Tag: Cellular Phone Power Management
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Summary
The new generation of mobile phones is not only small in size, but also integrates more functions such as PDA functions, receiving/sending e-mail, information services, and browsing Internet information on large, colorful displays. Some products also include FM radios, MP3 players, and even digital cameras.
Even with increased functionality, consumers still expect to maintain long-lasting battery life without increasing the size. Plugging in more features in a tighter dice while at the same time consuming less power puts a very demanding requirement on power management design. In response to the challenge, analog IC manufacturers are constantly developing smaller, higher performance power solutions.
Power management IC
A power management IC (PMIC) is beating in the heart of most wireless handsets. The PMIC takes on most of the power supply tasks and some other unit functions such as interfaces or audio. Some of the market's leading analog semiconductor vendors offer custom, semi-custom and/or standard devices for PMICs that typically use a 5V submicron BiCMOS process optimized for mixed-signal and power products. Any other functions in the phone that have not been integrated are attempted to be integrated into the PMIC, but it is usually recommended to use some more moderate solutions, as shown in Figure 1. Based on the following considerations, certain specific units should not be integrated: 1) the unit will be more cost-effective or smaller if it is designed with other different processes; 2) the unit may be different due to technological development or changes in customer requirements. Changes occur in the design version; 3) the unit may not be common on different platforms; 4) the unit may bring excessive design challenges/risks to the development schedule; 5) the unit may be due to performance reasons (such as noise Coupling) is not suitable for integration.
Undoubtedly, the goal of integrating as many functional units as possible into the PMIC is to save cost and size, especially when this integration solution is generally accepted by high volume handsets. Risk can be controlled by gradually increasing integration as the design improves.
Low dropout linear regulator
5 to 12 independent low dropout linear regulators (LDOs) are commonly used inside cellular phones. The large number of LDOs does not mean that there are the same number of voltage specifications inside the terminal, but because the LDO is still treated as An on/off switch with a certain power supply rejection ratio (PSRR) to prevent noise coupling. Most LDOs are integrated inside the PMIC, but there are still separate LDOs that are retained, mainly considering PCB layout/wiring, some special components (such as voltage-controlled oscillators) that are too sensitive to noise, or used to drive some non- The standard unit is, for example, an integrated digital camera. For many years, the 150mA LDO in the SOT23 package is the best choice for these discrete power supplies. At present, some of the latest ICs offer new performance in smaller sizes with new packages, new sub-micron processing and advanced design solutions. Now you can get a single 300mA LDO from SOT23, two 150mA LDOs in SOT23 package, or a single 120mA LDO in micro SC70 package, with both standard and ultra low noise (10?VRMS, 85dB PSRR) devices. In addition, the more advanced wafer-level package (UCSP) offers the largest possible size, while the QFN package allows for the largest wafer size in a 3mm x 3mm plastic package while providing higher thermal conductivity. Therefore, the QFN package enables higher current LDOs and a greater number of LDOs per package. It can contain three, four or even five LDOs, which narrows the gap between the split solution and the PMIC.
Buck converter for processor core
LDO has the characteristics of simple and small size, and its main drawback is low efficiency, especially when powering low voltage circuits. Due to the integration of PDA functions or Internet functions in the new generation of cellular phones, the data processing capability and computing power of the processor are required to be more powerful, and the nuclear voltage of the processor is reduced from 1.8V to 0.9V in order to reduce the power consumption of the processor. To reduce battery drain, a high-efficiency buck converter should be used to power the processor core. The main factors to consider in the design are: low cost, small size, high efficiency, low static (standby) current and fast transient response. In order to solve the above problems, not only rich simulation design experience, but also a certain original ability is required. For now, only a handful of leading analog semiconductor manufacturers are able to offer a suitable, SOT23 package, a buck converter with a switching frequency of 1 MHz or more that allows the use of tiny external inductors and capacitive components.
Buck converter for powering RF power amplifiers
In the more mature Japanese cellular phone market, the buck converter is also used to drive a CDMA RF power amplifier (PA), which dynamically adjusts the VCC supply voltage of the amplifier as the distance between the terminal and the base station changes. Considering the transmission probability density function, the buck converter saves an average of 40mA to 65mA of battery current. The amount of current savings depends on the number of stages of the output voltage, the characteristics of the PA, and whether voice or data is sent in urban or suburban areas. Based on Japan's successful experience and the early work of a European WCDMA manufacturer, South Korea, the United States and other European cellular phone manufacturers have begun to use this switching regulator for testing or design. This buck converter is required to have very small size, low cost, low output ripple and high efficiency. The SOT23 converter is again the preferred solution. To maintain the lowest possible voltage drop, a separate low RDS(ON) P-channel MOSFET is typically used to drive the PA directly from the battery at high transmit power. To further reduce overall size, the latest buck converters (such as the MAX8500 series) integrate this additional FET.
More and more LEDs
Among wireless phones with color displays, white LEDs have become mainstream in backlighting applications due to their simple circuit and very high reliability. The efficiency is higher than that of halogen lamps, but it is not comparable to fluorescent lamps. A new generation of cellular phones typically uses three or four white LEDs in the main display, two white LEDs in the secondary display (folding design), and six or more white or colored LEDs on the back of the keyboard. If integrated with a camera, at least four white LEDs are required for flash/strobe and MPEG image illumination. Thus, a total of 16 LEDs are used in a single mobile phone, all of which require constant current drive.
A few years ago, the first generation of color-display phones produced in Japan used less efficient 1.5 times the charge pump and ballast resistors (this solution actually wastes them through the hardships, using the buck converter Drive the 40mA current saved by the PA). Currently, inductor-based boost converters are used in most designs to achieve higher conversion efficiency. The new 1x/1.5x charge-charge pump achieves the same high efficiency and eliminates the need for external inductors, but requires many leads when connected to the LED. Since the market for LED power supplies is very large, countless ICs are designed for this purpose. Factors to consider in the design include: high efficiency, small external components, low input ripple (preventing noise coupling to other circuits), simple dimming interfaces, and other features that help reduce cost or increase reliability, such as Output overvoltage protection. Some PMICs include a white LED power supply, but typically cannot drive strobes from multiple displays or cameras, and may have problems with low efficiency or slow switching speeds. This requires large inductors and capacitors and produces large input ripple. Many designs often require a separate IC to work with the PMIC or directly use a highly integrated separation scheme.
Charging batteries
Almost all wireless phones charge a 3-cell NiMH battery or a 1-cell lithium battery with a simple linear charger. In many cases the charger is integrated into the PMIC, however, to simplify the design, the current-sense resistor and the regulator are still external. In order to ensure that the heat dissipation is within the allowable range, there are many options: 1) charging at a rate of C/4 or slower; 2) having the wall adapter have a certain resistance so that most of the voltage falls on it; Use pulse charging and current limiting wall adapters; 4) use feedback to adjust the wall adapter to keep the differential pressure on the adjustment tube constant; or 5) add a constant thermal control loop to maintain constant current by controlling the charging current Wafer temperature, which is only available when the adjustment tube is placed inside the PMIC. Separate charging ICs offer a lot of flexibility, but this advantage is greatly compromised in cell phones because integrated chargers are easily reprogrammed through the PMIC's serial interface to accommodate different battery chemistries or capacities.
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