Analyzing New Power Management Solutions in Portable Applications

Today's portable consumer electronics, such as mobile phones, smart phones, PDAs, and media players, are extremely feature-rich. They range from high-end, mid-range, and low-end, with a wide range of performance levels and sizes. In general, portable applications are becoming smaller, more feature-rich, and more powerful, but power consumption remains high.

Examples are numerous, such as high-resolution cameras in camera phones with more than 3 million pixels, single high-power flash LEDs with currents exceeding 1A or Xenon flash in digital cameras, advanced audio or amplifier systems in smart phones or media players, and high-resolution LCD displays in most portable applications.

Designers are faced with the challenge of meeting both static and dynamic power management requirements. As portable products become more feature-rich, applications place higher demands on single power supplies, resulting in significantly higher power consumption and correspondingly shorter battery life. In

addition, analog and digital baseband processor units, central processing unit hosts, and especially the new graphics and audio dedicated processors are constantly improving in terms of sophistication and integration. As product features increase, IC integration increases, requiring more power rails or applying higher supply currents on the same number of rails.

Most portable consumer products use standard high-performance lithium-ion batteries (usually in a single-cell configuration). Given the limited battery capacity, manufacturers have to make a choice between providing users with feature-rich applications but with a shorter battery life, or sacrificing the richness of the application's features to ensure a long battery life. But today's consumers want high-end products while requiring batteries with ultra-long life.

Dynamic voltage scaling in portable systems (Vbat greater than Vrail)

The most common voltage range in lithium-ion battery technology is 4.2V~3.0V. New battery or future chemical technologies will achieve voltages up to 4.5V on the one hand, and on the other hand, the discharge cut-off voltage needs to be reduced to 3V. This means that the available input voltage range becomes wider, so more voltage rails can be added within this range. Today's system voltage rails

are usually below 3V (such as processor core power, I/O power, and memory power) or above 5V. These voltage rails are usually generated by other sources such as discrete LDOs or low-power DC/DC converters, multi-channel power management units (PMICs), or analog baseband (ABB) units. The power management design provides the necessary voltage rails, the correct voltage and current levels for various processors. If the application switches to "off" or a predefined "power saving" mode, all processors and power management devices will usually enter light load or standby mode. In this way, the voltage level will be reduced and the current consumption will be minimized. In the best case, each IC consumes only a few uA of current. The above situation is static, and once the power management design is completed, the voltage rail is unlikely to be affected.

Recently introduced discrete low-power step-down DC/DC converters and highly integrated multi-channel power management units (PMICs) have serial I2C interface capabilities. With the use of serial interfaces in discrete power management devices, the impact on the voltage rail will be further reduced. By combining software tools, processor control functions and serial standard I2C interfaces, unprecedented high-performance information exchange between digital units and analog power management ICs is achieved. Real-time adjustment of voltage, current and power becomes a reality. In addition, software control of power management and monitoring can be implemented, so there can be multiple power saving modes between the existing full load and system standby mode.

The I2C interface has three different rate options: standard 100kbps, fast 400kbps and high-speed 3.4Mbps. Using a discrete low-power DC/DC converter or PMU, designers can now dynamically and accurately adjust the output voltage of the discrete power management device to adjust the core supply voltage of any processor unit. This design requires the use of a fast DC/DC converter. For example, a converter with a switching frequency of more than 3MHz can ensure the transient response of fast signals. In addition, the low-power DC/DC converter or PMU should have different operating modes, such as PFM or forced PFM, so that a certain working configuration can be entered through self-adjustment or through an I2C control signal.

The design can accurately meet the system performance requirements without sacrificing overall performance. Therefore, the power consumption of each operating condition or processor mode is minimized, thereby extending battery life, reducing device heat generation and enhancing overall system performance.

DC/DC Converters and Power Management ICs with I2C Interfaces

For example, the TPS62350, a single-channel, low-power DC/DC converter, supports all three I2C speed modes. The step-down converter in a tiny 12-ball chip-scale package (CSP) delivers up to 800mA of output current over the input voltage range of a single lithium-ion battery with up to 90% efficiency. The I2C interface allows the output voltage to be adjusted to support the latest generation of processors and power rails with a minimum output voltage of 0.6V with a "micro-step" of 12.5mV. Programmable DC/DC converters help extend battery life in 3G smartphones, PDAs, digital cameras, and other portable applications.

Another way to reduce power consumption with the I2C interface is to use a device like the TPS65020. This highly integrated PMIC has six output channels, three low-power DC/DC converters, and three LDOs with up to 97% efficiency.

The I2C can dynamically adjust and measure the output voltage of the main DC/DC converter that typically powers the processor core. The other two DC/DC converters can be used to power I/O supplies, memory, or other power rails. In addition, different building blocks (such as all three LDOs or DC/DC converters on the IC) can be switched between "on/off" through I2C to reduce the power consumption and heat generation of the entire PMU. "Shutting down" different building blocks can also dynamically reduce the consumption of static current.

Another method is to use the preset output voltage of the DC/DC converter. The TPS62400 is a dual-channel buck converter. The device does not have an I2C interface, but has a single-wire interface called "Easyscale". Through Easyscale, we can access and change the predefined output voltage stored in the device EEPROM during operation. Depending on the selected output voltage range (0.7V~6.0V), the voltage step can be as small as 25mV, 50mV or 100mV.

In short, dynamic voltage measurement can reduce overall power consumption, optimize system performance and extend battery life. The voltage size, frequency and power budget can be dynamically controlled according to device activity, operating mode and temperature changes to make the power system more flexible.

Buck-Boost DC/DC Converter in Portable Systems (Vbat Equals Vrail)

On the other hand, camera modules, audio amplifiers, memory cards, and other subsystems require supply voltages several times higher than 3.1V, 3.3V, or 3.6V. When the battery voltage exceeds the target voltage rail, the power stage by definition needs to step down the battery voltage; otherwise, it needs to step up the battery voltage. There are several solutions to solve
this problem, such as SEPC, Flyback Converter, or cascaded buck-boost converters. Each solution has its own advantages and disadvantages, but none can achieve the smallest size and highest efficiency at the same time.

The latest solution is the recently launched TPS63000, a highly integrated buck-boost DC/DC converter. The converter has 4 integrated main power FETs combined with a unique control design. By solving the efficiency degradation problem of existing solutions, the optimized efficiency can reach up to 96% when the battery voltage is the same or similar to the output voltage (Vbat=Vrail). What does this mean? First, compared with existing solutions, its efficiency is improved by 2% to 6%; second, and more importantly, this efficiency advantage can be reflected in the entire battery voltage range.

This maximizes the use of battery capacity, significantly extending battery life and ultimately leading to long system/application operating and standby times.

The second important issue to discuss is minimizing the size. The integrated converter uses a 3x3mm2 QFN package, which is the same size as a 2.2uH inductor. To reduce the number of passive components, the output voltage can be preset (such as 3.3V) to reduce the total number of components to 4: IC + inductor + 2 capacitors.

Summary of this article

Power management for portable applications is moving towards higher efficiency, smaller size and more flexibility. With the emergence of new interface functions, new control schemes, fast control of power rails, and communication between digital processors and their analog power management components will be comprehensively improved.

Real-time adjustment of power budgets, adjustment of processor power saving schemes, and optimization of voltage rails under load conditions will make batteries more intelligent. This is extremely helpful for improving application usage time and extending battery life, and significantly extending standby time, talk time or playback time under the premise that users use all system functions.