Power Electronics

Chip Cuts Battery Management Down to Size For Hybrid Vehicles

A battery stack monitor IC developed by Linear Technology addresses the daunting data acquisition and battery management challenges encountered in emerging high-voltage, Li-ion battery applications such as hybrid electric vehicles (HEVs), electric vehicles (EVs), and uninterruptible power supply (UPS) systems. The LTC6802 provides monitoring and cell balancing for up to 12 series-connected Li-ion cells in a battery stack (Fig 1).

Moreover, multiple LTC6802s can be stacked in series without optocouplers or isolators, for precision voltage monitoring of every cell in long strings of series-connected batteries.With these capabilities, the chips replace complex, high parts-count discrete solutions that are currently being used to manage high-voltage Li-ion battery stacks. The LTC6802 makes it possible to take quick and accurate measurements of all cell voltages, even in the presence of stack voltages over 1000 V.

To achieve this functionality, the LTC6802 integrates a 12-bit sigma-delta ADC, a precision (10 ppm) voltage reference, and a high-voltage input multiplexer (Fig 2). There’s also an interface for connecting multiple devices and that comes in two forms: In the LTC6802-1, the company offers a current-mode, 1-MHz serial interface that eliminates optocouplers between the devices. With the serial connection of devices only one isolator is needed at the bottom of the stack. On the other hand, Linear also offers the LTC6802-2 with a parallel interface. This version of the chip is advantageous in applications where designers can’t allow the loss of communications with one device to disable the whole battery stack (Fig 3).

One of the key challenges in designing a device like the LTC6802 is the need for millivolts of accuracy over such a wide common-mode voltage range and over a wide temperature range. These requirements were addressed in part with Linear’s choice of a sigma-delta architecture for the ADC. While other designs might employ a SAR architecture for its speed, the sigma delta architecture is beneficial in that, as a sampling device, it has built-in FIR filtering. That filtering is helpful because the measurements are typically taken in noisy environments.

In the LTC6802, the maximum total measurement error is guaranteed at less than 0.25% from -40°C to 85°C and all cell voltages in a battery stack can be measured within 13 ms. Each cell is monitored for undervoltage and overvoltage conditions, and an associated MOSFET switch is available to discharge overcharged cells. Each LTC6802 includes temperature sensor inputs, GPIO lines and the forementioned precision voltage reference.

The LTC6802 is fully specified for operation from -40°C to 85°C and offers diagnostics and fault detection. In addition, the device meets AEC Q100. The LTC6802 is packaged in an 8-mm x 12-mm, 44-lead SSOP.

Priced at $9.95 each in 1,000-piece quantities, samples, demonstration boards and the data sheet are now available at www.linear.com. The product will be available in production quantities in the fourth calendar quarter 2008.

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