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Use analog switches for multimedia cellphone design

Posted: 01 May 2008 ?? ?Print Version ?Bookmark and Share

Keywords:cellphone design? application-specific analog switch? reduce pop noise?

With increased market demand for feature-rich cellphones, analog switches with application-specific features continue to be adopted in end designs. This is done to reduce BOM cost, improve design performance and meet the time-to-market windows.

In this article, several switch applications are cited to guide system designers with reducing pop noise, detecting chargers and improving USB eye-pattern opening. Traditional solutions are also compared with integrated solutions to illustrate the benefit of adopting high-performance analog products, as the market trend moves toward multimedia designs in cellphones.

Pop noise continues to be a design challenge, particularly when the switch between the music and phone functions is invoked by end users. The annoying pop noise gives an unpleasant experience when end users activate music functions. The root cause is power on/off rush-current passing through the AC-coupling capacitors when the audio amps are powered on.

Eliminate pop
Different solutions are available to fix this problem. One of them is to add additional amplifiers to allow the audio output to have 0V bias to minimize the size of the AC-coupling capacitor immediately before the headphone. Since most headphone amplifiers are integrated into the baseband processor or power management unit, this additional amplifier not only adds to BOM costs, but also increases power dissipation.

Another method is to add a separate charge path to the audio-signal path, thus allowing the AC-coupling capacitor to be fully charged before switching to the headphone or main path. This can be controlled by the baseband IC's GPIO to allow the audio amplifier and switch to be powered first, with the main channel switches in the off state. The common-mode voltage of the audio output will begin to ramp from 0 to VCC/2. After some time (10ms as reference), with the two sides of the coupling capacitor charged to full and equal potential, the main channel is turned on with no rush current at all, since the potential difference between the two plates of the capacitor is now 0.

This switch is well-suited for cellphone and MP3/MP4 player applications with a single USB connector (D+/D- pins), which is shared by the headphone and USB data lines. Low total harmonic distortion (THD) is very important for the audio channel. Also, since the switch is placed after the AC-coupling capacitor, it has to handle large negative-signal swings with low THD under this condition. The ultralow off-capacitance of this kind of switch allows high-speed USB signals to be a "wired-OR" connection with this device. Low parasitic capacitance of the switches is critical for eye-pattern compliance testing to the high-speed USB 2.0 standard.

With the market migrating to a single USB charger/data port, application-specific USB switches have been widely adopted as a feature in phone designs with charger-detection capabilities. The figure is an example of the application of such a switch.

Detect charger
A switch with low on-capacitance is needed in this design for two major reasons. First, since the baseband processor and high-speed USB controller output share the same D+/D- pins at the connector side, the output capacitance of the baseband USB1.1/2.0 full-speed controller has to be cut off when the cellphone enters high-speed USB 2.0 mode for music downloads or flash-memory functions. Any extra capacitance on the D+/D- line hurts the eye opening of the high-speed USB signals with current output characteristics. Second, the extra trace hanging over the D+/D- line has to be cut off during high-speed mode to effectively avoid the signal reflection with the fast rise/fall edge rate for 480Mbit/s USB signals.

Driven by the desire to use a single USB port for both charger and data functions, the charger-detection function has been popular in recent designs. A traditional solution is to feed the D+/D- line to an internal ADC to determine if the D+/D- line is shorted or not. The major limitation of this solution is the high input capacitance of the GPIO ports of the baseband processors, which adds extra capacitance on the data lines. This extra capacitance has major impact on effective signal toggling at high data rates, which are part of the USB 2.0 compliance test (such as 480Mbit/s for USB 2.0 signals). Of course, another shortcoming of this method is that system ADC resources are occupied as well.

USB switches with ultralow internal capacitance-sensing circuitry are needed in these applications for both charger detection and full-speed USB-controller output-capacitance isolation. At the same time, the USB channel selection pin (S pin in the figure) has to be able to recognize both 1.8V and 3V logic inputs to determine which USB channel is selected to be output. Note that both 1.8V and 3V logic are quite commonly seen at the baseband-processor GPIO outputs.

USB switches with charger-detection features are suitable for high-speed USB applications.

A traditional switch-select pin can have an input "high" (Vih) level as great as 2.0V (TTL logic), which may cause significant leakage-current issues when the switch power-supply (VCC) is powered directly by the battery. With the capability to recognize 1.8V input logic levels, the need for an additional, external level-shifting device is eliminated.

- Jianhong Ju
Asia Business Development, Signal Path Group
Fairchild Semiconductor Corp.

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