proper placement and selecting CB’s value. 3. 4. 5. Rf Cs CB Application Information 20033201 * Refer to the section Proper Selection of External Components, for a detailed discussion of CB size. FIGURE 1. Typical Audio Amplifier Application Circuit Pin out shown for the LLP package. Refer to the Connection Diagrams for the pinout of the TSSOP package. www.national.com 8 LM4816 Application Information (Continued) BRIDGE CONFIGURATION EXPLANATION As shown in Figure 1, the LM4816 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.) External resistors Rf and Ri set the closed-loop gain of Amp1A, whereas two internal 20kΩ resistors set Amp2A’s gain at -1. The LM4816 drives a load, such as a speaker, connected between the two amplifier outputs, -OUTA and +OUTA. Figure 1 shows that Amp1A’s output serves as Amp2A’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 x (Rf / Ri) (1) The LM4816’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 LM4816’s TJMAX = 150˚C. In the MT (TSSOP) package, the LM4816’s θJA is 80˚C/W. At any given ambient temperature TJ\A, 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 LM4816’s maximum junction temperature. TA = TJMAX − 2 x PDMAX θJA (5) 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 For a typical application with a 5V power supply and an 8Ω load, the maximum ambient temperature that allows maximum stereo power dissipation without exceeding the maximum junction temperature is approximately 48˚C. TJMAX = PDMAX θJA + TA (6) Equation (6) gives the maximum junction temperature TJMAX. If the result violates the LM4816’s 150˚C, 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 twice the value given by Equation (3) exceeds the value given by Equation (4), then decrease the supply voltage, increase the load impedance, or reduce the ambient temperature. OUTPUT VOLTAGE LIMITER The LM4816’s adjustable output voltage limiter can be used to set a maximum and minimum output voltage swing magnitude. The voltage applied to the VLIM input (pin 20) controls the amount voltage limit magnitude. Without the limiter’s influence (VLIM = 0V), the LM4816’s maximum BTL output swing is nominally 2 x VDD When the limiter input voltage is greater than 0V, the BTL output voltage swing is VOUT-BTL = (2 x VDD) - (4 x VLIM) with a tolerance of ± 800 mV. 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 10µF in parallel with a 0.1µF filter capacitors to stabilize the regulator’s output, reduce noise on the supply line, and improve the supply’s transient response. However, their designing a Equation (2) for a singlevoltage and PDMAX = (VDD)2 / (2π2 RL) 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. The LM4816 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 an 8Ω load, the maximum single channel power dissipation is 0.633W or 1.27W for stereo operation. PDMAX = 4 x (VDD)2 / (2π2 RL) Bridge Mode (3) 9 www.national.com LM4816 Application Information (Continued) presence does not eliminate the need for a local 1.0µF tantalum bypass capacitance connected between the LM4816’s supply pins and ground. Do not substitute a ceramic capacitor for the tantalum. Doing so may cause oscillation in the output signal. Keep the length of leads and traces that connect capacitors between the LM4816’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 improves the amplifier’s PSRR. The PSRR improvements increase as the bypass pin capacitor value increases. Too large, however, increases turn-on time and can compromise 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 section, Proper Selection of External Components), system cost, and size constraints. MICRO-POWER SHUTDOWN The voltage applied to the SHUTDOWN pin controls the LM4816’s shutdown function. Activate micro-power shutdown by applying VDD to the SHUTDOWN pin. When active, the LM4816’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 0.6µA typical shutdown current is achieved by applying a voltage that is as near as VDD as possible to the SHUTDOWN pin. A voltage thrat 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 pull up resistor. TABLE 1. LOGIC LEVEL TRUTH TABLE FOR SHUTDOWN OPERATION SHUTDOWN Low High OPERATIONAL MODE Full power, stereo BTL amplifiers Micro-power Shutdown 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 Capacitor Value Selection Amplifying the lowest audio frequencies requires high value input coupling capacitor (Ci 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 large input capacitor. Besides effecting system cost and size, Ci has an affect on the LM4816’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 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 -3dB frequency. A 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 (7). (7) As an example when using a speaker with a low frequency limit of 150Hz, CI, using Equation (4), is 0.063µF. The 1.0µF CI shown in Figure 1 allows the LM4816 to drive high efficiency, full range speaker whose response extends below 30Hz. 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 LM4816 settles to quiescent operation, its value is critical when minimizing turn−on pops. The slower the LM4816’s outputs ramp to their quiescent DC voltage (nominally 1/2 VDD), 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 click-less and pop-less shutdown function. As discussed above, choosing Ci no larger than necessary for the desired bandwidth helps minimize clicks and pops. OPTIMIZING CLICK AND POP REDUCTION PERFORMANCE The LM4816 contains circuitry to minimize 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. While the power supply is ramping to its final value, the LM4816’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 10 SELECTING PROPER EXTERNAL COMPONENTS Optimizing the LM4816’s performance requires properly selecting external components. Though the LM4816 operates well when using external components with wide tolerances, best performance is achieved by optimizing component values. The LM4816 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 www.national.com LM4816 Application Information (Continued) VDD ≥ (VOUTPEAK + (VODTOP + VODBOT)) (9) 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. Here are some typical turn-on times for various values of CB: CB 0.01µF 0.1µF 0.22µF 0.47µF 1.0µF TON 20 ms 200 ms 440 ms 940 ms 2 Sec 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 LM4816 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 maximum power dissipation as explained above in the Power Dissipation section. After satisfying the LM4816’s power dissipation requirements, the minimum differential gain is found using Equation (10). (10) Thus, a minimum gain of 2.83 allows the LM4816’s to reach full output swing and maintain low noise and THD+N performance. For this example, let AVD = 3. The amplifier’s overall gain is set using the input (Ri) and feedback (Rf) resistors. With the desired input impedance set at 20kΩ, the feedback resistor is found using Equation (11). (11) Rf/Ri = AVD/2 The value of Rf is 30kΩ. The last step in this design example is setting the amplifier’s −3dB 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 fL = 100Hz/5 = 20Hz and an FH = 20kHzx5 = 100kHz (13) (12) 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". NO LOAD STABILITY The LM4816 may exhibit low level oscillation when the load resistance is greater than 10kΩ. This oscillation only occurs as the output signal swings near the supply voltages. Prevent this oscillation by connecting a 5kΩ between the output pins and ground. AUDIO POWER AMPLIFIER DESIGN Audio Amplifier Design: Driving 1W into an 8Ω Load The following are the desired operational parameters: Power Output: Load Impedance: Input Level: Input Impedance: Bandwidth: 1WRMS 8Ω 1VRMS 20kΩ 100Hz−20 kHz ± 0.25 dB As mentioned in the External Components section, Ri and Ci create a highpass filter that sets the amplifier’s lower bandpass frequency limit. Find the coupling capacitor’s value using Equation (14). 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 (4), 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 (8). The result in Equation (9). (14) the result is 1/(2π*20kΩ*20Hz) = 0.398µF (15) Use a 0.39µF capacitor, the closest standard value. 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 LM4816’s 3.5MHz GBWP. With (8) 11 www.national.com LM4816 Application Information (Continued) this margin, the amplifier can be used in designs that require more differential gain while avoiding performance-restricting bandwidth limitations. www.national.com 12 LM4816 1W Stereo Audio Amplifier + Adjustable Output Limiter Physical Dimensions inches (millimeters) unless otherwise noted 20-Lead Molded PKG, TSSOP, JEDEC, 4.4mm BODY WIDTH Order Number LM4816MT NS Package Number MTC20 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. 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