Electronic Design

Design A Linear Li-ion Battery Charger For Portable Systems

Energy-storage devices such as batteries continue to change how people live. Every year sees greater daily usage of battery- powered personal electronic devices. Moreover, demands for longer run times and smaller sizes are driving continuous growth in both the battery and semiconductor industries.

When the time to develop next-generation batteries takes longer than Moore’s Law, the need arises for highly integrated, feature-rich ICs that deliver better performance. It’s important to learn how to design with these types of ICs to simplify the development of new systems.

A battery converts chemical energy into electric potential, or voltage. If the energy can be restored, the battery is considered a secondary or rechargeable battery. Nickelmetal- hydride (NiMH) and lithium-ion (Li-ion) batteries are common in portable applications. Compared to NiMH batteries, Li-ion batteries offer a higher nominal voltage per cell, lower self-discharge rate, and energy density in mass and volume that make them attractive for powering lightweight and space-sensitive applications (Table 1).

Li-ion batteries are relatively safe when designers use caution working with them. Table 2 shows some typical applications of Li-ion battery-powered systems. Single- and dual-cell applications comprise approximately 70% of the Li-ion battery market. Recent trends in space, cost, and weight reduction when designing small tools, digital camcorders, and similar devices are driving some dual-cell applications to become single-cell.

A single Li-ion cell can replace three NiMH battery cells in devices (Table 1, again). One advantage of reducing the number of battery cells in a system is to avoid extra design work for balancing multiple cells.

With the widely used Universal Series Bus (USB), Li-ion batteries are able to be charged from USB ports on a majority of computers. A nominal voltage of 5 V makes the USB protocol attractive for single Li-ion cell applications. The USB specification defines the voltage drop budget in the range of 4.75 to 5.25 V for both host and/or hub, and no less than 4.45 V is allowed at the connector of host and/or hub.

Li-ion batteries typically use the constant-current constant-voltage (CCCV) algorithm for charging. When a charge voltage of 4.2 V per cell is met, the charger maintains a constant voltage until the termination condition is satisfied. A battery’s voltage should be carefully designed with tolerance to avoid premature termination and hazard. The USB voltage range is well suited for simple step-down charger designs with a typical Li-ion voltage-regulation value of 4.2 V.

Two common step-down topologies are linear (low dropout, or LDO) converters and switching (buck) converters. Ideally, a switching topology offers 100% efficiency. After considering areas of power loss, efficiency may fall between 85% and 95%. Equation 1 calculates LDO efficiency:

When IGND is much smaller than IOUT, it can be ignored. Thus, the efficiency of an LDO-based Li-ion battery charger can be simplified to the ratio of VOUT to VIN:

During a typical constant-current (CC) charging mode, the efficiency moves from 60% to 84%. The efficiency will stay at 84% for the constant-voltage (CV) charging mode. Thus, an LDO topology works well in single-cell Li-ion battery-charger designs when the input voltage is about 5 V.

An LDO topology also reduces cost by omitting inductors, and it avoids electromagnetic interference (EMI) challenges associated with switching topologies. But, if a fast-charging current above 1 A is required, a switching topology should be considered. Equation 4 presents a powerdissipation calculation that illustrates this:

In this example, a battery-charging current of 2 A and a battery voltage of 3 V are selected to show the worst condition in CC mode. An input voltage of 5 V is selected to simplify the calculation. When designing a system, the worst condition that’s based on a given tolerance should be considered.

Even for a 35°C/W thermal-rated 4- by 4-mm quad flat no-lead (QFN) package, it’s difficult to dissipate 4 W:

A room temperature of 25°C with an additional 144°C introduces a temperature of 169°C in a system. A junction temperature of 169°C is over the thermal-shutdown threshold of a typical die temperature. Well-designed Li-ion charging-management ICs should include thermal feedback that reduces the charge current when temperature begins to rise to threshold levels.

Baseline linear Li-ion battery chargers are usually low-cost and have a low pin count and low passive-component requirements. They’re often available in packages such as SOT-23, MSOP, and DFN. With the maturation of semiconductor technology, most baseline linear battery chargers are fully integrated. The typical pin count ranges from five to 10 pins.

Charging a Li-ion battery safely is usually the primary and only goal for baseline chargers. No fancy features are required. Figure 1 depicts a simple five-pin battery charger that requires a minimum of three components to operate—an input capacitor, an output capacitor, and a programming resistor. Additional pins may be available for functions such as extra status indicators, power-good indicators, battery temperature monitoring, timer, and logic current control.

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In addition to linking peripherals and computers, the USB protocol also delivers high speed at an economic cost. Connecting devices and peripherals through USB ports to a computer has become the most popular method. With a voltage range of 4.75 to 5.25 V, USB is an excellent candidate for restoring energy back to singlecell Li-ion battery cells or packs as previously discussed. There are many methods for charging single-cell Li-ion batteries.

Table 3 lists a few basic methods for designing a single-cell Li-ion battery charger from USB ports. The first method utilizes a low-power USB port for a fixed charging current. This method usually ends up below the absolute maximum current of a low-speed USB port, which is 100 mA. Due to the resistor’s tolerance, charge current, and supply current, this charge current is typically under 90 mA. This simply treats a USB port as a 5-V, 100-mA-rated power supply.

To take advantage of high-speed USB ports, an external MOSFET can be used to set two different charging currents when driving the gate low or high. A high-speed USB port allows an absolute maximum current of 500 mA, but a port should always start at low speed until verification is complete.

An integrated MOSFET for setting two different charging currents simplifies this design and offers either a preset or resistor- programmable charge current. Figure 2 shows an example that offers three different charge-current settings and can seamlessly switch between a wall wart (ac-dc adapter) and a USB port.

When a wall wart is present, the maximum charging current can easily be higher than 500 mA from a high-speed USB port. When just a USB cable is applied, the charge current will be based on the logic level high or low. Some designs require only one input-power rail, but a different input type can be set by communication between interfaces.

Typically, the preset USB charging current is below 450 mA for a high-speed USB port for the same reason as it is in a low-speed USB port. Proper design methods should also limit the amount of input current drawn from the USB port for safety, as well as to meet USB specifications.

As today’s portable devices become more feature-rich, requirements for proper battery management increase. In spaceconstrained applications, highly integrated power-rail controls advance a designer’s experience. Each power rail must be well managed for seamless switching among the input power path, system load, and battery cell.

Figure 3 demonstrates a typical application circuit of a Li-ion battery charger with system load-sharing and power-path management features that can switch between power sources. One advantage to using this design instead of a traditional method is that each power rail is managed and the battery is in support mode when the input voltage is insufficient to keep the output voltage steady. Sometimes, additional features such as low-power indicators or controls, as well as power-source selection, offer functionality beyond just restoring energy back to batteries.

Increased use of Li-ion batteries leads to a broader range of safety and functionality requirements. These requirements may come from internal organizations that promote hazard-free design guidance; local governmental regulations or policies; regional product-manufacturer preference; battery-manufacturer specifications; a designer’s level of experience; or an enduser’s habits. Common functions include timers for each charging stage, input overvoltage protection, communication protocols, multiple channels of regulated outputs, and battery authentication.

Figure 4 shows an input overvoltage protection feature of a single-cell Li-ion battery charger. The output-charge current terminates when the input voltage passes the protection threshold, and it resumes once the input voltage falls back to the designed range (Fig. 5). Since December 2006, this technique has been recommended for mobile devices as a technical requirement and test method of charger interfaces for mobile telecommunication terminal equipment.

Limiting the input voltage for a linear battery charger keeps end users from incorrectly using wall-wart or ac-dc adapters. It also prevents voltage spikes. Recall Equation 4:

Assuming the charge current is 1 A, if the input and output voltages (battery voltage) increase, power dissipation grows. Therefore, when the differences between input and battery voltages jump to 4 V, the power dissipation is 4 W.

Green technology is always a hot topic. Engineers and scientists constantly work to improve existing designs and offer better solutions for society. Li-ion batteries can be designed with fuel cells, photovoltaic solar cells, hydro power, and wind power as storage, backup, or supportive power. Highly integrated linear solutions may overcome hurdles in low-power designs, such as compactness and simplicity.

When intelligence, efficiency, or power dissipation are concerns, designers should survey their solutions thoroughly and understand the tradeoffs between platforms that are available. When designing with batteries or any power systems, safety is always the first priority.

For more information, visit www.microchip.com/battery. Also, visit www.analogtalk.com to view Microchip’s analog technology blog.

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