Power Electronics

Single-Cell Li-Ion Chargers Get Smaller and Smarter

As the number of portable devices powered by a single Li-ion or Li-polymer cell continues to grow, semiconductor vendors maintain their quest to develop ever-more compact and robust battery-charger circuits. These battery-charger ICs target numerous high-volume consumer applications including smart phones, wireless headsets, portable media players, GPS systems, digital still cameras, toys, and point-of-sale and diagnostic equipment.

Although for some of these applications vendors have developed large power-management ASICs that integrate the charging function with voltage regulators and other functions, there is still demand for dedicated charger chips. These devices build on advances made in previously introduced ICs such as the ability to take power from either a USB port or ac adapter; the ability to optimize charge rates based on thermal conditions; and various fault-protection and diagnostic features.

In addition, functions such as the pass transistor, reverse-blocking diode and current-sensing circuits continue to be integrated on chip. The newest single-cell Li-ion/Li-polymer charger chips refine many of these capabilities, shrink the chip or overall design and reduce cost as illustrated by a few recent product introductions.

Higher Power Density

In April 2007, Advanced Analogic Technologies introduced a linear battery charger for single-cell Li-ion/Li-polymer batteries that offers very high power density. The AAT3697 delivers up to 2 A of charge current using a 12-pin, 3-mm × 3-mm TDFN package. According to the vendor, this linear charger offers the industry's highest charge-current to solution footprint ratio. In addition, a thermal loop feature enables the host system to use a high-current linear charger for fast charging times and avoid the risk of pulse charging the battery.

In the AAT3697, an intelligent digital thermal loop control system continuously measures internal circuit temperature and automatically reduces the fast charge current when the device exceeds a preset die temperature threshold. This closed-loop thermal management system maximizes charge current and minimizes charge time, even under harsh high-temperature operating conditions.

The AAT3697 also continuously monitors battery temperature and charge state for fault conditions. If an overvoltage or overtemperature condition occurs, the device automatically shuts down to protect the charging device, the control system and the battery under charge. A battery-charge timer also protects the battery and the system by terminating charge if the user-programmable charge time is exceeded.

The new battery charger also features two status monitor output pins capable of driving external LEDs to indicate charge status. A serial interface output can be used to report any of 12 charge states to the system microcontroller. The chip sells for $1.16 each in 1000-piece quantities. For more information, see www.analogictech.com.

Another device that helps shrink the single-cell Li-ion charger is Linear Technology's LTC4095, which was introduced in March 2007. This fully integrated linear charger in a 2-mm × 2-mm DFN delivers up to 950 mA without overheating the device or its surrounding components.

This USB-compatible IC offers digital high power select (charge current set to 100% or 20% of programmed value) and suspend mode input pins, in addition to an NTC thermistor input, trickle charge, timer termination with C/10 detection and automatic recharge.

Stand-alone operation eliminates the need for a microprocessor to terminate charging. A complete charger solution with the LTC4095 requires only two discrete components (input capacitor and a charge current program resistor) to complete the charger circuit (see the figure).

A proprietary thermal regulation scheme prevents the junction temperature of the IC from exceeding 105°C by reducing charge current during high-power or high ambient-temperature operation. The NTC thermistor input may be used for temperature-qualified charging. Charge current is adjustable with a standard off-the-shelf resistor. The device is designed to work with supplies up to 5.5 V, including those within USB specifications and from 5-V wall adapters.

During suspend mode, the supply current is only 11 µA. The LTC4095 also features four charge indication states including blinking, bad battery detection, a charge current monitor output for gas gauging, a 4.2-V 0.6% accurate float voltage and 5% charge accuracy. Pricing starts at $1.15 each in 1000-piece quantities. For more information, see www.linear.com.

The MCP73811 and MCP73812 from Microchip Technology are fully integrated Li-ion/Li-polymer charge-management controllers that offer a combination of small size and low cost. Packaged in 5-pin SOT-23s, which have a footprint of about 3 mm × 3 mm, these chips provide compact charging designs with selectable or programmable charge currents up to 500 mA. The chips are priced starting at $0.57 each in quantities of 10,000.

The MCP7381X charge-management controllers can be powered off a USB port, and feature on-chip and thermal regulation. However, the remaining feature set is minimized to provide a very cost-effective charging solution.

With their charge-enable input, the MCP7381X charge-management controllers provide a simple interface. The MCP73811 has a digital input and selectable USB charge currents of either 85 mA or 450 mA, meaning it provides charge currents without intervention from external components. The MCP73812 offers a user-programmable charge current via an external resistor, enabling designers to optimize charge currents for their particular application. For more information, see www.microchip.com.

In November 2006, Intersil introduced the ISL9214, a fully integrated, low-cost single-cell Li-ion charger that accepts two power inputs, one from a USB port and the other from a desktop cradle. The chip features 28-V-rated inputs for the cradle and USB, which offers an extra level of protection to a Li-ion battery from charging system failures. Moreover, the 28-V rating enables the chip to work safely with low-cost adapters that have loose output tolerance.

The ISL9214's charge current is programmable for the cradle input with a small resistor. The end-of-charge current is also programmable by another external resistor. Thermal protection is also included in chip. If the die temperature rises above a typical value of +125°C, a thermal foldback function reduces the charge current automatically.

The ISL9214 is available in a 10-lead DFN and is priced at $1.25 in 1000-unit quantities. For more information, see www.intersil.com.

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