Okay, you want to design an electric car. Whatever
kind of motor you decide on, you’re going to want
to run it at a pretty high voltage. That means stacking
many batteries in series to get to that voltage,
which introduces interesting challenges in monitoring and
charging circuits as potentials at the negative electrodes rise
above system ground.
This is not a new problem. But as long as it’s been confined
to products like golf carts and nuclear submarines,
engineers have had to attack it on an ad-hoc basis The
potential product volumes of the electric car have changed all
that. Ever alert to new business opportunities, Linear Technology
has staked a claim to the market with the LTC6802.
This highly integrated multicell battery monitoring IC can
handle up to 12 individual cells all by itself. Multiple LTC6802s
can be stacked in series without optocouplers or isolators for
precision voltage monitoring of every cell in longer strings of
series-connected batteries (see the figure). Ultimately, that
lets you run your motor at more than 1000 V. The LTC6802
was designed with an eye on fussy lithium-ion (Li-ion) batteries,
but the chips really don’t care what kind of batteries
you’re using.
There are three modes of operation. Standby mode is a
power-saving state where everything except the serial interface
is turned off. In measure mode, the chip records cell voltages and stores the results in memory. Measurements
are based on an internal precision voltage reference. In measurement
mode, the chip also monitors each cell overvoltage
(OV) and undervoltage (UV) conditions. In monitor mode, it just checks the cells for UV and OV conditions. (UV and OV
are reported on a status pin. The registers are read out ondemand
over the SPI bus.)
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. There is a control
pin for an external MOSFET switch, which you’d need
in a practical system to discharge overcharged cells. The
LTC6802 comes in an 8- by 12-mm surface-mount package.
Pricing starts at $9.95 each in 1000-piece lots. Samples
and demonstration boards are available.
DON TUITE
LINEAR TECHNOLOGY •
www.linear.com