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USB and audio convergence enabled through analog switches

(Note : This article originally appeared at the EE Times-Europe Analog home page.)

Mobile devices appear to be converging on set of standard hardware designs with standard hardware interfaces. The two most ubiquitous external interfaces on handsets today are the USB interface and the 3.5 mm audio jack. A new class of analog switches is emerging which can optimize both interfaces and provide a richer experience for the end user while providing great savings to the hardware designer.

The USB interface is typically used for data transmission with a host PC and is increasingly being used to charge the mobile device. This interface is often a micro-USB or mini-USB connector that provides a space saving, robust interface to the host PC.  Most mobile devices include a 3.5 mm audio jack for connection to headphones, headsets and speakers.

A new class of analog switches provides a solution that allows the audio output to be shared with the USB interface. These switches are specially designed to be able to share both high-speed USB data and high fidelity audio on a single interface. This allows a single user interface on the mobile device or another interface that the end user of the mobile device can enjoy their audio on.

Handset designers are always looking to add more features in a smaller form factor with each generation of a mobile device. By standardizing to a single micro or mini USB interface, the 3.5 mm headphone jack can be eliminated. This saves valuable space and reduces cost. This has many benefits to the handset designer in both area savings and true cost savings. In addition to physically removing the 3.5 mm headphone jack, additional components can be removed like ESD protection devices and filters.

To enable the transition to a single USB interface for both USB data and audio, an analog switch must be chosen to switch between the two sources. The audio and USB paths have very different requirements which must be taken into account when looking at the switch parameters. For the USB path, maintaining signal integrity and eye compliance is critical for all high speed USB interfaces. The on-resistance and on-capacitance specifications of analog switches are important to meet the demanding requirements of the USB protocol. 

Analog switches have to balance the requirements of low on-resistance and low on-capacitance.  As the on-resistance decreases, the switches tend to get larger and this increases the on-capacitance. Conversely, to get low on-capacitance, the switches become smaller and the  on-resistance increases.

Figure 1 shows an eye-compliance diagram for a high-speed USB interface.


Figure 1: High-speed USB eye-compliance diagram.
(Click on image to enlarge)

If the on-resistance of the USB data path of the analog switch is too large, the signal will be attenuated by the analog switch and cause sections B and D in Figure 1 to collapse and fail eye compliance. If the on-capacitance of the USB data path of the analog switch is too large then switch will roll the edges of the USB signals and cause sections A and C in Figure 1 to fail eye compliance. Ideally, the USB switch path would have very low on-resistance and on-capacitance. USB compliance requires switches than have lower than 4 pf of on-capacitance which often means the on-resistance is around 4 ohms. This combination provides a good overall solution for the USB data path.

For the audio path, ensuring the switch does not have a negative effect on the audio quality is important. Today’s consumers demand very high quality audio in almost any mobile device. The key parameter for the audio path is the THD+N performance. The THD+N of the switch is ideally less than the THD+N performance of the driving audio source which ensures that the quality of the audio signal is not negatively effected by adding the switch in the path. THD+N performance of the switch is dependent on the on-resistance and the on-resistance flatness. Figure 2 shows an on-resistance plot of an analog switch.


Figure 2: On-resistance plot of typical analog switch.
(Click on image to enlarge)

There are two key parameters when evaluating the on-resistance of analog switches. The first parameter is the actual on-resistance of the switch at the voltage level that the switch will typically be operated at. The on-resistance will usually be lowest at the highest operating voltage of the analog switch. For the audio path, having the lowest on-resistance is important because this resistance is in series with the load of the audio amplifier. This means that if the on-resistance is too high, the switch will limit the output power of the amplifier. Typically audio switches have on-resistance which is lower than 1 ohm, often as low as 0.4 ohms.

The second key parameter when looking at analog switches for audio signals is the on-resistance flatness of the switch. Having a flat on-resistance over the voltage swing of the audio signals is critical to having low THD+N. As shown in Figure 2, most analog switches will have what is commonly called a “bathtub plot” for on-resistance. Designing the switches so that this on-resistance curve is as flat as possible ensures that the audio signals are distorted as little as possible while passing through the switch. On-resistance flatness of 100 milliohms is typical is audio switches.

Another important feature for the audio path of the switch is if AC-coupled audio sources can be switched through the audio path.  AC-coupled audio is becoming more common in handsets because it does not require the large and expensive DC blocking capacitors. The DC blocking capacitors form a high pass filter with the impedance of the headset. The impedance of headphones is low, 16 to 32 ohms, which requires large capacitors to ensure that the low frequencies of the audio range are not filtered out of the audio path.

Often these capacitors are 220 μF, which cost about 0.04 US Dollars and are 3 mm × 2 mm. By going to an AC-coupled audio source, these capacitors are no longer needed and the headphones can be driven directly by the amplifier.  Another benefit of directly driving the headphones is that the overall bass response is improved. The analog switch must be able to pass negative swing signals with high fidelity to ensure that the THD+N remains low and that AC-coupled audio signals can be passed cleanly.

Another key consideration is if the switch itself can actually generate the AC-coupled audio output. This would allow designs with DC coupled audio to realise all the benefits of AC-coupled audio with minimal changes to the architecture and maintain all existing audio infrastructure. These new switches incorporate a high quality audio amplifier inside the switch which allows removal of the DC blocking capacitors in a very cost effective manner. In an application where audio can be output on both the 3.5 mm audio jack and the USB interface, the removal of the DC blocking caps can be a tremendous cost and space savings.

The requirements of the switch are different for the audio path compared to the high speed USB path. Often handset designers are faced with selecting an analog switch which is meant for either audio or USB. The analog switches that are meant for audio will have very low on-resistance but will have higher on-capacitance which can limit the speed performance of the USB path. The analog switches that are meant for USB will have very low on-capacitance but will have higher on-resistance and will not have as flat an on-resistance as the audio switches.

A new type of analog switch has been developed which combines the audio switch with the USB switch which gives handset designers the optimal solution. In addition to combing both USB and audio in a single switch, new switches are incorporating the audio amplifier into the switch, offering a cost effective way to remove the expensive and large DC blocking capacitors while providing a richer and better sounding audio output.

How to control the switch to select either the USB or audio path can be a concern in handset designs. Utilizing an additional GPIO off the baseband processor might not be a viable solution. Creating the new software to control that GPIO might be another hurdle preventing either the adoption of a single USB interface for the USB and audio paths or providing audio on both the 3.5 mm audio jack and the USB interface.

This can be overcome by selecting switches that automatically detect what is connected to the USB interface connector and selecting the proper path. One technique is monitoring the Vbus signal on the USB connector to determine if a USB interface is connected and automatically switching to the USB data path on the handset. This provides complete control of the switch without an additional GPIO or any software overhead.

Finally, due to the location of the USB and audio switch to the USB connector, the ESD performance of the switch becomes critical. Ideally, the switch can handle all ESD requirements of the system so costly external ESD are not required.

Summary
A new class of analog switches have emerged which are optimized to switch between a high quality audio output and a high speed USB data path. These switches allow mobile device designers to eliminate the 3.5mm audio connector or provide an audio output on both interfaces.

This article detailed the many competing parameters that these switches must balance to offer an optimal solution. By properly selecting an analog switch to switch between the audio outputs and the high speed USB path, mobile device hardware designers can offer an improved solution to their customer and realise both space savings and cost savings. The elimination of expensive DC blocking capacitors both saves cost and valuable PCB board area.

About the author

Erik Maier is a senior applications engineer at Fairchild Semiconductor Corp., where he is responsible for analog switch products within Fairchild including all audio switches. Maier has 10+ years of experience in the semiconductor industry and prior to support for analog switches, supported the interface products within Fairchild.

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