The first product in a family of programmable lithium-batterycharger ICs for portable consumer applications is built around a 1-A current-mode step-down switching supply. Intended for USB or ac adapter power sources, Summit Microelectronics' SMB135 operates on input voltages from 4.35 to 6.0 V. (For applications where the ac adapter may be poorly regulated and/or an aftermarket product, the SMB135 can tolerate 10-V inputs without damage.)
Charging the lithium-ion (Li-ion) batteries in portable devices is getting to be a major challenge. They're moving to higher capacities, going beyond even 1 Ah per cell. Battery manufacturers specify allowable charge rate in terms of that capacity, designated C. Usually, lithium batteries can be charged at 0.7 to 1.0 C. Allowing for the shape of the charging curve, a 1-Ah battery could be fully charged in an hour or two from a charger that delivers a maximum dc current of 1 A or more.
For these batteries, controlling the shape of the charging current is necessary to minimize reliability-reducing "hot spots" in the portable devices. Power dissipation is equal to the voltage difference between the charger and the battery times the charging current. So, sending a lot of current to a fully discharged battery is a bad idea.
But look at the problem through the other end of the telescope. Many consumer devices are moving to providing battery charging via the USB port. USB can only provide 500 mA maximum, and more often 100 mA maximum from a host-powered port. And if the user is going to use the portable device while it's being charged, there's even less current available for charging. As a result, it can take a very long time to charge 1-Ah batteries from a simple linear charger.
That's why a switching regulator makes sense. Since a buck regulator steps down voltage and steps up current, it's possible to deliver up to 750-mA charging current from a standard 500-mA USB port. This is more than double the charge current typically available from chargers based on linear regulators. The benefit of this is shorter charging time, and end-users' perceptions of battery charge time is an important-factor in repeat sales and word-ofmouth advertising.
Additionally, switch-mode operation reduces power dissipation by as much as 90% compared to linear-mode chargers. High efficiency eliminates the need for thermal current foldback, which extends charge time in linear-based chargers. Low power dissipation also allows the SMB135 to be housed in a 1.3- by 2.1-mm chip-scale package for the smallest footprint of any present battery-charger IC.
Since the programmable switching frequency ranges from 750 kHz to 1.25 MHz, external component size is in line with the chip package size. At the same time, a switching supply facilitates flexibility in programming charging voltage and current (see the figure).
The chip provides a programmable algorithm for controlling voltage and current to Li-ion and Li-polymer cells. To protect deeply discharged cells, the SMB135 has a 3-mA "trickle charge" mode below 2.0 V. It permits safe, controlled recovery of the deeply discharged cell until it can accept normal charging currents. As future battery packs use cell float voltages other than 4.1 or 4.2 V, the charging algorithm in the Summit chip can be dynamically adapted (up to 4.62 V), providing a platform that can be used through multiple product generations.
All charging parameters, including pre-charge/fast-charge/chargetermination current, cell float/pre-charge voltage, and battery temperature/timer safety limits, are I2C/SMBusconfigurable. Designers also can custom program the SMB135 statically, using nonvolatile registers.
The SMB135 costs $0.98 each in quantities of 10,000 units. For development, Summit Microelectronics provides evaluation boards and a menu-driven Windows GUI software development platform. The design kit software can be downloaded directly from the company's Web site.