[Design View / Design Solution]
Design A Linear Li-ion Battery Charger For Portable Systems
Lithium-ion batteries help designers meet their goals of getting greener, whether used for storage or backup power purposes, or in highly integrated solutions to develop low-power solutions.
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).
WHY USE SINGLE-CELL LI-ION? 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 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.
Please give me the details Can charge the NiMh-1.2V.2100mAh batteries with Constant Voltage Source.
Nagendra -May 11, 2009
Your Comments:
Enter the text from the image below
Please refresh the page if you have trouble reading this text.
Search Electronic Design
Email Newsletter
Sponsored By:
The Find Power Products monthly newsletter brings you the most important new developments within the world of power design. The newsletter includes exerpts from industry leader Sam Davis's exclusive blog, as well as overviews of the latest new products.
Enter Email to Subscribe
Web Seminar
Sponsored By:
Title: Exploring How Good GUIs Drive Adoption in the Digital Power Management Space
Reprints
