Power architecture for portable wireless devices goes green

A battery-powered portable wireless device presents a number of key issues that system designers must overcome. One of the most important is how to get the heat out of the device, since such devices typically do not have fans for cooling purposes. As a result, the power conversion and management ICs that may be used in such devices must be thermally efficient, since a major byproduct of inefficient power conversion is heat.

This heat is generated by the power lost within the regulator during energy delivery. Additionally, within many portable devices, there is limited air flow for cooling purposes, and heat sinks are limited by their size and the space available within the device, so the densely packed printed circuit board must handle this heat. However, this heat translates into an increase in the product's internal operating (ambient) temperature, which can have a detrimental effect on long-term reliability.

The conversion efficiency of a DC/DC converter can be calculated as output power divided by input power, or in other words, load power divided by input power. System designers must carefully consider the type of regulator to use based on the heat generated during the power conversion process. Therefore, it is common practice for many battery-powered portable wireless device manufacturers to use switching regulators instead of simpler linear low-dropout regulators because switching regulators operate more efficiently.

Power Architecture Trends

Figure 1 shows a typical power conversion and management architecture for a battery-powered portable wireless device. Not all products include an integrated battery charger, as some manufacturers prefer to place the battery charger in an accessory charging cradle. These cradles allow the device to simultaneously communicate with the host computer and provide the charging current necessary to recharge the battery . Alternatively, some manufacturers do not want to incur the cost and design time of designing and manufacturing a battery charger into their product and simply choose to power their product with one or more standard cylindrical AA or AAA batteries, regardless of battery chemistry.

Figure 1. A typical power architecture for a battery-powered portable wireless device.

It is common to have several different voltage rails in almost any type of portable wireless device , which may have multiple input power sources in addition to some type of battery. These voltage rails typically include a 3.0V or 3.3V main system bus, a 1.2V microprocessor or DSP core voltage, 1.8V for I/O, 2.8V for RF power, 5V for USB OTG support or to power audio circuits, and an LED driver to power an LED array for display backlighting. However, a common problem remains, which is how to best manage the available power from the various different input power sources to optimize the functionality of the end product while charging the battery (if a battery is present). What is needed to solve this complex problem is simple and effective power path control circuitry.

Power Path Control is an automatic load priority handling circuit that autonomously and seamlessly manages the power path between multiple input power sources such as USB ports, AC adapters and batteries while giving priority to powering the system load. In traditional battery-fed charging systems, users must wait until there is sufficient battery charge and voltage before they can get system power. In contrast, Power Path Control allows the end product to work immediately after plugging in, regardless of the battery's charge state, which is often referred to as "instant-on" operation.

The PowerPath control circuit can be implemented using both linear and switching topologies. The benefits of the linear PowerPath topology are ease of implementation and cost-effectiveness. However, the switch-mode PowerPath topology improves the efficiency of delivering power to the system load and battery. The switch-mode PowerPath topology does this by eliminating the power lost in the linear battery charger unit, which is particularly important when the battery voltage is low and/or the input power is limited (for example, when powered from a limited USB port), giving this topology excellent thermal characteristics. A second outstanding advantage is its ability to draw up to 700mA of battery charging current from a standard USB port (about 2.3W) when the battery voltage is low. This is made possible because the switching topology has a conversion efficiency of more than 90%, while the linear topology has a nominal conversion efficiency of only 60%. Fortunately, there are many analog semiconductor suppliers that offer both standalone and highly integrated PowerPath control ICs.

Devices powered by AA or AAA batteries require special consideration

In addition to Li-Ion batteries, many portable wireless devices are still powered by two rechargeable or non-rechargeable AA or AAA batteries (using nickel, alkaline or new cylindrical lithium chemistries) for reasons of convenience, availability and cost. However, as already mentioned, managing the power path into handheld devices is an increasingly complex task due to the presence of multiple supply voltages in the product, very limited space and the need for optimal efficiency. As is often the case, these factors have led to the development of highly integrated power management ICs (PMICs) for many battery-powered devices.

However, when using a portable wireless device powered by two AA or AAA batteries and a 5V AC adapter or a 5V USB port, one of the biggest obstacles is providing a fixed 3V or 3.3V output for the main power rail and a 1.2V output to power the microprocessor or DSP core voltage. When the device is powered by a 5V AC adapter or a 5V USB port, only a buck DC/DC converter is required . However, when the device is powered by batteries, a buck- boost DC/DC converter is often required to provide 3V or 3.3V for the main power rail, while a buck DC/DC converter is required to provide 1.2V for the large digital processor core. This is because the discharge curve of two AA (nickel or alkaline) batteries is from 3.2V to 1.8V, but with "new" cylindrical lithium AA and AAA batteries, this range has shifted upward by about 0.4V, so a buck-boost regulator is needed to regulate 3.0V or 3.3V over the entire battery discharge range with higher efficiency. In addition, a second step-down channel is often required to power the memory at a nominal 1.8V.

Battery-powered wireless portable devices also need green power

The concept of "going green" has been in the news a lot over the past year, and we will see even more of it in 2009. As a result, many suppliers or power management and conversion ICs have made great strides in improving power conversion efficiency across a wide load range.

In addition, it is widely believed that energy conservation is needed regardless of whether the product is plugged into a wall socket or runs on battery power. This is because as a country's population increases, the demand for energy also increases, and people need energy to power their home heating/cooling systems, lighting, and household appliances. Not only does it cost a lot of money to build new power generation facilities, but it also costs a lot of money to transmit the electricity after it is generated to the user. It has been observed that reducing the current energy consumption of most household appliances by 15% to 20% is more cost-effective than building new power generation facilities.

In the case of battery-powered portable wireless products, a similar concept applies, however, in the case of multiple AA or AAA batteries, the disposal of these batteries containing hazardous chemical materials has a negative impact on our environment. Obviously, anything that can be done to extend the life of these batteries in the end product will minimize the frequency of battery replacement, thereby reducing the harmful pollutants that need to be recycled.

As a result of the high costs associated with building new power generation facilities or hazardous chemical material recycling facilities, many countries have adopted "green policies" that encourage manufacturers to use energy-saving technologies in their end products. Therefore, for a power management and conversion IC to be used in any type of energy-saving device, any DC/DC converter used inside must have two main characteristics. First, they must have very high conversion efficiency over a wide load current range. Second, they must have very low quiescent current in standby and shutdown modes. As a result, many battery-powered portable products are incorporating power management and conversion products with these two key characteristics.

New green power conversion products

The LTC3101 is the latest PMIC in a family of versatile, compact power management solutions for battery-powered and battery-backup applications. It integrates a low-loss PowerPath controller, three high-efficiency synchronous switching regulators (one buck - boost and two bucks), a 200mA current-limited VMAX output (tracking the voltage of the higher input supply), a protected 100mA Hot Swap output, push-button on/off control, a programmable processor reset generator, and an always-on LDO, all in a compact, low-profile 4mm × 4mm QFN-24 package .

The LTC3101 has a wide input operating voltage range of 1.8 to 5.5V, compatible with 2 or 3 AA or AAA batteries using nickel, lithium or alkaline chemistries, standard single-cell lithium-ion/polymer prismatic batteries, and USB or 5V AC adapter input power (see Figure 3). In addition, the device's low-loss power path control seamlessly and automatically manages the power path between the above multiple input power sources. The "keep-alive" VMAX and LDO outputs power critical functions or additional external regulators. Internal sequencing and independent enable pins provide flexible power-up options.

Figure 2 Efficiency curve of LTC3101

Figure 3 LTC3101 schematic diagram

The LTC3101's buck-boost regulator can continuously deliver up to 800mA of current when the input voltage is above 3V, making it ideal for efficiently regulating 3.0V or 3.3V outputs over the entire input voltage range of 1.8 to 5.5V. The LTC3101's two buck regulators operate at 100% duty cycle, each capable of delivering 350mA of output current with an adjustable output voltage as low as 0.6V. The LTC3101's internal low RDS(ON) switches achieve up to 95% buck-boost efficiency and up to 93% buck regulator efficiency, maximizing battery run time (see Figure 2).

It is not uncommon for portable wireless instruments such as handheld meters and medical diagnostic equipment to be powered by 3 or 4 AA batteries due to the need to perform a lot of data processing tasks. The synchronous buck-boost converter LTC3534 is designed for this purpose. The device has an extended input voltage range of 2.4 to 7V and can provide up to 500mA of output current to a fixed regulated output. Its input can be higher, equal to or lower than the output. The topology used by the LTC3534 provides continuous delivery mode in all operating modes, making it very suitable for 3 or 4 alkaline battery applications.

For example, consider an application with a 4-cell alkaline (AA or AAA) battery with an input voltage range of 3.6 to 6.4V to provide a fixed 5V output (see Figure 4). In many cases, using the LTC3534 can extend battery run time by 25% when compared to the more traditional SEPIC approach. The LTC3534's 1MHz constant switching frequency provides low output noise while minimizing external component size. The tiny external components combined with the 3mm × 5mm DFN (or SSOP-16) package provide a tiny solution footprint that is ideal for many handheld devices.

Figure 4 LTC3534 schematic diagram

The LTC3534 contains two N-channel and two P-channel MOSFETs (215mΩ/275Ωm and 260mΩ, respectively), providing up to 94% efficiency. Burst Mode operation requires only 25μA of quiescent current, while shutdown current is less than 1μA to further extend battery run time. If the application is noise sensitive, the PWM pin can also be configured to provide forced continuous operation, thereby reducing noise and potential RF interference. Other features include soft-start, current limit, thermal shutdown and output disconnect.