End users continue to demand not only longer battery life, but also more accurate battery monitoring known as "gas-gauging." As battery chemistries evolve from NiMH to Lilon to LiPolymer, battery management 1C vendors must continually evolve the gas gauges. To that end, a pair of devices, one from Microchip and one from Intersil, push the battery-monitoring envelope.
Microchip's PS501 merges the technologies between itself and PowerSmart, an acquired power management specialist. The part is a two- to four-cell Lilon and LiPolymer fuel gauge, using a PIC18 microcontroller core. (A single-cell device is on the way.) A 16-bit sigma-delta converter handles the current, voltage, and temperature integration, while an on-board oscillator maintains an accuracy of ±0.35% over temperature.
The 501 has a direct-connect input to each of the cells. To enhance the support for LiPolymer cells, the direct-connect inputs contain a firmware-controlled cell-balancing circuit. According to the company, the number of requests for a LiPolymer device is increasing, mostly because of the form factor that the cells can provide since it is smaller and can adapt to curved configurations. The cell-balancing is mostly a safety issue and may not be needed down the road as the technology matures. In some cases, the safety features are integrated into the system's protection circuitry. The 501 eliminates an extra layer and can be embedded into an embedded or a removable battery.
By taking advantage of Microchip's process for flash, the amount of memory was dramatically increased (by about Zx to 16 kbytes without increasing the required area. That bump in memory size let the Microchip designers enhance their Accuron proprietary monitoring algorithms. The algorithms can be optimized for specific cells and specific applications, looking at the variables of voltage, current, and temperature and their impact on the application environment and the cell itself. Also included is a state-of-health algorithm that looks beyond just the battery's capacity. This algorithm helps the battery determine where it stands in its lifecycle (how many cycles remain) and to look at other criteria to determine an average state of health based on multiple criteria.
The increased memory space also allows for an expanded flexibility of the general-purpose I/O (GPIO). There are 13 GPIOs, nearly twice the number from the previous generation.
A unique set of development tools are bundled with the 501. Called the PowerToolZ, the Windows-based tool allows for simple user configuration of the GPIOs. The fuel gauge can be configured for custom applications using a wizard-driven menu. Those same tools can then be used as production tools, further reducing time-to-market. the Microchip part, Intersil's ISL6253 is a highly integrated battery two- to four-cell charger/ controller for Lilon/LiPolymer batteries. In this device, a high level of efficiency is achieved using a synchronous buck topology and low-side MOSFET (rather than a diode) to select between the adapter and battery. The MOSFET emulates a diode at light loads to improve the light-load efficiency and prevent system bus boosting.