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SSM2250RM-REEL Datasheet(PDF) 10 Page - Analog Devices |
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SSM2250RM-REEL Datasheet(HTML) 10 Page - Analog Devices |
10 / 12 page SSM2250 Rev. A | Page 10 of 12 SUPPLY VOLTAGE (V) 1.6 0.8 1.0 1.2 1.4 0.6 0.4 0.2 0 1.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 RL = 4Ω RL =8Ω RL = 16Ω Figure 18. Maximum BTL Output Power vs. VS Use Figure 18 to find the minimum supply voltage needed to achieve a specified maximum undistorted output power. The output power in SE mode is exactly one-fourth the equivalent output power in BTL mode. This is because twice the voltage swing across the two BTL outputs results in 4 × the power delivered to the load. Figure 19 shows the maximum output power in SE mode vs. supply voltage for various headphone loads. SUPPLY VOLTAGE (V) 100 25 50 75 0 1.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 RL = 32Ω RL = 128Ω RL = 64Ω Figure 19. Maximum SE Output Power vs. VS Example An application requires only 500 mW to be output in BTL mode into an 8 Ω speaker. By inspection, the minimum supply voltage required is 3.3 V. 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 efficiency of a speaker is typically rated at 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 one meter at 1 W of power. Table 5 shows a com- parison of the relative loudness of different sounds. Table 5. 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 produce quite a bit of acoustic energy. Shutdown Feature The SSM2250 can be put into a low power consumption shutdown mode by connecting Pin 3 to VDD. In shutdown mode, the SSM2250 has low supply current of 60 μA. Pin 3 should be connected to ground for normal operation. Connecting Pin 3 to VDD shuts down all amplifiers and puts all outputs into a high impedance state, effectively muting the SSM2250. A pull-up or pull-down resistor is not required. Pin 3 should never be left floating, as this could produce unpre- dictable results. PC99-Compliant Computer Audio Reference Design The schematic shown in Figure 20 is a reference design for a complete audio system in a computer. The design is compliant with the PC99 standard for computer audio. The AD1881A is an AC’97, version 2.1, audio codec, available from Analog Devices. The stereo output from the AD1881A is coupled into the SSM2250, which is used to drive a mono internal speaker and stereo headphones. The internal speaker switching is controlled by the SSM2250 through the normaliz- ing pin on the headphone jack. The AD1881A controls the shutdown pin on the SSM2250, and is activated through the power management software drivers installed on the computer. For more information, refer to the AD1881A data sheet. |
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