Portable CE devices are achieving improved performance and increasing functionality, thus requiring maximized runtimes out of each battery charge cycle. The addition of a growing number of features is also creating a demand for higher capacity batteries.

Li-ion batteries are ideal for a variety of portable-electronic applications because of their high cell voltage, high density, long shelf life and maintenance free nature. In addition to the popular 4.2V charge voltage regulation 1C maximum charge/ discharge rate, new technology in Li-ion batteries may require different charge voltages and deliver higher C-rates.

This article discusses some new trends in Li-ion batteries and shows how portable product designers can design a flexible Li-ion battery charge management system using an MCU-controlled pulse-width modulation (PWM) module or a stand-alone integrated battery charge-management controller-based solution.

Figure 1: Charge voltage regulation is important to maximize capacity after each charge cycle. An undercharged battery voltage of 0.6 percent can result in a 5 percent capacity loss for Li-ion batteries.

Design difficulties
Challenges in portable power design with Li-ion batteries include safety concerns, battery chemistries, available space and required features. For rechargeable Li-ion batteries, one must also consider the charge/discharge rate, life cycles, maintenance and charge algorithm. To maximize the capacity after each charge cycle, charge voltage-regulation accuracy is important.

Figure 1 above shows that an undercharged battery voltage of 0.6 percent can result in a 5 percent capacity loss for Li-ion batteries. However, overcharging a Li-ion battery is not recommended and can be dangerous.

Battery manufacturers recommend undercharging a 4.2V regulated Li-ion battery at 4.1V to extend its life for backup energy applications.

Production challenges are often associated with time-to-market, total system cost and reliability. Time-to-market is significant for consumer products that have a short product life cycle.

Fast response to market changes is important in today's fast-paced world. The short time permitted from concept to final product also minimizes the resources used and reduces cost by saving design time.

Additional components may result in extra failure factor points and, in some cases, increase in costs. Although it is not true in all applications, saving space by creating highly-integrated solutions may cost more than a system built from discrete components. Thus, reliability should always come first when designing a product, if performance is the trade-off.

Figure 2: A typical SEPIC topology multi-cell, multi-chemistry charge-management system using the MCP1631 high-voltage PWM and the PIC12F683 general-purpose MCU is shown.

Using an MCU
If flexibility is important for product development and the ability to make changes during a project is necessary, then an MCU directed PWM controller battery charge-management system is a perfect fit.

Figure 2 above demonstrates a typical single ended primary inductor converter (SEPIC) topology multicell, multichemistry charge-management system using a high voltage PWM  and a general purpose 8-bit MCU..

Advanced MCUs are available for GPIOs and ADCs for additional sensing and output status. SEPIC is a switching topology that delivers better efficiency and less power dissipations when I/O voltage difference is wide and current flow is significant.

For example, operating a 9V input voltage when VBAT is 4V and ICHARGE is 1A, the power dissipation for popular linear solutions is (9V - 4V) * 1A = 5W and the same condition for a switching solution with 90 percent efficiency is only 4W * (0.1/0.9) = 0.44W.

Cooling a 1/2W solution is much easier than cooling a 5W system. The following equations show the calculations for linear and switching power dissipations that are applied in the above example:

Figure 3 below depicts a typical charge profile of an MCU-directed PWM controller for a single-cell 1,700mAh Li-ion battery with constant current/constant voltage (CC-CV) algorithm at 1A charge rate. The algorithm starts with precondition if the battery voltage is below the pre-conditioning threshold.

Once it passes the pre-conditioning stage, the system goes into constant-current stage until a regulated voltage is detected. The charge termination value in this example is 200mA.

The system continues monitoring the battery voltage and recharges when it falls below the recharge threshold voltage, to limit the number of charge/discharge cycles and prolong the battery's life, while keeping its voltage at a safe level.

Figure 3: A typical charge profile of an MCU-directed PWM controller for a single-cell 1700mAh Li-ion battery with CC-CV algorithm at 1A charge rate is shown.

Stand-alone system
The main reasons that designers select fully integrated, single-chip battery charge-management systems are compact size, low cost and minimum design time/effort/ resources.

The stand-alone Li-ion battery charger IC, especially for the linear topology, may require only SMD capacitors to maintain AC stability and provide compensation when a battery load is not present. Thus, the required PCB space and associated components are minimized when using an integrated solution.

Figure 4 below shows a typical application circuit when a fully integrated battery management controller is applied as a stand-alone battery charger. Since the charge algorithm and housekeeping circuits are built into the IC, no firmware is required and the design is straightforward.

Figure 4: The required PCB space and associated components are minimized when using an integrated battery-management controller.

Semiconductor companies typically deliver good product support in the form of detailed datasheets and application notes to help designers implement the battery charger IC into the system.

This saves time-to-market and reduces cost by shortening development time and eliminating software development. On the flip side, inflexibility is a major barrier to standalone charge management ICs in today's rapidly-changing battery world.