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Audio designers of portable computers are committed to continuously improving system audio performance. In space-constrained designs, the better solution is to use a 2.1-channel configuration that uses two stereo satellite speakers to handle intermediate and high frequencies (typically 150 Hz and higher) and a woofer to handle low frequencies ( Typical values ​​are 150 Hz and lower). This application note presents a 2.1-channel audio power amplifier system with a single 5V power supply with 2 x 2W and 1 x 9W output power using Maxim's professional audio and power ICs.Traditional solution
The main problem facing audio system designers is that satellite speakers and woofer have different output power requirements. Typically, the woofer requires 4 to 5 times more output power than the satellite speakers to achieve the proper sound balance. There are many audio amplifier solutions available when powered from a single 5V supply, but all have drawbacks.
1. The most common solution is to use two sets of stereo amplifiers with the same output power. One is for driving satellite speakers and the other is for driving woofer. Satellite speakers use 8Ω speakers, while woofers are 4Ω speakers. This constitutes a 2.1 channel solution for 2 x 1W satellites and 1 x 2W woofer. Although the solution is simple, it does not provide enough power for the woofer to produce a realistic bass effect. In addition, the use of 8Ω satellite speakers does not maximize the sound pressure level (SPL) of satellite speakers. Therefore, the overall sound effect of this scheme is very limited.
2. By replacing the speakers in the above scheme, using a 4Ω satellite speaker and a 2Ω woofer, a 2.1 solution for 2×2W satellite speakers and 1×4W woofer can be constructed. This solution doubles the output power and increases the sound pressure level. However, purchasing 2Ω speakers and power amplifiers that drive such speakers is very difficult and costly. In addition, the supply current demand will probably double, which will reduce the efficiency of the solution, especially in systems where board space is limited, which may cause thermal issues.
3. Better than the above two solutions is to use 2 × 2W amplifier for satellite speakers, 1 × 9W amplifier for woofer. In this configuration, the satellite speaker is 4Ω and takes full advantage of the 5V supply voltage; while the woofer is 8Ω, it produces enough bass at 9W. However, the 9W woofer amplifier requires a 12V power supply, adding to the complexity of the solution. For a system that can only provide one 5V power supply, it is necessary to generate a 12V power supply.
Traditional program analysis
The advantage of using the 2.1 channel speaker system is that it can produce a "shock" sound effect from a small space. To achieve this, the woofer amplifier is at least 4 to 5 times more powerful than the satellite speaker amplifier. For a 2W satellite speaker amplifier, the output power of the woofer amplifier is at least 8W to 10W.
The solutions #1 and #2 mentioned above are easy to implement because only one single 5V power supply is required. However, these two solutions cannot solve the problem because they lack sufficient power to drive the woofer. Solution #3 is ideal, but only if you can easily get an extra 12V power supply.
Maxim's solutions and their advantages
Figure 1 shows Maxim's complete solution for a 2.1 channel speaker system. The solution uses the MAX9789, a 2×2W stereo amplifier with a stereo headphone driver, the MAX9768, a 1×10W mono Class D amplifier, and the MAX8740, a low noise step-up DC/DC converter.
The MAX9789 integrates a stereo 2W Class AB speaker amplifier and a 100mW stereo DirectDrive headphone amplifier that drive satellite speakers in a 2.1-channel system in a single device. The MAX9789 is designed for use on portable computer systems with the Windows Vista operating system and is fully compatible with the Windows Vista specification. The headphone amplifier features Maxim's patented DirectDrive architecture that produces a ground-referenced output from a single supply without the need for an output DC blocking capacitor. This DirectDrive architecture saves cost, reduces board space, reduces device height, and eliminates the need for output DC blocking capacitors to eliminate clicks and pops. In addition, the MAX9789 integrates a 1.21~4.75V output-adjustable LDO to provide a clean power supply for audio codecs or other analog circuits.
The MAX9768 is a Class D amplifier that requires no filtered output and can deliver 9W of output power to an 8Ω load from 10% THD+N on a 12V supply. Its Class D modulation eliminates the need for an output filter, which reduces cost and provides enough 9W power to the woofer in a 2.1 channel system. The MAX9768 is 87% efficient and requires no heat sink. In addition, its spread-spectrum modulation mode allows the device to pass FCC EMI limits with low-cost ferrite beads and capacitors on each output when using a 0.5m cable.
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NINGBO YULIANG TELECOM MUNICATIONS EQUIPMENT CO.,LTD. , https://www.yltelecom.com