Wireless device power architectures go green
26 March 2010
The thermal efficiency of power management ICs is a key factor in its green credentials as Tony Armstrong explains.
There are many critical aspects to the design of a battery-powered portable wireless device which a system designer must overcome. One of the most significant being how to get heat out of the device since it typically has no fans for cooling purposes. As a result, the power conversion and management ICs used inside such a potential device must be thermally efficient since the primary byproduct of poor power conversion efficiency is heat.
This heat is generated from the power lost inside the regulators during the energy transfer process. Furthermore, as there is limited airflow for cooling purposes inside many portable devices and the fact that heat sinks are limited due to their size and available space within the product itself, it is the densely packed printed circuit board that must deal with this heat. However, this heat equates to a rise in the internal operating (ambient) temperature inside the product that could adversely impact long-term reliability.
The conversion efficiency of a DC/DC converter can be calculated as the output power divided by the input power, or put another way, power to the load divided by power from the input. System designers must carefully consider the type of regulator that should be used depending on the heat generated during the power conversion process. Thus, a common trend by many manufacturers of battery-powered portable wireless devices is to adopt a switching regulator over the simpler linear low dropout regulator because of their higher efficiency operation.
Power Architecture Trends
Figure 1 represents a typical battery-powered portable wireless device’s power conversion and management architecture. Not all products contain an integrated battery charger since some manufacturers prefer to have their battery charger inside an accessory-charging cradle. These cradles allow for the simultaneous communication of the device to a host computer and provide the necessary charge current with which to recharge the battery. Furthermore, some manufactures do not want to incur the added cost and design time to implement a battery charger in their product and simply elect to power their products with single, or multiple, standard cylindrical AA or AAA form factor batteries regardless of their type of chemistry.
It is very common for several different voltage rails to be required in almost any type of portable wireless device, which could also have multiple input sources in addition to some type of battery. These rails typically include a 3.0 V or 3.3 V main system bus, a 1.2 V microprocessor or DSP core voltage, 1.8 V for I/O, 2.8 V for RF power, 5 V for USB OTG support or power audio circuits and a LED driver to power an array of LEDs for the backlighting of a display. However, a common problem still exists and that is how to best manage the available power from the variety of different input sources in order to optimize the end products functionality while simultaneously charging the battery - if one is present. What is needed to tackle this complex issue is a simple and effective power path control circuit.
Power path control is an automatic load prioritisation circuit that offers the ability to autonomously and seamlessly manage power flow between multiple input sources such as USB ports, wall adapters and the battery, all while preferentially providing power to the system load. In a traditional battery-fed charging system, the user must wait until there is sufficient battery charge and voltage level to obtain system power. Conversely, power path control allows the end product to operate immediately when plugged in, regardless of the battery’s state of charge, commonly referred to as “instant-on” operation.
Power path control circuits can be implemented in both linear and switching topologies. Benefits of a linear power path topology include simple and cost-effective implementation. However a switch-mode power path topology improves power delivery efficiency to the system load, as well as to the battery. It does this by eliminating the power lost in the linear battery charger element, especially critical when the battery voltage is low and/or input power is limited, such as being powered by a limited USB port, giving it excellent thermal properties. A second big advantage is its ability to extract up to 700 mA battery charge current from a standard USB port (~2.3 W) when battery voltages are low. This is made possible due to the switching topologies 90% plus conversion efficiency versus the linear topologies nominal 60% conversion efficiencies. Fortunately, there are a number of analogue semiconductor suppliers that offer both standalone and highly integrated power path control ICs.
AA or AAA battery-powered devices need special consideration
In addition to Li-Ion batteries, a variety of portable wireless devices are still powered from two AA or two AAA cell form factor batteries (in Nickel, Alkaline or new Lithium-cylindrical chemistries), non-rechargeable and rechargeable, for convenience, availability and cost reasons. However, as already mentioned, managing the flow of power into a handheld device is an increasingly complex task because of the presence of multiple power sources, multiple supply voltages within the product, demands for optimum efficiency and very limited space. It has been common for these factors to drive the development of highly integrated power management ICs (PMICs) for many battery-powered devices.
However, one of the biggest obstacles when using a portable wireless device powered by either two AA or AAA cells and a 5 V AC adapter or a 5 V USB port, is being able to deliver both a fixed 3 V or 3.3 V output for the main power rail and a 1.2 V output to power a microprocessor or DSP core voltage. When the device is powered from either a 5 V wall adapter or a 5 V USB port, then only a step-down (buck) DC/DC converter is needed. However, when the device is battery-powered, a buck-boost DC/DC converter is usually required to deliver the 3 V or 3.3 V for the main power rail, whereas a step-down DC/DC converter is needed to supply the 1.2 V for the large-scale digital processor core voltage. This is due to the fact that the discharge profile of two AA cells (Nickel or Alkaline) is 3.2 V down to 1.8 V; however, this range has shifted approximately 0.4 V higher with “new” Lithium cylindrical AA and AAA cells, thereby requiring a buck-boost to more efficiently regulate either a 3.0 V or 3.3 V rail across the entire battery discharge range. Additionally, a second buck channel is usually needed to power memory at a nominal 1.8 V.
Green power needed in battery-powered wireless portables
The popularisation of the concept of “Green Environment Protection” was in the news a great deal during the course of last year and we will continue to see more in 2009. As a result, many suppliers or power management and conversion ICs have made a lot of progress in improving power efficiency across a wide load range.
Furthermore, it is generally accepted that most industrialised nations recognise the need to conserve energy regardless of whether the product is plugged into a wall socket or operated by battery power. This is due to the fact that as a nation’s population increases, so does the demand for energy to power new homes with heating/cooling systems, lighting and electrical appliances. It costs a great deal of money not only to build new power-generating facilities, but also to deliver this power to the users once it is generated. It has been observed that it is more cost effective to cut the current energy consumption of most electrical appliances by 15% to 20% than it is to build new power facilities.
For portable wireless products powered by batteries, a similar concept also applies; however, in the case of multiple AA or AAA form factor batteries, it is the disposal aspects of these batteries, with their hazardous chemical content, that has a negative impact on our environment. Clearly, anything that can be done to extend their useful life within an end product will minimise the frequency of replacement and thereby reduce the level of harmful contaminants needing to be recycled.
As a result of the high costs associated with the building of either new power-generating facilities or hazardous chemical recycling facilities, many countries have adopted a “Green Policy” whereby they encourage manufacturers to incorporate energy saving techniques into their end products. Thus, for a power management and conversion ICs to be used in any type of energy-saving device, any DC/DC converters used internally must have two main attributes. Firstly, they must posses very high efficiency of conversion properties across a wide range of load currents. And secondly, they must have very low quiescent current in both standby and shutdown modes. As a result, many types of battery-powered portable products are incorporating power management and conversion products with both of these key attributes.
New green conversion products
The LTC3101 is the latest PMIC in a family of multifunction, compact power management solutions for battery-powered and battery backup applications. The micropower LTC3101 integrates a low loss PowerPath controller, three high efficiency synchronous switching regulators (one buck-boost and two bucks), a current limited 200 mA VMAX output (which tracks the higher voltage input supply), a protected 100mA Hot Swap output, pushbutton On/Off control, a programmable processor reset generator and an always-on LDO, all in a compact, low-profile 4 x 4 mm QFN-24 package.
The LTC3101 features a wide input operating range of 1.8 V to 5.5 V, compatible with 2 or 3 AA or AAA form factor battery cells in Nickel, Lithium or alkaline chemistries, standard 1-cell Li-Ion/Polymer prismatic batteries, plus USB or 5 V wall adapter input power (see figure 3). Additionally, the device’s low loss PowerPath control seamlessly and automatically manages power flow between these multiple input sources. The “always-alive” VMAX and LDO outputs provide power for critical functions or additional external regulators. Internal sequencing and independent enable pins provide flexible power-up options.
The LTC3101’s buck-boost regulator can deliver up to 800 mA continuously for input voltages above 3 V and is ideal for efficiently regulating a 3.0 V or 3.3 V output over the full 1.8 V to 5.5 V input voltage range. The LTC3101’s two buck regulators feature 100% duty cycle operation and are capable of delivering output currents of 350 mA each, with adjustable output voltages down to 0.6 V. The LTC3101’s internal low RDS(ON) switches enable buck-boost efficiencies up to 95% and buck regulator efficiencies up to 93%, maximizing battery run time – see figure 2 below.
In addition, Burst Mode operation optimises efficiency at light loads with a total IC quiescent current of only 38 uA with all regulators enabled and only 15 uA in standby with the LDO and VMAX outputs active. The high 1.27 MHz switching frequency allows the use of tiny low cost capacitors and inductors less than 1 mm in height. Furthermore, all regulators are stable with ceramic output capacitors, achieving very low output voltage ripple. See figure 3 for the LTC3101 detailed schematic.
It is not uncommon for portable wireless instrumentation, such as handheld meters and medical diagnostic equipment to be powered by three or four AA cells due to the large amount of data processing they are required to perform. As a result, Linear Technology has recently introduced the LTC3534, a synchronous buck-boost converter with an extended 2.4 V to 7 V input voltage range that can deliver up to 500 mA of output current to a regulated fixed output. As with all of Linear Technology’s single inductor buck-boost converters, the input can be above, below or equal to the output. The topology incorporated in the LTC3534 provides a continuous transfer mode through all of the operating modes making it ideal for three or four cell alkaline applications which must maintain a constant output voltage even as the battery voltage declines below the output.
For example, consider a four cell alkaline (AA or AAA) application that has an input voltage range of 3.6 V to 6.4 V powering a fixed 5 V output, as shown in figure 4. In many cases, using the LTC3534 can add up to 25% more battery run-time when compared to a more traditional SEPIC approach. The LTC3534’s constant 1 MHz switching frequency offers low output noise while minimising the size of external components. This combination of tiny externals and a 3 x 5 mm DFN (or SSOP-16) package provides a tiny solution footprint, ideal for many handheld devices. See application schematic below for further details.
The LTC3534 includes two N-Channel, as well as and two P-Channel MOSFETs (215 mOhm/275 mOhm and 260 mOhm, respectively) to deliver efficiencies of up to 94%. Burst Mode operation requires only 25 µA of quiescent current while shutdown current is less than 1 µA to further extend battery run-time. If the application is noise sensitive, the PWM pin can also be configured to provide forced continuous operation to reduce noise and potential RF interference. Other features include soft-start, current limiting, thermal shutdown and output disconnect.
Tony Armstrong is Director of Product Marketing for the Power Products group at Linear Technology Corporation
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