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LM4671 Datasheet(PDF) 10 Page - National Semiconductor (TI) |
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LM4671 Datasheet(HTML) 10 Page - National Semiconductor (TI) |
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10 / 18 page  Typical Performance Characteristics (Continued) Gain vs Supply Voltage R in = 150k Ω Supply Current vs Supply Voltage R L = 15µH+8 Ω+15µH 20107319 20107320 Shutdown Current vs Supply Voltage R L = 15µH+8 Ω+15µH 20107321 Application Information GENERAL AMPLIFIER FUNCTION The LM4671 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 LM4671 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 LM4671 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 LM4671 and in the transducer load. The amount of power dissipation in the LM4671 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 www.national.com 10 |
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