Power Electronics

Battery-Charger ICs Are Tailored for Latest Applications

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As the use of rechargeable batteries grows in consumer applications, chip vendors continue to develop application-specific charging ICs. In recent years, many charging ICs have been developed for single-cell and multicell Li-ion applications. Many of these were developed with cell phones, PDAs, digital still cameras and notebook computers in mind. However, new electronic devices such as Bluetooth headsets are now also inspiring the development of Li-ion charger ICs. At the same time, new uses for other chemistries like NiMH are driving chip manufacturers to develop different types of charging circuits.

One new charger IC for NiMH batteries is the DS2715 from Dallas Semiconductor/Maxim Integrated Products (Sunnyvale, Calif.). This battery-pack charger controller can charge up to 10 NiMH cells in series and may be configured as a switched dc charger, a linear current regulator or a switch-mode current source. Charge rates in any configuration can be user-selected with an external sense resistor, which enables rates between 0.15 C and a fast 2 C.

The DS2715 is similar to another NiMH charge controller, the DS2711, which the company introduced last year. The DS2711 was designed to charge up to two cells in series, but also had the ability to charge up to four cells in parallel. Because the newer IC is designed to charge a longer string of cells, it requires a higher voltage rating and employs a different internal architecture. The maximum charging voltage on the DS2715 is 16.5 V; however, this chip does not have the capability to charge cells in parallel. In addition, the DS2711 monitored the cell impedance to identify when a defective or wrong cell type (such as alkaline primary cells) was connected to the charger. That function is not offered on the DS2715.

The DS2715 implements various charge states to ensure safe and reliable charging. Throughout the entire charge cycle, the IC continuously monitors cell voltage and pack temperature. The charge cycle begins with a precharge qualification to prevent either fast charging of deeply depleted cells or charging under extreme temperature conditions. When the cells are ready, fast charging begins and proceeds as long as temperature and cell voltage remain within specified ranges.

To terminate charging, the DS2715 uses the dT/dt technique. The charger monitors the cell temperature as measured by a thermistor. If the change in cell temperature exceeds the prescribed value, charging is terminated. For added safety, a secondary charge timer backs up the dT/dt termination method. The duration for timing a charge termination is user selectable with an external resistor from 30 min up to 6 hr.

The DS2715 targets applications such as portable DVD players and notebook computers that are adopting NiMH batteries rather than Li-ion to reduce cost. The charge controller also may be applied to charge the NiMH batteries that power electrically assisted bicycles. The device is available in a 16-pin SOP, priced starting at $2 each in quantities of 100. For more information, see www.maxim-ic.com.

Two battery-charger ICs from Intersil (Milpitas, Calif.) address the requirements of NiMH as well as NiCd and Li-ion batteries. The ISL6251 and ISL6255 are fully programmable, chemistry-independent battery chargers that provide highly accurate charging. Operating with an input voltage range of 7 V to 25 V, the chips permit a battery-voltage setpoint up to 17.64 V and accommodate 2-, 3- and 4-cell Li-ion battery packs.

The chips maintain a charge voltage accuracy of ±0.5% over the -10°C to 100°C temperature range. In addition, these chargers feature a fully programmable charger current limit from zero to full scale, including trickle-charge currents as well as a programmable adapter current limit and charge voltage. Accuracy on the input current limit is ±3%, while the battery-charge current limit is accurate to within ±3% (ISL6255) or ±5% (ISL6251). As an option, the trickle-charge current limit is accurate to ±25%. In addition, input current limit response is less than 100 µs.

To achieve high efficiency (>96%), both chargers use synchronous buck converters and employ a MOSFET switch rather than a diode to select either the adapter or the battery as the power source. In the ISL6255, additional circuitry performs automatic selection between ac-dc adapter and battery. The ISL6255 also contains a dc adapter monitor to support aircraft power applications with the option of no battery charging.

The ISL6251 and the ISL6255 are available in 28-pin 5-mm × 5-mm QFN packages and also in QSOPs. In the QFN packaging in quantities of 1000, pricing is $2.25 for the ISL6251HRZ and $2.75 for the ISL6255HRZ. For more details, see www.intersil.com.

With USB ports now being used as a power source for charging, some charge controllers offer the ability to select either the USB port or an ac adapter as the source. Two new single-cell Li-ion battery chargers from Linear Technology (Milpitas) — the LTC4075 and LTC4075X dual-input battery chargers — offer this option. After sensing the voltage at each input, the chargers will automatically select the ac adapter if both sources are present and if sufficient power is available at the input. To satisfy the “sufficient power” requirement, the supply voltage must be greater than the UVLO threshold and at least 50 mV above the battery voltage.

Using a CC/CV algorithm, the LTC4075 and LTC4075X dual-input battery chargers can deliver up to 950 mA of charge current from a wall adapter supply, or up to 500 mA of charge current from a USB supply. The final float voltage accuracy is ±0.6% in either case.

The LTC4075X disables the trickle-charge function, benefiting systems requiring more charge current during low-battery conditions. Applications include PDAs, MP3 players, lightweight portable medical and test equipment, and large color screen cell phones.

The LTC4075X is equipped with a thermal regulation scheme that maximizes the charging rate while guarding against overheating. Wall adapter supply-charge current, USB supply-charge current and charge- termination current are resistor programmable.

The LTC4075X is offered in a 10-lead 3-mm × 3-mm DFN package. The total solution footprint is less than 25 mm2 since no external MOSFET, sense resistor or blocking diode are required. The 1000-piece pricing for the LTC4075/XEDD begins at $1.88 each. For more information, see www.linear.com.

Another single-cell Li-ion charger offering autonomous power-source selection from ac adapter or USB port is the bq25010 from Texas Instruments (Dallas). This IC integrates a power FET and current sensor to deliver up to 500-mA, while the USB charge control limits the USB current to 100 mA or 500 mA. The bq25010, which targets applications such as Bluetooth headsets, also incorporates a 100-mA stepdown dc-dc converter. Under nominal load current, operating at a fixed switching frequency of 1 MHz, the device achieves up to 95% efficiency. When the input supply is removed, the converter can operate directly from the battery and regulate the output voltage.

The bq25010's dc-dc converter has an adjustable output voltage of 0.7 V to 4.2 V. Another family member, the bq25012, provides a fixed-output voltage of 1.8 V. Both devices are packaged in a 3.5-mm × 4.5-mm QFN and are priced at $2.20 each in quantities of 1000 units. For more information, see www.ti.com/sc05077.

National Semiconductor Corp., Santa Clara, Calif., www.national.com CIRCLE 350
Linear Technology Corp., Milpitas, Calif., www.linear.com CIRCLE 351
Fairchild Semiconductor, San Jose, Calif., www.fairchildsemi.com CIRCLE 352
Intersil Corp., Milpitas, Calif., www.intersil.com CIRCLE 353
Maxim Integrated Products, Sunnyvale, Calif., www.maxim-ic.cm CIRCLE 354
Texas Instruments Inc., Dallas, Tx., www.ti.com CIRCLE 355
Catalyst Semiconductor, Sunnyvale, Calif., www.catalyst.com CIRCLE 356

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