Figure 6 - TI's DRV60x family, which integrates a bipolar op amp and charge pump.

This is one of the newest methods used to generate a higher-voltage signal swing output and maintaining a ground-biased output.

DirectPath technology patented by TI integrates a charge pump into the device, creating an internal negative supply. This allows the amplifier in the device to run in a bipolar (dual supply) mode, creating a potential 6-V+ swing with a ground-biased output.

Figure 7 - DirectPath gives bipolar performance with single-supply ease of integration.

Such technology offers the best of both worlds. The performance and the advantages of a ground-biased output offered by a bipolar power supply gives pop-free performance, along with a direct connection to the next device in the chain - with no DC-blocking caps. Bass response is also a lot more natural. Furthermore, running from a +3.3-V supply means the device can run from a typical system voltage (usually found in digital processors, etc). No need now for dedicated higher voltage rails, or, usually overlooked, the need to lay out extra power traces across your system to the output amplifier.

For the majority of home audio systems, an audio performance of 107 dB is in excess of any DACs in their systems. It's worth pointing out that some systems may need to move to a traditional dual supply output driver design. Typical systems mostly would be deemed "professional" AV receivers whose converters are running in the 110-dB+ range. A large range of op amps are available for these applications as well.

Conclusions:

• Advantages: The best of both worlds - pop free, clear bass response, easy-to-lay-out device - ideal for 95% of the home audio market.
• Disadvantages: Performance limitation at around 107-dB dynamic range - usually not a problem in the set-top box, portable DVD player and gaming console markets.

Many designers are finally beginning to ask themselves why they need higher voltage rails in their systems, if all they are doing is driving the output line drivers. New products coming on the market with single-supply devices and a built-in charge pump to generate a negative supply rail, such as with DirectPath, are rapidly changing the landscape of audio and video products.

References:
For more information on TI audio solutions, visit: http://www.ti.com/audio.

About the Author:
Dafydd Roche is the Home Entertainment and Professional Audio Marketing Manager for TI's High Performance Analog group. A graduate from the University of York (UK), Dafydd pours his passion and knowledge of audio and music making into his work, doing his part to help enable audio design engineers to make products that end users can't wait to use. In between a hectic life of customer visits, internal meetings and tradeshows, Dafydd still manages to find time to make and record music with fellow musicians in the Dallas area.

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