Several changes in the power electronics industry are converging to create a growing problem. The complexity and performance requirements of power systems continue to increase, and time-to-market pressures intensify. But at the same time, the number of competent power system designers in the industry is diminishing.
During the last couple decades, universities have churned out graduates skilled in digital design, but very few of these graduates are capable of complex analog design. These factors combine to produce an “experience gap” in power electronics. Because of this gap, many systems will go to market late and with poorly designed power systems.
Designers of today’s portable electronic products are faced with unprecedented challenges. Among these are the demands for high performance power management systems to accommodate growing system complexity and higher power budgets. These systems strive for an optimum balance among competing objectives including long battery runtime, compatibility with multiple power sources, high power density, small size and effective thermal management.
While new lithium-based battery technologies promise incremental gains in energy density, these gains are now being offset by increased safety concerns which are driving charging strategies that include lower charge (float) voltages, thermal regulation and temperature qualified charging. New battery chemistries will enable longer runtimes, but their voltage discharge profiles are extended, with significant energy available at battery voltages below 3V. This characteristic will influence the related power conversion system, making synchronous buck-boost regulators necessary to generate outputs as low as 3V.
The demand for higher power solutions is largely driven by the increased computational power required in today’s advanced handheld products. At the same time, the number of power supply rails continues to grow. This trend increasingly dictates the use of multi-output switching regulator solutions to meet size and efficiency goals and to keep thermals in check. Further, DC/DC converters with high efficiency over a wide range of load current and low standby (quiescent) current are a must to manage battery runtimes.
CHALLENGESThe most challenging issue facing a system designer of portable battery-powered products today is how to best architect his power management system to optimize overall performance and maximize batter run time. Until recently, designers of portable power systems have had two basic approaches for addressing these challenges. One choice is to architect the system using individual components, each optimized for a single function. This approach yields maximum flexibility in design, layout and thermal management while achieving the appropriate level of performance for each function. But this choice has the major disadvantage of being relatively costly and taken together; these components require substantial board space to address the growing list of functional requirements.
At the other extreme, designers may choose from a variety of very highly integrated power management ICs (PMICs). These devices typically support a superset of the functionality needed for most applications, including unwieldy combinations of switching DC/DC controllers, monolithic switchers and numerous LDOs along with unrelated mixed signal functions like touch screen controllers, audio CODECs and more. As a result, they can be cumbersome to use and most require a substantial investment in firmware just to turn them on. These products tend to favor integration over performance, and often complicate thermal management by concentrating heat in a single “hot spot” within the product. Ironically, these highly integrated solutions also require relatively large board area. This is because they are housed in large, high pin-count packages, and because they force board layout heroics in order to accommodate all the related external components (MOSFETs, inductors, diodes and assorted passive components) as well as all the routing needed to get from the PMIC to the various loads across the system.
SOLUTIONSLinear Technology has a wide portfolio of products targeted at portable power management applications. These products feature high switching frequencies (as high as 8MHz); high efficiency conversion to minimize thermal issues; high efficiencies even at light loads; very low quiescent currents in standby modes (as low as 9µA); and high levels of integration, including integrated MOSFETs and Schottky diodes.
The company recently introduced new family of PMICs that provide a meaningful level of integration, with no performance compromises and without all the complexity that can get in the way. These products provide the functionality that can make or break a portable device, including hard-to-do, “performance matters” functions like USB OTG, and automotive-compatible power path control, autonomous battery charging and high efficiency monolithic DC/DC power conversion. Other features include high charge currents, high switching frequencies and independent I/O or I2C control. The family is offered in packages that take just a fraction of the board space of traditional PMICs – from just 9mm˛ to 20mm˛, while requiring few external components and no tantalum capacitors.
Synchronous step-down converters have offered a substantial improvement in battery runtime over traditional linear regulators due to their increased conversion efficiencies. These converters offer 90 percent to 95 percent efficiency and virtually eliminate the need for any heat sinking. This higher efficiency comes at a cost of extra circuit board space for an additional inductor for each channel, so it is paramount to keep the total solution footprint to a minimum. By combining multiple channels in a synchronous step-down solution, they can all operate from a single input capacitor keeping the solution footprint minimal.
Linear Technology recently introduced a family of multi-output synchronous buck regulators that provided ultra-compact, high efficiency solutions. As an example, consider an application using a single cell Li-ion battery (VBATT starts a 4.2V and declines to 2.7V) that requires 1.2V, 1.5V, 1.8V and 0.8V rails for the DSP Core, I/O, and memory power. A single LTC3544 (Figure 1), one of the latest product releases from Linear Technology, could accommodate all of these voltage requirements from a single 3x3mm QFN with a single input capacitor—providing a solution footprint of less than one-third of that required by four single output synchronous bucks, with a standby quiescent current of only 70µA.
Low IC quiescent and operating currents across a wide load range, combined with high switching regulator conversion efficiencies, help preserve battery run time in portable electronics. Linear Technology’s PMICs with PowerPath control, ultra-low quiescent and standby currents, and with integrated programmable synchronous buck switching regulators solve these design challenges simply and easily.
The LTC3554 PMIC for portable Li-ion/polymer battery-based applications integrates a USB-compatible linear PowerPath manager, a stand-alone battery charger, two high efficiency synchronous buck regulators and pushbutton control. A pin-selectable standby mode reduces battery drain current to just 10µA with all outputs on. The LTC3554’s PowerPath manager with automatic load prioritization seamlessly manages the transition between multiple input sources to power the load, while delivering up to 400mA battery charge current from a USB port or 5V wall adapter supply. The input current limit is pin-selectable and internally set. The device is compatible with inputs up to 5.5V (7V absolute maximum transient for added robustness), and its “instant-on” operation ensures system load power even with a fully discharged battery. Autonomous operation simplifies design, eliminating the need for an external microprocessor for charge termination. The internal 240m ideal diode provides a low loss power path to the load when input current is limited or unavailable. The LTC3554 is well suited for low power portable device applications, including personal navigation devices (PNDs), small fitness portable electronic devices, media players and handheld medical & industrial devices.
In conclusion, the products discussed above offer clean and compact solutions to the most difficult power management challenges faced by designers of portable power systems.
About the authorTony Armstrong is the Director of Product Marketing, Power Products, for Linear Technology Corp. He can be reached at tarmstrong@linear.com.
Caption
Linear Technology’s LTC3554 PMIC for portable Li-ion/polymer battery-based applications integrates a USB-compatible linear PowerPath manager, a stand-alone battery charger, two high efficiency synchronous buck regulators, and pushbutton control.
