A variety of linear and switch-mode chips continues to emerge in the IC battery charger market, triggered by shrinking real estate requirements and the need for improved thermal management in charging lithium-ions, today's battery of choice.
One-chip simplicity, while a design virtue for chargers, is at the same time giving rise to two kinds of integration. The first kind strives to minimize the number of external components in dedicated, standalone chargers that work without the need for microprocessor controllers. The second kind stresses functionality and adds ancillary building-block functions for chips that run the gamut from charger/controller to those approaching power-management ICs-that is, designs that reflect a previously custom part whose functions are gaining ground in the mainstream. Both types of integrated devices tend to feature more interconnectivity to facilitate system monitoring and control, usually through an 12C or SMBus, the two-wire buses that share functionality and are usually compatible.
Charger ICs generally are classified as linear or switch-mode devices. A third type, pulse mode, is much less common for Li-ion applications. Charger ICs may also be classified by applications that range from one- or two-cell chargers working off 5 V for wireless and handheld devices, to those that accept a wide range of inputs to charge two or more cells for large, portable computing devices.
The newest linear arrivals include the LTC4054 from Linear Technology (Milpitas, Calif.), an 800- milliamp charger for one-cell lithium-ions, with on-chip power MOSFET in a ThinSOT package that the company says is half the size of competing offerings. It incorporates thermal-feedback regulation to limit the die temperature during high-power operation or high ambient temperature. It's possible to power up this charger from a USB port. Another USB-compatible charger is the company's LTC-4053, which also can power up from an ac wall adapter. The standalone charger offers up to 1.25 A and features a charge accuracy of 7 percent. Linear's latest device, the LTC4058, teams temperature management with USB features and provides charge rates of 800 mA in a 9-mm2 footprint.
Maxim Integrated Products' MAX1507/08 are linear devices that offer integrated constant-temperature regulation for one-cell, handheld equipment applications. These 3 x 3-mm TDFN devices, which the company is promoting as the smallest complete temperature-regulated linear products in the market, integrate a power MOSFET, reverse-blocking diode and current-sense resistor (basically becoming an industry-standard complement). The devices fast-charge at up to 800 mA and, from a nominal input of 4.25 to 7 V, are protected against input transients up to 14 V from loosely regulated ac adapters.
Maxim (Sunnyvale, Calif.) also offers a dual-input device, the MAX1874, that enables users to charge the device from a Universal Serial Bus or ac adapter. This dual-input feature, fast becoming popular, enhances a self-contained device that was designed for single-cell Li-ion batteries. The MAX1874 works from a maximum input of 6.5 V and can be protected from inputs as high as 18 V with an external PFET. It contains battery-to-dc power switchover circuitry to allow a system to draw power from the ac mains in lieu of the battery. The MAX1874 will reduce charging current, rather than shut down, when it approaches thermal limits. The chip provides an additional layer of safety by monitoring ambient or battery temperature with an external thermistor, Maxim said. The MAX1874 is available in a 5 x 5-mm, 16-pin thin dual-in-line flat nonleaded package.
Other linear chips with the same bent toward multiple power sources, and a claimed advantage in thermal performance, include the AnyAdapter ISL6292 from Intersil Corp. (Milpitas), which works from a 4.3- to 7-V input and is especially suited to smart phones and PDAs. The company bills the chip, in a 3 x 3-mm DFN, or 4 x 4-mm or 5 x 5-mm QFN, as the industry's smallest and most thermally safeguarded IC for Li-ion/polymer batteries. Intersil claims its Thermaguard circuitry cuts heat in the thermally controlled charging sequence by several hundred percent more than competing solutions, which means less charging time in similar operating conditions. The device can power up from a current-limited adapter, in which case it will operate as a linear charger but with thermal efficiencies typically found in switch-mode or pulse-mode chargers, the company said.
The bqTiny BQ24014 and bqTinyII BQ24022 from Texas Instruments Inc. (Dallas) are linear Li-ion chargers for portable applications such as wireless phones, PDAs and digital cameras. The BQ24014 includes integrated power FET and current sensor, as well as a plethora of protection and status indicators, all in a 10-pin SON package. The BQ24022 includes an integrated USB port and an ac-adapter input and will automatically select between the two. This device can be preset to deliver 100 or 500 mA.
Semtech (Camarillo, Calif.) claims its SC803 is a complete, highly integrated charger IC for Li-ion/polymer and is promoting it as a battery-charger management system-on-a-chip. Suitable for charging up batteries from a USB port, the device works from a 4.2- to 7-V input and combines a controller, thermally protected power MOSFET, reverse-blocking diode and sense resistor elements in a 4 x 4 MLP footprint. The device delivers a fast-charge of up to 1.5 A for PDAs, handheld computers, digital cameras and desktop battery-charging stations. It can also be used as a general-purpose programmable current source. A battery-in-place monitor detects the presence of the battery, and Kelvin sensor inputs enable monitoring battery terminal voltage. An output current-mode monitor allows for connecting to an A/D converter or external microcontroller to direct charging functions.
National Semiconductor Corp. (Santa Clara, Calif.) characterizes its LP3945 and LP3946as a complete charge-management system for Li-ion and NiMH batteries (see March 17, page 48). The LP3946 is a standalone charger, preprogrammed at the factory. The LP3945 requires a controller chip. Edging toward the power-management IC, the company's LP3941 contains 11 low-dropout regulators, two battery chargers, 12C interface and LED drivers. The device was among the first of similar integrations, the most recent of which was TI's TPS65010 (see Aug. 4, page 63).
Switch-mode chargers provide higher efficiency than linear types and are often used for higher-powered applications. The trade-off: They are more expensive and tend to take up more real estate because they have external components that often include power MOSFETs. At the low-power end, Linear Technology's LTC4002 is a standalone switch-mode charger that works off an operating source up to 24 V for single Li-ions in general-purpose applications. At the low voltages, the device can power up from a simple 5-V wall adapter. It is intended for use with a p-channel MOSFET that will deliver up to 2 A to the battery. The LTC4002 works at 500 kHz, allowing use of smaller inductor and capacitors as compared with chargers that employ a hysteretic topology.
The company's multifunction LTC1980 is a 300-kHz PWM controller IC that combines a Li-ion battery charger (or, suitably programmed, an NiMH battery charger) and dc/dc converter. It will charge a battery and establish a regulated output from that battery for digital cameras, handheld computers and PDAs. With an external LDO switch, it can simultaneously charge the battery while powering system load from an unregulated ac wall adapter. The LTC1980's charger and dc/dc sections share its discrete components to minimize size and cost of discrete-controller sections. It comes in a 24-pin SSOP package.
Linear Technology's LTC1760 system manager, in a 48-pin TSSOP, is billed as a standalone charger with SMBus 1.1 interface for dual smart batteries. Intended for use in portable computers, instruments and battery backup systems, it contains a sequencer for managing system power, battery charger and power path controllers, 10-bit current D/A and 11-bit voltage D/A. Working from a nominal 6- to 28-V input, it's basically a buck charger that's fit for charging and monitoring the status of two two- to six-cell Li-ion batteries, six- to 18-cell NiMH or NiCd batteries, and a 6- to 28-V lead acid battery. No host microcontroller and associated firmware are needed.
The BQ24702 and BQ24703 multichemistry battery charge controller and system power selector from TI are fixed-frequency PWM devices. Operating in a buck configuration, they use so-called dynamic power management to minimize battery charge time for PC and power tool-type applications. A host-controlled system adjusts the battery charge current based on the total available adapter supply current that is available. These devices come in a 24-pin TSSOP.
Another IC working as a pulse-mode charger is the ST3S01LED from STMicroelectronics (Lexington, Mass.). It's a dedicated chip intended for use with an MCU to charge one Li-ion or three NiMH cells.
Maxim Integrated Products
National Semiconductor Corp.
Texas Instruments Inc.