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SSM2211P Datasheet(PDF) 9 Page - Analog Devices |
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SSM2211P Datasheet(HTML) 9 Page - Analog Devices |
9 / 16 page SSM2211 REV. 0 –9– Driving a speaker differentially from a bridged output offers an- other advantage in that it eliminates the need for an output cou- pling capacitor to the load. In a single supply application, the quiescent voltage at the output is 1/2 of the supply voltage. If a speaker were connected in a single ended configuration, a cou- pling capacitor would be needed to prevent dc current from flowing through the speaker. This capacitor would also need to be large enough to prevent low frequency roll-off. The corner frequency is given by: f RC dB LC − = 3 1 2 π (4) Where RL is the speaker resistance and, CC is the coupling capacitance For an 8 Ω speaker and a corner frequency of 20 Hz, a 1000 µF capacitor would be needed, which is quite physically large and costly. By connecting a speaker in a bridged output configura- tion, the quiescent differential voltage across the speaker be- comes nearly zero, eliminating the need for the coupling capacitor. Speaker Efficiency and Loudness The effective loudness of 1 W of power delivered into an 8 Ω speaker is a function of the efficiency of the speaker. The effi- ciency of a speaker is typically rated as the sound pressure level (SPL) at 1 meter in front of the speaker with 1 W of power applied to the speaker. Most speakers are between 85 dB and 95 dB SPL at 1 meter at 1 W. Table I shows a comparison of the relative loudness of different sounds. Table I. Typical Sound Pressure Levels Source of Sound dB SPL Threshold of Pain 120 Heavy Street Traffic 95 Cabin of Jet Aircraft 80 Average Conversation 65 Average Home at Night 50 Quiet Recording Studio 30 Threshold of Hearing 0 It can easily be seen that 1 W of power into a speaker can pro- duce quite a bit of acoustic energy. Power Dissipation Another important advantage in using a bridged output configu- ration is the fact that bridged output amplifiers are more effi- cient than single ended amplifiers in delivering power to a load. Efficiency is defined as the ratio of power from the power supply to the power delivered to the load η = P P L SY . An amplifier with a higher efficiency has less internal power dissipation, which results in a lower die-to-case junction temperature, as compared to an amplifier that is less efficient. This is important when considering the amplifier device’s maximum power dissi- pation rating versus ambient temperature. An internal power dissipation versus output power equation can be derived to fully understand this. The internal power dissipation of the amplifier is the internal voltage drop multiplied by the average value of the supply cur- rent. An easier way to find internal power dissipation is to take the difference between the power delivered by the supply voltage source and the power delivered into the load. The waveform of the supply current for a bridged output amplifier is shown in Figure 40. IDD, AVG TIME T T TIME VOUT VPEAK ISY IDD, PEAK Figure 40. Bridged Amplifier Output Voltage and Supply Current vs. Time By integrating the supply current over a period T, then dividing the result by T, IDD,AVG can be found. Expressed in terms of peak output voltage and load resistance: I V R DD AVG PEAK L , = 2 π (5) therefore power delivered by the supply, neglecting the bias cur- rent for the device is, P VV R SY DD PEAK L = 2 π (6) Now, the power dissipated by the amplifier internally is simply the difference between Equation 6 and Equation 3. The equa- tion for internal power dissipated, PDISS, expressed in terms of power delivered to the load and load resistance is: P V R PP DISS DD L LL = × − 22 π (7) The graph of this equation is shown in Figure 41. |
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