A high voltage battery monitor for hybrid electric and electric vehicles, and other high voltage, stacked-battery systems has been announced by Linear Technology . The company’s LTC6804 can measure up to 12 series connected battery cells at voltages up to 4.2V with 16 bit resolution and better than 0.04 percent accuracy.
This high precision is maintained over time, temperature and operating conditions by a sub-surface Zener voltage reference similar to references used in precision instrumentation. When stacked in series, this device enables the measurement of every battery cell voltage in large high voltage systems.
Six operating modes are available to optimise update rate, resolution and the low pass response of the built-in 3rd order noise filter. In the fastest mode, all cells can be measured within 290µsec.
Multiples of these devices can be interconnected over long distances and operated simultaneously, the company’s 2-wire isoSPI interface.
Each of these devices has an isoSPI interface that provides high RF noise immunity up to 1Mbps and up to 100 meters of cable, using only twisted pair. Two communication options are available: with the LTC6804-1, multiple devices are connected in a daisy chain with one host processor connection for all devices; with the LTC6804-2, multiple devices are connected in parallel to the host processor with each device individually addressed.
The LTC6804 was designed to minimise power consumption, especially during long-term storage where battery drain is unacceptable. In sleep mode, draws less than 4µA from the batteries. General purpose I/O pins are available to monitor analogue signals, such as current and temperature, and can be captured simultaneously with the cell voltage measurements.
Additional features include passive balancing for each cell with a programmable balancing timer for up to 2 hours, even when this device is in sleep mode.
This solution interfaces with external I2C devices such as temperature sensors, ADCs, DACs and EEPROM. Local EEPROM can be used to store serialization and calibration data, enabling modular systems.
The LTC6804 isspecifically designed to withstandthe environmental, reliability and safety demands of automotive and industrial applications. It is fully specified for operation from -40°C to 125°C. It has been engineered for ISO 26262 (ASIL) compliant systems and a full set of self-tests ensure that there are no latent fault conditions. To accomplish this, the LTC6804 includes a redundant voltage reference, extensive logic test circuitry, open wire detection capability, a watchdog timer and packet error checking on the serial interface.
Along with the LTC6804, the company has also introduced an isoSPI transceiver. The LTC6820 enables bidirectional transmission of the Serial Peripheral Interface bus (SPI) across an isolated barrier up to 100 meters.
With this device, the SPI data is encoded into a differential signal, which is then transmitted via twisted pair and a simple, inexpensive Ethernet transformer. It supports SPI data rates up to 1MHz, using matched source and sink currents to eliminate the need for a transformer centre tap and to reduce EMI.
The drive currents and the comparator thresholds are set with two resistors, allowing the system to be optimised for cable length and signal-to-noise performance. The LTC6820 is a companion to the LTC6804 high voltage battery monitor with its built-in isoSPI interface. Battery management systems can interface to external components, such as microcontrollers.
Linear Technology Corporation
