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LM4953 データシート(PDF) 8 Page - National Semiconductor (TI) |
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LM4953 データシート(HTML) 8 Page - National Semiconductor (TI) |
8 / 11 page Application Information (Continued) Since the LM4953 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 large internal power dissipation, the LM4953 does not require heat sinking over a large range of ambient tempera- tures. The maximum power dissipation point obtained must not be greater than the power dissipation that results from Equation 2: P DMAX =(TJMAX -TA)/( θ JA) (2) Depending on the ambient temperature, T A, 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 T A reduced. 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. 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 3V power supply typi- cally use a 4.7µF capacitor in parallel with a 0.1µF ceramic filter capacitor to stabilize the power supply’s output, reduce noise on the supply line, and improve the supply’s transient response. Keep the length of leads and traces that connect capacitors between the LM4953’s power supply pin and ground as short as possible. AUTOMATIC STANDBY MODE The LM4953 features Automatic Standby Mode circuitry (patent pending). In the absence of an input signal, after approximately 3 seconds, the LM4953 goes into low current standby mode. The LM4953 recovers into full power operat- ing mode immediately after a signal, which is greater than the input threshold voltage, is applied to either the left or right input pins. The input threshold voltage is not a static value, as the supply voltage increases, the input threshold voltage decreases. This feature reduces power supply current con- sumption in battery operated applications. To ensure correct operation of Automatic Standby Mode, proper layout techniques should be implemented. Separat- ing PGND and SGND can help reduce noise entering the LM4953 in noisy environments. It is also important to use correct power off sequencing. The device should be in shut- down and then powered off in order to ensure proper func- tionality of the Auto-Standby feature. While Automatic Standby Mode reduces power consumption very effectively during silent periods, maximum power saving is achieved by putting the device into shutdown when it is not in use. MICRO POWER SHUTDOWN The voltage applied to the SD controls the LM4953’s shut- down function. When active, the LM4953’s micropower shut- down feature turns off the amplifiers’ bias circuitry, reducing the supply current. The trigger point is 0.3*CPV DD for a logic-low level, and 0.7*CPV DD for logic-high level. The low 0.01µA (typ) shutdown current is achieved by applying a voltage that is as near as ground a possible to the SD pins. A voltage that is higher than ground may increase the shut- down 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 Ω pull-up resistor between the SD pins and V DD. Connect the switch between the SD pins and ground. Select normal amplifier operation by opening the switch. Closing the switch connects the SD pins to ground, activating micro-power shutdown. The switch and resistor guarantee that the SD pins will not float. This prevents unwanted state changes. In a system with a microprocessor or microcontroller, use a digital output to apply the control voltage to the SD pins. Driving the SD pins with active circuitry eliminates the pull-up resistor. SELECTING PROPER EXTERNAL COMPONENTS Optimizing the LM4953’s performance requires properly se- lecting external components. Though the LM4953 operates well when using external components with wide tolerances, best performance is achieved by optimizing component val- ues. Charge Pump Capacitor Selection Use low ESR (equivalent series resistance) (<100m Ω) ce- ramic capacitors with an X7R dielectric for best perfor- mance. Low ESR capacitors keep the charge pump output impedance to a minimum, extending the headroom on the negative supply. Higher ESR capacitors result in reduced output power from the audio amplifiers. Charge pump load regulation and output impedance are affected by the value of the flying capacitor (C1). A larger valued C1 (up to 3.3uF) improves load regulation and mini- mizes charge pump output resistance. Beyond 3.3uF, the switch-on resistance dominates the output impedance for capacitor values above 2.2uF. The output ripple is affected by the value and ESR of the output capacitor (C2). Larger capacitors reduce output ripple on the negative power supply. Lower ESR capacitors mini- mize the output ripple and reduce the output impedance of the charge pump. The LM4953 charge pump design is optimized for 2.2uF, low ESR, ceramic, flying, and output capacitors. Input Capacitor Value Selection Amplifying the lowest audio frequencies requires high value input coupling capacitors (C i in Figure 1). A high value ca- pacitor can be expensive and may compromise space effi- ciency in portable designs. In many cases, however, the speakers used in portable systems, whether internal or ex- ternal, have little ability to reproduce signals below 150Hz. Applications using speakers with this limited frequency re- sponse reap little improvement by using high value input and output capacitors. Besides affecting system cost and size, C i has an effect on the LM4953’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. As shown in Figure 1, the internal input resistor, R i and the input capacitor, C i, produce a -3dB high pass filter cutoff frequency that is found using Equation (3). Conventional headphone amplifiers require output capacitors; Equation (3) can be used, along with the value of R L, to determine to- wards the value of output capacitor needed to produce a –3dB high pass filter cutoff frequency. www.national.com 8 |
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同様の説明 - LM4953 |
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