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LTC6803

Multicell Battery Stack Monitor

Linear Technology 

LTC6803-1/LTC6803-3

OPERATION

when V+ = 44V. All circuits are turned off except the serial

interface and the voltage regulator. For the lowest possible

standby current consumption all SPI logic inputs should

be set to logic 1 level. The LTC6803 can be programmed

for standby mode by setting the comparator duty cycle

configuration bits, CDC[2:0], to 0. If the part is put into

standby mode while ADC measurements are in progress,

the measurements will be interrupted and the cell voltage

registers will be in an indeterminate state. To exit standby

mode, the CDC bits must be written to a value other than 0.

MEASURE MODE

The LTC6803 is in measure mode when the CDC bits are

programmed with a value from 1 to 7. When CDC = 1 the

LTC6803 is on and waiting for a start ADC conversion

command. When CDC is 2 through 7 the IC monitors each

cell voltage and produces an interrupt signal on the SDO

pin indicating all cell voltages are within the UV and OV

limits. The value of the CDC bits determines how often

the cells are monitored, and how much average supply

current is consumed.

There are two methods for indicating the UV/OV inter-

rupt status: toggle polling (using a 1kHz output signal)

and level polling (using a high or low output signal). The

polling methods are described in the Serial Port section.

The UV/OV limits are set by the VUV and VOV values in the

configuration registers. When a cell voltage exceeds the

UV/OV limits a bit is set in the flag register. The UV and

OV flag status for each cell can be determined using the

Read Flag Register Group.

An ADC measurement can be requested at any time when

the IC is in measure mode. To initiate cell voltage measure-

ments while in measure mode, a Start A/D Conversion is

sent. After the command has been sent, the LTC6803 will

indicate the A/D converter status via toggle polling or level

polling (as described in the Serial Port section). During

cell voltage measurement commands, the UV and OV flags

(within the flag register group) are also updated. When

the measurements are complete, the part will continue

monitoring UV and OV conditions at the rate designated

by the CDC bits.

Operating with Less than 12 Cells

If fewer than 12 cells are connected to the LTC6803, the

unused input channels must be masked. The MCxI bits in

the configuration registers are used to mask channels. In

addition, the LTC6803 can be configured to automatically

bypass the measurements of the top 2 cells, reducing power

consumption and measurement time. If the CELL10 bit is

high, the inputs for cell 11 and cell 12 are masked and only

the bottom 10 cell voltages will be measured. By default,

the CELL10 bit is low, enabling measurement of all 12 cell

voltages. Additional information regarding operation with

less than 12 cells is provided in the applications section.

ADC RANGE AND OUTPUT FORMAT

The ADC outputs a 12-bit code with an offset of 0x200

(512 decimal). The input voltage can be calculated as:

VIN = (DOUT – 512) • VLSB; VLSB = 1.5mV

where DOUT is a decimal integer.

For example, a 0V input will have an output reading

of 0x200. An ADC reading of 0x000 means the input

was –0.768V. The absolute ADC measurement range is

–0.768V to 5.376V. The resolution is VLSB = 1.5mV = (5.376

+ 0.768)/212. The useful range is –0.3V to 5V. This range

allows monitoring super capacitors, which could have small

negative voltage. Inputs below –0.3V exceed the absolute

maximum rating of the C pins. If all inputs are negative

then the ADC range is reduced to –0.1V. Inputs above 5V

will have noisy ADC readings (see Typical Performance

Characteristics curves).

ADC MEASUREMENTS DURING CELL BALANCING

The primary cell voltage ADC measurement commands

(STCVAD and STOWAD) automatically turn off a cell’s

discharge switch while its voltage is being measured. The

discharge switches for the cell above and the cell below will

also be turned off during the measurement. For example,

discharge switches S4, S5 and S6 will be off while cell 5

is being measured.

680313f

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