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Application Note AN-5064
Low-ICCT Analog Switches for Ultra-Portable Designs
Summary
Fairchild Semiconductor has revolutionized and enhanced the design of analog switches to meet the demand for ultraportable products, such as cell phones. Greater integration and improvements in theprocess technology involves using different voltage rails to power and control the analog chipset. To solve this problem, Fairchild, a leading analog switch supplier, has introduced new analog switches and begun to offer modified versions of existing products. This new generation of analog switches offers an expanded control input range, while maintaining low current consumption and rail-to-railsignaling. This application note discusses the underlying reasons behind the shift in design styles and how the new solutions meet the needs of these ultra-portable systems. Additionally, it discusses a new generation of low-ICCT analog switches specifically designed to prolong battery life in ultra-portable devices and the design trade-offs made to improve performance over mixed voltage rails andenhance the total system performance. Ultra-portable products, such as cell phones, PDAs, or MP3 players, use analog switches in a variety of applications, from USB port sharing and isolation to audio switching. After choosing a switch relative to configuration and application-specific requirements, there are several key specifications that ultra-portable designers consult regardless of the endapplication. Ultra-portables rely on a battery supply, making power consumption a major factor in the selection of an analog switch. In most ultra portable systems, there are multiple supply rails available, and designers use a power management IC to detect which supplies are present. The power management IC chooses to supply the analog directly from the battery or from a regulated supply. Dependingupon the situation, the supply voltage could range from 2.7V to 3.6V, in the case of a regulated supply from the wall, or 4.3V VCC when powered from a fully charged battery. Until recently, the on-board General Purpose Input Output (GPIO) control voltage levels corresponded to the supply voltage rail powering the analog switch. This resulted in very little current consumption by the switch. Underthese conditions, it is standard for analog switches to have less than 1µA of current consumption. When the ultra-portable product is operating from the battery supply, total current consumption becomes very important. In a standard configuration (Control = 0V or VCC), analog switches are well within the typical power budget with less than 1µA of current consumption. Newer ASIC designs havemigrated to smaller process geometries, limiting their voltage handling capabilities. As a result, system designers have had to step down the ASIC supply voltage from the power management IC to an acceptable level. In many cases, the ASIC requires a 2.6V to 2.8V supply, which limits the maximum output voltage for the GPIO signal. The GPIO signal is generally used to drive the analog switch control pin.When the standard analog switch is powered directly by the battery and the GPIO voltage is in the 2.6V to 2.8V range, this leads to excessive current consumption by the switches. The excess current can be as high as several milliamps, depending on the design of the particular switch. For portable devices already operating on a tight power budget, several milliamps of current consumption isunacceptable. Most standard analog switches specify only ICC consumption of the typical condition where the input control voltage is equal to the supply voltage. This has caused confusion among many system designers who were surprised to find milliamps of current as they migrated to lower-voltage ASICs. Low-ICCT analog switches are designed to operate in this type of application. Figure 1 relates the...