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APA3544KI-TR Datasheet(PDF) 8 Page - Anpec Electronics Coropration |
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APA3544KI-TR Datasheet(HTML) 8 Page - Anpec Electronics Coropration |
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8 / 13 page  Copyright ANPEC Electronics Corp. Rev. B.1 -Apr., 2003 APA3541/4 www.anpec.com.tw 8 Application Note Input Capacitor , Ci In the typical application an input capacitor , Ci , is required to allow the amplifier to bias the input signal to the proper DC level for optimum operation . In this case , the external capacitor Ci and the internal re- sistance Ri form a high-pass filter with the corner fre- quency determined in the follow equation: fc (highpass)= 1/ (2 πRiCi) (1) The value of Ci is important to consider as it directly affects the low frequency performance of the circuit. Consider the APA3541 where Ri is 180k Ω and APA3544 is 90k Ω internal fixed . Equation is reconfigured as follow: Ci= 1/(2 π*180kΩ*fc) for APA3541 Ci= 1/(2 π*90kΩ*fc) for APA3544 (2) And the ceramic capacitor is recommanded. Bias Capacitor , Cb As with any power amplifier , proper supply bypass- ing is critical for low noise performance and high power supply rejection . The capacitor location on both the bypass and power supply pins should be as close to the device as possible . The effect of a larger half supply bias capacitor is improved PSRR due to increased half-supply stability . Typical applications employ a 5V regulator with 10 µF and a 0 . 1µF bias capacitors which aid in supply filtering . This does not eliminate the need for bypassing the supply nodes of the APA3541/4 . The selection of bias capacitors , especially Cb , is thus dependent upon desired PSRR requirements , click and pop per- formance . The capacitor is fed from a 95k Ω source inside the amplifier . To keep the start-up pop as low as possible , the relationship shown in equation should be maintained . 1/(Cb*95k Ω)≤ 1/{Ci*Ri} (3) As an example , consider a circuit where Cb is 4. 7µF, Ci is 1µF and APA3541 Ri is 180kΩ . Inserting these values into the equation we get 2.24 ≤ 5.55 which satisfies the rule . Bias capacitor , Cb , values of 2.2 µF to 10µF ceramic or tantalum low-ESR ca- pacitors are recommended for the best THD and noise performance . Output Coupling Capacitor, Cc In the typical single-supply SE configuration , an out- put coupling capacitor (Cc) is required to block the DC bias at the output of the amplifier thus preventing DC currents in the load . As with the input coupling capacitor , the output coupling capacitor and imped- ance of the load form a high-pass filter governed by equation . fc(highpass)= 1/(2 πR LCc) (4) For example , a 220 µF capacitor with an 32Ω speaker would attenuate low frequencies below 22Hz . The main disadvantage , from a performance standpoint , is the load impedance is typically small , which drives the low-frequency corner higher degrading the bass response . Large values of Cc are required to pass low frequencies into the load . Optimizing Depop Circuitry When the amplifier is in mute mode , both of the out- put stage and input bypass continues to be biased . And no pop noise will be heard during the transition out of mute mode . Power Supply Decoupling, Cs APA3541/4 is a high-performance CMOS audio am- plifier that requires adequate power supply decoupling to ensure the output total harmonic distortion (THD) is as low as possible . Power supply decoupling also prevents the oscillations causing by long lead length between the amplifier and the speaker . The optimum decoupling is achieved by using two different type capacitors that target on different type of noise on the power supply leads . For higher frequency tran- sients , spikes , or digital hash on the line , a good low equivalent-series-resistance (ESR) ceramic capacitor, typically 0.1 µF placed as close as possible to the device V DD lead works best . For filtering lower- frequency noise signals , a large aluminum electro- lytic capacitor of 10 µF or greater placed near the audio power amplifier is recommended . |
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