LTC6803-2/LTC6803-4
13
680324fa
OPERATION
THEORY OF OPERATION
The LTC6803 is a data acquisition IC capable of mea-
suring the voltage of 12 series connected battery cells.
An input multiplexer connects the batteries to a 12-bit
delta-sigma analog-to-digital converter (ADC). An internal
8ppm/°C voltage reference combined with the ADC give
the LTC6803 its outstanding measurement accuracy. The
inherent benefits of the delta-sigma ADC versus other types
of ADCs (e.g., successive approximation) are explained
in Advantages of Delta-Sigma ADCs in the Applications
Information section.
Communication between the LTC6803 and a host processor
is handled by a SPI compatible serial interface. Multiple
LTC6803s can be connected to a single serial interface.
As shown in Figure 1, the LTC6803-2s or LTC6803-4s
are isolated from one another using digital isolators. A
unique addressing scheme allows all the LTC6803-2s or
LTC6803-4s to connect to the same serial port of the host
processor. Further explanation of the LTC6803-2/LTC6803-
4 can be found in the Serial Port section of the data sheet.
The LTC6803 also contains circuitry to balance cell voltages.
Internal MOSFETs can be used to discharge cells. These
internal MOSFETs can also be used to control external
balancing circuits. Figure 1 illustrates cell balancing by
internal discharge. Figure 3 shows the S pin controlling
an external balancing circuit. It is important to note that
the LTC6803 makes no decisions about turning on/off
the internal MOSFETs. This is completely controlled by
the host processor. The host processor writes values to
a configuration register inside the LTC6803 to control the
switches. The watchdog timer on the LTC6803 can be used
to turn off the discharge switches if communication with
the host processor is interrupted.
Since the LTC6803-4 separates C0 and V
–
, C0 can have
higher potential than V
–
. This feature is very useful for
super capacitors and fuel cells whose voltages can go to
zero or slightly negative. In such a case, the stacked cells
can’t power the LTC6803-4. In Figure 1, an isolated 36V
and –3.6V provides power to each LTC6803-4. This allows
the C1 to C12 pins to go up to 3.6V below C0.
The LTC6803 has three modes of operation: hardware
shutdown, standby and measure. Hardware shutdown is
a true zero power mode. Standby mode is a power saving
state where all circuits except the serial interface are turned
off. In measure mode, the LTC6803 is used to measure
cell voltages and store the results in memory. Measure
mode will also monitor each cell voltage for overvoltage
(OV) and undervoltage (UV) conditions.
HARDWARE SHUTDOWN MODE
The V
+
pin can be disconnected from the C pins and the
battery pack. If the V
+
supply pin is 0V, the LTC6803 will
typically draw less than 1nA from the battery cells. All
circuits inside the IC are off. It is not possible to com-
municate with the IC when V
+
= 0V. See the Applications
Information section for hardware shutdown circuits.
STANDBY MODE
The LTC6803 defaults (powers up) to standby mode.
Standby mode is the lowest supply current state with a
supply connected. Standby current is typically 12µA 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 measure-
ments 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.
