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LM4673TM bảng dữ liệu(PDF) 10 Page - National Semiconductor (TI) |
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LM4673TM bảng dữ liệu(HTML) 10 Page - National Semiconductor (TI) |
10 / 18 page Application Information GENERAL AMPLIFIER FUNCTION The LM4673 features a filterless modulation scheme. The differential outputs of the device switch at 300kHz from V DD to GND. When there is no input signal applied, the two outputs (V O1 and VO2) switch with a 50% duty cycle, with both outputs in phase. Because the outputs of the LM4673 are differential, the two signals cancel each other. This re- sults in no net voltage across the speaker, thus there is no load current during an idle state, conserving power. With an input signal applied, the duty cycle (pulse width) of the LM4673 outputs changes. For increasing output volt- ages, the duty cycle of V O1 increases, while the duty cycle of V O2 decreases. For decreasing output voltages, the con- verse occurs, the duty cycle of V O2 increases while the duty cycle of V O1 decreases. The difference between the two pulse widths yields the differential output voltage. POWER DISSIPATION AND EFFICIENCY In general terms, efficiency is considered to be the ratio of useful work output divided by the total energy required to produce it with the difference being the power dissipated, typically, in the IC. The key here is “useful” work. For audio systems, the energy delivered in the audible bands is con- sidered useful including the distortion products of the input signal. Sub-sonic (DC) and super-sonic components (>22kHz) are not useful. The difference between the power flowing from the power supply and the audio band power being transduced is dissipated in the LM4673 and in the transducer load. The amount of power dissipation in the LM4673 is very low. This is because the ON resistance of the switches used to form the output waveforms is typically less than 0.25 Ω. This leaves only the transducer load as a po- tential "sink" for the small excess of input power over audio band output power. The LM4673 dissipates only a fraction of the excess power requiring no additional PCB area or cop- per plane to act as a heat sink. DIFFERENTIAL AMPLIFIER EXPLANATION As logic supply voltages continue to shrink, designers are increasingly turning to differential analog signal handling to preserve signal to noise ratios with restricted voltage swing. The LM4673 is a fully differential amplifier that features differential input and output stages. A differential amplifier amplifies the difference between the two input signals. Tra- ditional audio power amplifiers have typically offered only single-ended inputs resulting in a 6dB reduction in signal to noise ratio relative to differential inputs. The LM4673 also offers the possibility of DC input coupling which eliminates the two external AC coupling, DC blocking capacitors. The LM4673 can be used, however, as a single ended input amplifier while still retaining it’s fully differential benefits. In fact, completely unrelated signals may be placed on the input pins. The LM4673 simply amplifies the difference be- tween the signals. A major benefit of a differential amplifier is the improved common mode rejection ratio (CMRR) over single input amplifiers. The common-mode rejection charac- teristic of the differential amplifier reduces sensitivity to ground offset related noise injection, especially important in high noise applications. PCB LAYOUT CONSIDERATIONS As output power increases, interconnect resistance (PCB traces and wires) between the amplifier, load and power supply create a voltage drop. The voltage loss on the traces between the LM4673 and the load results is lower output power and decreased efficiency. Higher trace resistance between the supply and the LM4673 has the same effect as a poorly regulated supply, increased ripple on the supply line also reducing the peak output power. The effects of residual trace resistance increases as output current increases due to higher output power, decreased load impedance or both. To maintain the highest output voltage swing and corre- sponding peak output power, the PCB traces that connect the output pins to the load and the supply pins to the power supply should be as wide as possible to minimize trace resistance. The use of power and ground planes will give the best THD+N performance. While reducing trace resistance, the use of power planes also creates parasite capacitors that help to filter the power supply line. The inductive nature of the transducer load can also result in overshoot on one or both edges, clamped by the parasitic diodes to GND and V DD in each case. From an EMI stand- point, this is an aggressive waveform that can radiate or conduct to other components in the system and cause inter- ference. It is essential to keep the power and output traces short and well shielded if possible. Use of ground planes, beads, and micro-strip layout techniques are all useful in preventing unwanted interference. As the distance from the LM4673 and the speaker increase, the amount of EMI radiation will increase since the output wires or traces acting as antenna become more efficient with length. What is acceptable EMI is highly application specific. Ferrite chip inductors placed close to the LM4673 may be needed to reduce EMI radiation. The value of the ferrite chip is very application specific. POWER SUPPLY BYPASSING As with any power amplifier, proper supply bypassing is critical for low noise performance and high power supply rejection ratio (PSRR). The capacitor (C S) location should be as close as possible to the LM4673. Typical applications employ a voltage regulator with a 10µF and a 0.1µF bypass capacitors that increase supply stability. These capacitors do not eliminate the need for bypassing on the supply pin of the LM4673. A 4.7µF tantalum capacitor is recommended. SHUTDOWN FUNCTION In order to reduce power consumption while not in use, the LM4673 contains shutdown circuitry that reduces current draw to less than 0.01µA. The trigger point for shutdown is shown as a typical value in the Electrical Characteristics Tables and in the Shutdown Hysteresis Voltage graphs found in the Typical Performance Characteristics section. It is best to switch between ground and supply for minimum current usage while in the shutdown state. While the LM4673 may be disabled with shutdown voltages in between ground and supply, the idle current will be greater than the typical 0.01µA value. Increased THD may also be observed with voltages less than V DD on the Shutdown pin when in PLAY mode. The LM4673 has an internal resistor connected between GND and Shutdown pins. The purpose of this resistor is to eliminate any unwanted state changes when the Shutdown www.national.com 10 |
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