LTC6803-1/LTC6803-3
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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 V
UV
and V
OV
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. Note that there is a 5µs window during
each UV/OV comparison cycle where an ADC measure-
ment request may be missed. This is an unlikely event.
For example, the comparison cycle is 2 seconds when
CDC = 7. Use the CLEAR command to detect missing
ADC commands.
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:
V
IN
= (D
OUT
– 512) • V
LSB
; V
LSB
= 1.5mV
where D
OUT
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 V
LSB
= 1.5mV = (5.376
+ 0.768)/2
12
.
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. The UV/OV comparison conversions in
