Maxim's DirectDrive II Headphone Amplifier Extends Audio Playback Time in Portable Devices

Today's portable consumer electronics support numerous multimedia functions that put a strain on battery life. This problem is only exacerbated with each new design cycle, as designers are pressed to integrate camera modules, HD video capabilities, support for web browsing, and other differentiating features into the latest generation of a device. To keep up with this feature creep, system designers must find new ways to conserve battery life.



Proper amplifier selection is critical to extending the power budget. Class AB amplifiers, for instance, are notoriously inefficient. They always use the maximum supply voltage, regardless of audio signal level. This approach results in typical efficiencies in the 13% to 15% range at normal listening levels (2mW to 5mW into a 16ohm load).



In contrast, Class H amplifiers (called Class G in Europe) switch between multiple internally generated supply rails based on the amplitude of the output signal. By quickly modulating the supply voltage, Class H amplifiers improve efficiency while minimizing dynamic distortion at the output stage.



This approach enables the MAX97200 to improve efficiency by up to 97% compared to Class AB amplifiers. Over the 2mW to 5mW range, the MAX97200 achieves efficiencies between 20% and 29%. Figure 1 compares the efficiency of Maxim's Class H amplifier to a Class AB amplifier (MAX9725).



DirectDrive II Technology Enables the Lowest Power Class H Architecture

Maxim's DirectDrive II technology utilizes an integrated dual-mode charge pump to generate ±PVIN and ±PVIN/2 from an externally regulated 1.8V supply. The MAX97200 uses ±PVIN/2 for low-amplitude output signals, only commuting to the ±1.8V rails when more output power needs to be delivered to the load. Since most audio signals are of the low-amplitude variety, the MAX97200 achieves power consumption similar to a traditional ±0.9V DirectDrive amplifier.



To further conserve battery power, it consumes less than 1microamp in shutdown and only 1.15mA (typ) during normal operation.





Glitch-Free Audio Performance

The challenge when implementing a Class H architecture is to switch the supply just before the output signal commutes to a higher level; otherwise, the audio output will suffer from a noticeable clipping effect and create dynamic distortion.



Maxim's Class H technology employs a unique tracking feature to anticipate changes in audio signal level and, then, seamlessly adjust the supply. This renders a continuous signal without the glitches seen in competitive amplifiers. Figure 2 juxtaposes the MAX97200's distortion-free performance with the clipping produced by a competitive Class G amplifier.

The MAX97200 also provides industry-leading click-and-pop performance both into and out of shutdown. This is guaranteed by a very low output-offset voltage. Because the DirectDrive II architecture produces a ground-referenced output from a single supply, it eliminates the large DC-blocking capacitors normally required between the amplifier output and the headphone load. This feature allows the end user to enjoy a better listening experience without annoying clicks and pops.



Additionally, the MAX97200 features a high signal-to-noise ratio (SNR = 105dB) to maintain system fidelity. The output noise is only 5.6microvolts, which improves the overall audio noise-floor quality and avoids the annoying hiss normally generated in the absence of a signal.



Optimized for Small Form Factors

The MAX97200 is the smallest high-efficiency headphone amplifier on the market. Offered in a tiny (1.27mm x 1.65mm), 12-bump wafer-level package (WLP), the IC is 25% smaller than the closest competitor. The total solution size is 30% smaller than competing devices, since the MAX97200 does not require an inductor (Figure 3).