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MAX9728B Datasheet(PDF) 10 Page - Maxim Integrated Products |
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MAX9728B Datasheet(HTML) 10 Page - Maxim Integrated Products |
10 / 19 page Lineout Amplifier and Filter Block The MAX9728A can be used as an audio line driver capable of providing 2VRMS into 10k Ω loads with a sin- gle 5V supply (see Figure 3 for the RMS Output Voltage vs. Supply Voltage plot). 2VRMS is a popular audio line level, first used in CD players, but now common in DVD and set-top box (STB) interfacing standards. A 2VRMS sinusoidal signal equates to approximately 5.7VP-P, which means that the audio system designer cannot simply run the lineout stage from a (typically common) 5V supply—the resulting output swing would be inade- quate. A common solution to this problem is to use op amps driven from split supplies (±5V typically), or to use a high-voltage supply rail (9V to 12V). This can mean adding extra cost and complexity to the system power supply to meet this output level requirement. Having the ability to derive 2VRMS from a 5V supply can often sim- plify power-supply design in some systems. When the MAX9728A is used as a line driver to provide outputs that feed stereo equipment (receivers, STBs, notebooks, and desktops) with a digital-to-analog con- verter (DAC) used as an audio input source, it is often desirable to eliminate any high-frequency quantization noise produced by the DAC output before it reaches the load. This high-frequency noise can cause the input stages of the line-in equipment to exceed slew-rate lim- itations or create excessive EMI emissions on the cables between devices. To suppress this noise, and to provide a 2VRMS stan- dard audio output level from a single 5V supply, the MAX9728A can be configured as a line driver and active lowpass filter. Figure 4 shows the MAX9728A connected as 2-pole Rauch/multiple feedback filter with a passband gain of 6dB and a -3dB (below passband) cutoff frequency of approximately 27kHz (see Figure 5 for the Gain vs. Frequency plot). Layout and Grounding Proper layout and grounding are essential for optimum performance. Connect PGND and SGND together at a single point on the PC board. Connect PVSS to SVSS and bypass with a 1µF capacitor. Place the power-sup- ply bypass capacitor and the charge-pump hold capacitor as close to the MAX9728 as possible. Route PGND and all traces that carry switching transients away from SGND and the audio signal path. The thin QFN package features an exposed paddle that improves thermal efficiency. Ensure that the exposed paddle is electrically isolated from PGND, SGND, and VDD. Connect the exposed paddle to SVSS only when the board layout dictates that the exposed paddle cannot be left floating. 60mW, DirectDrive, Stereo Headphone Amplifiers with Shutdown 10 ______________________________________________________________________________________ LEFT AUDIO INPUT RIGHT AUDIO INPUT OUTL INL INR OUTR MAX9728A RIN RIN RF RF Figure 2. Gain Setting for the MAX9728A RMS OUTPUT VOLTAGE vs. SUPPLY VOLTAGE SUPPLY VOLTAGE (V) 5.0 2.7 2.9 3.1 3.3 3.5 3.7 3.9 4.1 2.5 4.5 5.5 fIN = 1kHz RL = 10k Ω 1% THD+N RL = 1k Ω 1% THD+N Figure 3. RMS Output Voltage vs. Supply Voltage |
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