As the market demand for feature-rich mobile phones continues to grow, analog switches with application-specific performance continue to be favored in final designs. This not only reduces bill of materials (BOM), but also helps improve design performance and meet time-to-market requirements. This article will guide system designers on how to reduce impact noise (pop noise), detect chargers, and improve eye opening through several real-world use cases.
At the same time, this article also compares traditional solutions with integrated solutions to illustrate the benefits of adopting such high-performance analog products as the mobile phone market moves 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, the power supply on/off surge current through the AC coupling capacitor is the culprit for the impact noise when the audio amplifier is working, and the audio common-mode voltage at this time will increase sharply.
There are currently several solutions on the market. One of them is to add an additional amplifier to give the audio output a "0V" bias, thereby minimizing the size of the AC coupling capacitor immediately before the earphone. Because 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 an alternative approach that adds a separate charging path in the audio signal path, allowing the AC coupling capacitor to be fully charged before switching to the headphone or main channel. This can be controlled by the general purpose I/O of the baseband processor, so that the audio amplifier and switch are powered up first, while the main channel switch is closed. The common mode voltage of the audio output will begin to rise from 0 to VCC/2. After a period of time (10ms as a reference), 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 terminals of the capacitor is now 0V.
Figure 1: An audio switch with low THD and negative swing function can eliminate audio surge noise.
This switch is suitable for mobile phones and MP3/MP4 players where a single USB connector (D+/D- pins) is shared by the headphone and USB data lines. Low total harmonic distortion (THD) is very important for the audio channel. Additionally, because the switch is placed after the AC coupling capacitors, it must handle large reverse signal swings at low THD. The ultra-low off capacitance of this switch allows high-speed USB signals to be wired-ORed with the device. 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 single USB charger/data ports, application-specific USB switches have become a common feature 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.
There are two main reasons why low on-capacitance switches are required in this design. 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 signal. Second, when in high-speed USB mode, the additional traces 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 signal.
Since a single USB port is used for both charger and data functions, charger detection function has become very popular in current designs. The traditional solution is to feed the D+/D- lines to an internal A/D converter to determine whether the D+/D- lines are shorted. As mentioned previously, 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 signaling). Of course, another disadvantage of this approach is that it also uses system A/D converter resources.
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 baseband processor GPIO outputs).
Traditional switch select pins can accept input "high" (Vih) levels as high as 2.0 V (TTL logic), which can cause significant leakage current when the switch power supply (VCC) is directly taken from the battery. The ability to recognize 1.8 V input logic levels can also eliminate external level-shifting 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 can well meet the needs of USB data path switch design.
Summary of this article
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. In addition, these switches can reduce bill of materials costs and significantly shorten product time to market.
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