HV automotive battery stack monitor ups accuracy, meets ISO26262

November 19, 2015 // By Graham Prophet
LTC6811 is a high voltage battery stack monitor, a drop-in replacement for the LTC6804 with higher performance and 25% lower price. The LTC6811 is a complete battery measuring IC for hybrid/electric vehicles that incorporates a deep buried Zener voltage reference, high voltage multiplexers, 16-bit delta-sigma ADCs and a 1Mbps isolated serial interface.

An LTC6811 can measure up to 12 series-connected battery cell voltages with better than 0.04% accuracy. With eight programmable 3rd order low pass filter settings, the LTC6811 provides a high level of noise reduction. In the fastest ADC mode, all cells can be measured within 290 µsec.

For large battery packs, multiple LTC6811s can be interconnected and operated simultaneously, using Linear Technology’s 2-wire isoSPI interface. This built-in interface provides electrically isolated, high RF-noise-immune communication for data rates up to 1 Mbps. Using twisted pair, many LTC6811s can be connected in a daisy chain to one host processor, enabling the measurement of hundreds of cells in high voltage battery stacks.

A number of features support ISO 26262-compliant design. Sum of cell (SC) measurement is improved three-fold; comparison with individual cell measurements confirms accuracy to within ±0.25%. VREF2 (2nd reference) is improved by a factor of two - comparison with the primary reference confirms accuracy to within ±5mV. Simultaneous C6 and C7 measurement using both ADCs allows a comparison of results to confirm operation of both ADCs to within 0.01%. Simultaneous measurements through both digital filters: comparison of digital results confirms operation of digital filters.

The part follows three generations of battery monitor ICs, designed to surpass the environmental, reliability and safety demands of automotive and industrial applications. LTC6811 is fully specified for operation from ˗40°C to 125°C. It has been engineered for ISO 26262 (ASIL) compliant systems with extensive fault coverage via its redundant voltage reference, logic test circuitry, cross-channel testing, open wire detection capability, a watchdog timer and packet error checking on the serial interface.

For existing designs using Linear Technology’s LTC6804 battery stack monitor, the LTC6811 is a drop-in replacement with additional filter cutoff frequencies, added passive and active balancing control features, new ADC commands and enhanced fault coverage for functional safety. Available filter responses have been augmented by two additional instances, at 422 and 845 Hz. The LTC6811 is fully supported by Linear’s Linduino technology, an Arduino-based microcontroller board and software library. The microcontroller board includes an electrically isolated USB port and directly connects to the LTC6811 demonstration board, providing a platform for evaluating and developing the LTC6811.

The LTC6811 is packaged in an 8 x 12 mm surface mount SSOP at $8.19 (1,000)

Linear Technology; www.linear.com/product/LTC6811

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