Improving Portable Designs with Application-Specific Analog Switches

As the market demand for feature-rich mobile phones continues to grow, analog switches with application-specific performance continue to be favored in the final design. This not only reduces the bill of materials (BOM), but also helps improve the design performance and meet the time-to-market requirements. This article will guide system designers through several real-world use cases on how to reduce impact noise (pop noise), detect chargers , and improve eye opening.

At the same time, this article also compares traditional solutions with integrated solutions to illustrate the benefits of adopting this high-performance analog product in the mobile phone market's move towards multimedia design.

Reducing impact noise

Impact noise caused by inrush current remains a difficult challenge for designers, especially when the end user activates the switch between music and call functions. As long as the end user turns on the music function, this annoying noise will bring an unpleasant experience. As shown in Figure 1, when the audio amplifier is operating, the power supply on/off surge current through the AC coupling capacitor is the culprit for the impact noise, and the audio common-mode voltage will increase sharply at this time. There are currently several solutions on the market. One of them is to add an extra amplifier to give the audio output a "0V" bias, thereby minimizing the size of the AC coupling capacitor just before the headphone. Since most headphone amplifiers are integrated into the baseband processor or power management unit (PMU), adding this amplifier not only increases the bill of materials cost, but also increases power consumption. Figure 1 shows another approach, which adds a separate charging path in the audio signal path, allowing the AC coupling capacitor to be fully charged before being switched to the headphone or main channel. This can be controlled using the general-purpose I/O of the baseband processor , so that the audio amplifier and switch are powered up first, and the main channel switch is closed at this time. The common-mode voltage of the audio output will begin to rise from 0 to VCC/2. After a period of time (referenced to 10ms), the coupling capacitors are charged to the same potential, and there will be no inrush current when the main channel is turned on again, because the voltage difference between the two electrodes of the capacitor is now 0V. Figure 1: Audio switch with low THD and negative swing function can eliminate audio surge noise. This switch is well suited for cell phones and MP3/MP4 players where a single USB connector (D+/D- pins) is shared by headphones and USB data lines. Low total harmonic distortion (THD) is very important for the audio channel. In addition, because the switch is placed after the AC coupling capacitors, it must handle large reverse signal swings with low THD. The ultra-low off capacitance of this switch allows high-speed USB signals to be "wired-OR" connected through this device. And low parasitic capacitance is also key to compliance testing of the high-speed USB 2.0 standard.

Application-Specific USB Switches

With the current market trend toward a single USB charger / data port, application-specific USB switches have become a common configuration in cell phone designs with charger



detection. Figure 2 shows an example of such a switch application. Figure 2: USB switches with charger detection are ideal for high-speed USB applications where the USB power and data ports are shared.

A low on-capacitance switch is required in such designs for two main reasons. First, since the baseband processor and high-speed USB controller outputs share the same D+/D- pins on the connector side, the output capacitance of the baseband USB1.1/2.0 full-speed controller must be reduced when the phone enters high-speed USB 2.0 mode (such as music download or flash memory functions). Any additional capacitance on the D+/D- lines will degrade the eye opening of the high-speed USB signals. Second, in high-speed USB mode, the extra traces left hanging on the D+/D- lines must be cut off to effectively avoid signal reflections caused by the fast rising/falling edges of the 480Mbps USB signals.

Since a single USB port is used for both charger and data functions, charger detection has become very popular in current designs. The traditional approach is to feed the D+/D- lines to an internal A/D converter to determine if the D+/D- lines are shorted. As mentioned earlier, the main limitation of this approach is that the high input capacitance of the baseband processor GPIO port will add additional capacitance to the data line. This added capacitance will have a very negative impact on the effective triggering of the signal at the high data rate that is part of the USB 2.0 compliance test (for example, 480 Mbps for USB 2.0 signals). Of course, another disadvantage of this approach is that it also occupies the resources of the system A/D converter.

In these applications, USB switches with ultra-low internal capacitance detection circuits are required to achieve charger detection and isolation of the output capacitance of the full-speed USB controller. At the same time, the USB channel select pin (S pin in Figure 2) used to determine which USB channel is selected as the output must be able to recognize 1.8 V and 3 V logic inputs (note: 1.8 V and 3 V are quite common in the baseband processor GPIO output).

Traditional switch select pins can accept input "high" (Vih) levels up to 2.0 V (TTL logic), which can result in significant leakage current when the switch power (VCC) is taken directly from the battery . The ability to recognize 1.8 V input logic levels can also eliminate external level translation devices, allowing designers to further reduce bill of materials costs. For example, ICs such as Fairchild's FSUSB45 have ultra-low on-capacitance (7pF) and small size (1.4×1.8 mm) as well as charger detection and 1.8 V control logic recognition, which are well suited to USB data path switch design needs.

Summary

Analog switch applications have been evolving from simple audio switching functions to more advanced products that can provide both value-added design features and strong I/O-to-ground ESD capabilities. As multimedia features such as MP3/MP4 players and GPS/WiFi functions become more prevalent in end applications, designers need more application-specific switches that can not only provide low-distortion switching channels but also address the design challenges faced by standard compliance testing. Additionally, these switches reduce bill of materials costs and significantly shorten time to market.