Datasheet
LTC6802-1
11
68021fa
TIMING DIAGRAM
D3D3 D2 D1 D0 D7
…
D4
D3D3D4 D2 D1 D0 D7
…
D4
t
6
t
7
t
3
t
5
t
4
t
1
t
8
t
2
PREVIOUS COMMAND CURRENT COMMAND
SCKI
SDI
CSBI
SDO
68021 TD
OPERATION
THEORY OF OPERATION
The LTC6802-1 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
10ppm voltage reference combined with the ADC give the
LTC6802-1 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 LTC6802-1 and a host pro-
cessor is handled by a SPI compatible serial interface. As
shown in Figure 1, the LTC6802-1’s can pass data up and
down a stack of devices using simple diodes for isolation.
This operation is described in Serial Port in the Applica-
tions Information section.
The LTC6802-1 also contains circuitry to balance cell volt-
ages. 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 4 shows the S pin controlling
an external balancing circuit. It is important to note that
the LTC6802-1 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 LTC6802-1 to control
the switches. The watchdog timer on the LTC6802-1 will
turn off the discharge switches if communication with the
host processor is interrupted.
OPEN CONNECTION DETECTION
When a cell input (C pin) is open, it affects two cell mea-
surements. Figure 2 shows an open connection to C3,
in an application without external filtering between the C
pins and the cells. During normal ADC conversions (that
is, using the STCVAD command), the LTC6802 will give
near zero readings for B3 and B4 when C3 is open. The
zero reading for B3 occurs because during the measure-
ment of B3, the ADC input resistance will pull C3 to the
C2 potential. Similarly, during the measurement of B4, the
ADC input resistance pulls C3 to the C4 potential.
Timing Diagram of the Serial Interface