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.
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