Dallas Semiconductor introduces the DS2790, a programmable 1-cell Li-ion (Li+) fuel gauge and protector. With its integrated MAXQ microcontroller, generous program and data memory, and accurate measurement system for battery current, voltage, and temperature, the DS2790 provides a platform for customizing single-cell-battery fuel-gauging algorithms. The DS2790 is a truly unique device. Optimized for a single-cell Li+ battery pack, the DS2790 microprocessor-based solution enables the fuel-gauging capability and customization traditionally reserved for devices that serve battery packs with much higher cell counts and more complexity, such as those found in notebook computers.
Inside
At the heart of the DS2790's computing core is the low-power 16-bit MAXQ20 microcontroller with its advanced, accumulator-based (MAC), 16-bit RISC architecture. Highly efficient, its fetch and execution operations are completed in one cycle without pipelining, because the instruction contains both the operation code and data. The processing core is supported by a 16-level hardware stack, which enables fast subroutine calling and task switching. Data can be manipulated with three internal data pointers. Multiple data pointers allow more than one function to access data memory without having to save and restore data pointers each time.
Three types of memory
To allow the user to program proprietary algorithms, the DS2790 contains three types of memory: programming memory, data EEPROM, and data RAM. The memory is arranged in an Harvard architecture, with separate address spaces for program and data memory. The 16kB of programming memory consists of 8kB of password-protected EEPROM and 8kB of ROM. The inclusion of EEPROM allows the devices to be reprogrammed, which simplifies and reduces the cost of development and field upgrades. The ROM contains routines that allow reprogramming over the I2C interface, SHA-1 authentication, and support for in-circuit debugging. The data EEPROM consists of 128 bytes, and is available for storing important data such as charge parameters, cell characteristics, and manufacturing data that should remain unaffected by sever battery depletion, accidental shorts, or ESD events. The data RAM is 512 bytes, and is used for temporary data storage.
More functions
The DS2790 also provides precise current, accumulated current, voltage, and temperature measurements. The 12-bit-plus-sign current measurements are an average of 128 individual current samples. The current measurements are internally summed to produce the accumulated current with accuracy within ±2% of full-scale measurement, ±4µV over a range of ±64mV. Using a 15m sense resistor, this current accuracy translates to within ±2% of full scale, ±267µA over a 4.2A range. Standby currents are measured with an accuracy of ±195µA. The DS2790 measures voltage as a 10-bit-plus-sign value over a 0 to 4.75V range with a resolution of 4.8mV. An on-chip temperature sensor measures the temperature of the battery and reports the results as a 10-bit-plus-sign value with a resolution of 0.125 degrees Celsius. All measured data and any password-protected reprogramming of the EEPROM memory can be communicated to the host through the DS2790's I2C communication interface. The DS2790's 1-cell Li+ protection circuitry is comprised of an autonomous state machine that provides the overvoltage, undervoltage, and over/underdischarge protection. Safety and reliability are increased because the protection function does not rely on the CPU and, therefore, does not depend on its loading to perform other functions.
Availability
Typical applications for the DS2790 include wireless handsets, high-end PDAs, digital video camcorders, and digital still cameras. The device is available in 28-pin TSSOP and TDFN packages with prices starting at $2.50 (1000-up, FOB USA).
