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LM4925SD Folha de dados(PDF) 11 Page - National Semiconductor (TI) |
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LM4925SD Folha de dados(HTML) 11 Page - National Semiconductor (TI) |
11 / 15 page Application Information (Continued) POWER DISSIPATION Power dissipation is a major concern when designing a successful amplifier, whether the amplifier is bridged (BTL) or single-ended. A direct consequence of the increased power delivered to the load by a bridge amplifier is an increase in internal power dissipation. Since the LM4925 has two operational amplifiers in one package, the maximum internal power dissipation in BTL mode is 4 times that of a single-ended amplifier. The maximum power dissipation for a given application can be derived from the power dissipation graphs or from Equation 1. P DMAX =4*(VDD) 2 /(2 π2R L) (1) When operating in single ended mode, Equation 2 states the maximum power dissipation point for a single-ended ampli- fier operating at a given supply voltage and driving a speci- fied output load. P DMAX =(VDD) 2 /(2 π2R L) (2) Since the LM4925 has two operational amplifiers in one package, the maximum internal power dissipation point is twice that of the number that results from Equation 2. The maximum power dissipation point obtained from either Equations 1, 2 must not be greater than the power dissipa- tion that results from Equation 3: P DMAX =(TJMAX -TA)/ θ JA (3) For package MUB10A, θ JA = 175˚C/W. TJMAX = 150˚C for the LM4925. Depending on the ambient temperature, T A,of the system surroundings, Equation 3 can be used to find the maximum internal power dissipation supported by the IC packaging. If the result of Equation 1 or 2 is greater than that of Equation 3, then either the supply voltage must be de- creased, the load impedance increased or T A reduced. For the typical application of a 3.0V power supply, with an 16 Ω load, the maximum ambient temperature possible without violating the maximum junction temperature is approximately 129˚C provided that device operation is around the maxi- mum power dissipation point. Thus, for typical applications, power dissipation is not an issue. 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 dissi- pation information for lower output powers. POWER SUPPLY BYPASSING As with any amplifier, proper supply bypassing is important for low noise performance and high power supply rejection. The capacitor location on the power supply pins should be as close to the device as possible. Typical applications em- ploy a battery (or 3.0V regulator) with 10µF tantalum or electrolytic capacitor and a ceramic bypass capacitor that aid in supply stability. This does not eliminate the need for bypassing the supply nodes of the LM4925. A bypass ca- pacitor value in the range of 0.1µF to 4.7µF is recom- mended. MICRO POWER SHUTDOWN The voltage applied to the SHUTDOWN pin controls the LM4925’s shutdown function. Activate micro-power shut- down by applying a logic-low voltage to the SHUTDOWN pin. When active, the LM4925’s micro-power shutdown fea- ture turns off the amplifier’s bias circuitry, reducing the sup- ply current. A voltage that is higher than ground may in- crease 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 mi- crocontroller. When using a switch, connect an external 100k Ω pull-up resistor between the SHUTDOWN pin and V DD. Connect the switch between the SHUTDOWN pin and ground. Select normal amplifier operation by opening the switch. Closing the switch connects the SHUTDOWN pin to ground, 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 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. Shutdown enable/disable times are controlled by a combination of C bypass and VDD. Larger values of Cbypass results in longer turn on/off times from Shutdown. Longer shutdown times also improve the LM4925’s resistance to click and pop upon entering or returning from shutdown. For a 3.0V supply and C bypass = 4.7µF, the LM4925 requires about 2 seconds to enter or return from shutdown. This longer shutdown time enables the LM4925 to have virtually zero pop and click transients upon entering or release from shutdown. Smaller values of C bypass will decrease turn-on time, but at the cost of increased pop and click and reduced PSRR. When the LM4925 is in shutdown, the outputs be- come very low impedance (less than 5 Ω to GND). MUTE The LM4925 also features a mute function that enables extremely fast turn-on/turn-off with a minimum of output pop and click. The mute function leaves the outputs at their bias level, thus resulting in higher power consumption than shut- down mode, but also provides much faster turn on/off times. Providing a logic low signal on the MUTE pin enables mute mode. Threshold voltages and activation techniques match those given for the shutdown function as well. PROPER SELECTION OF EXTERNAL COMPONENTS Proper selection of external components in applications us- ing integrated power amplifiers is critical to optimize device and system performance. While the LM4925 is tolerant of external component combinations, consideration to compo- nent values must be used to maximize overall system qual- ity. The LM4925 is unity-gain stable that gives the designer maximum system flexibility. The LM4925 should be used in low gain configurations to minimize THD+N values, and maximize the signal to noise ratio. Low gain configurations require large input signals to obtain a given output power. Input signals equal to or greater than 1Vrms are available from sources such as audio codecs. Very large values should not be used for the gain-setting resistors. Values for Ri and Rf should be less than 1M Ω. Please refer to the section, Audio Power Amplifier Design, for a more complete explanation of proper gain selection. Besides gain, one of the major considerations is the closed-loop bandwidth of the amplifier. To a large extent, the bandwidth is dictated by the choice of external components shown in Figures 2 and 3. The input coupling capacitor, Ci, forms a first order high pass www.national.com 11 |
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