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TPA721 Datasheet(PDF) 17 Page  Texas Instruments 

TPA721 Datasheet(HTML) 17 Page  Texas Instruments 
17 / 28 page www.ti.com Midrail Bypass Capacitor, CB 10 C B 250 k Ω v 1 R F ) R I C I (10) USING LOWESR CAPACITORS 5V VERSUS 3.3V OPERATION HEADROOM AND THERMAL CONSIDERATIONS P dB + 10LogP W + 10Log 700 mW + –1.5 dB TPA721 SLOS231E – NOVEMBER 1998 – REVISED JUNE 2004 The midrail bypass capacitor, CB, is the most critical capacitor and serves several important functions. During startup or recovery from shutdown mode, CB determines the rate at which the amplifier starts up. The second function is to reduce noise produced by the power supply caused by coupling into the output drive signal. This noise is from the midrail generation circuit internal to the amplifier, which appears as degraded PSRR and THD + N. The capacitor is fed from a 250k Ω source inside the amplifier. To keep the startup pop as low as possible, the relationship shown in Equation 10 should be maintained. This insures the input capacitor is fully charged before the bypass capacitor is fully charged and the amplifier starts up. As an example, consider a circuit where CB is 2.2 µF, CI is 0.47 µF, RF is 50 kΩ, and RI is 10 kΩ. Inserting these values into the Equation 10 results in: 18.2 ≤ 35.5 which satisfies the rule. Recommended value for bypass capacitor CB is 0.1µF to 2.2µF, ceramic or tantalum lowESR, for the best THD and noise performance. LowESR capacitors are recommended throughout this applications section. A real (as opposed to ideal) capacitor can be modeled simply as a resistor in series with an ideal capacitor. The voltage drop across this resistor minimizes the beneficial effects of the capacitor in the circuit. The lower the equivalent value of this resistance, the more the real capacitor behaves like an ideal capacitor. The TPA721 operates over a supply range of 2.5 V to 5.5 V. This data sheet provides full specifications for 5V and 3.3V operation, as these are considered to be the two most common standard voltages. There are no special considerations for 3.3V versus 5V operation with respect to supply bypassing, gain setting, or stability. The most important consideration is that of output power. Each amplifier in TPA721 can produce a maximum voltage swing of VDD –1 V. This means, for 3.3V operation, clipping starts to occur when VO(PP) = 2.3 V as opposed to VO(PP) = 4 V for 5V operation. The reduced voltage swing subsequently reduces maximum output power into an 8 Ω load before distortion becomes significant. Operation from 3.3V supplies, as can be shown from the efficiency formula in Equation 4, consumes approximately twothirds the supply power of operation from 5V supplies for a given outputpower level. Linear power amplifiers dissipate a significant amount of heat in the package under normal operating conditions. A typical music CD requires 12 dB to 15 dB of dynamic headroom to pass the loudest portions without distortion as compared with the average power output. The TPA721 data sheet shows that when the TPA721 is operating from a 5V supply into an 8 Ω speaker, 700 mW peaks are available. Converting watts to dB: Subtracting the headroom restriction to obtain the average listening level without distortion yields: –1.5 dB – 15 dB = –16.5 (15dB headroom) –1.5 dB – 12 dB= –13.5 (12dB headroom) –1.5 dB – 9 dB = –10.5 (9dB headroom) –1.5 dB – 6 dB = –7.5 (6dB headroom) –1.5 dB – 3 dB= –4.5 (3dB headroom 17 
