Datasheet

LTC6802-2
12
68022fa
Figure 2. Open Connection
4. Issue a RDCV command and store all cell measurements
into array CELLB(n).
5. For each value of n from 1 to 11:
If CELLB(n + 1) – CELLA(n + 1) ≥ +200mV,
then Cn is open, otherwise it is not open.
The 200mV threshold is chosen to provide tolerance for
errors in the measurement with the 100µA current source
connected. Even without an open connection there is al-
ways some difference between a cell measured with and
without the 100µA current source because of the IR drop
across the finite resistance of the MUX switches. On the
other hand, with capacitors larger then 0.1µF remaining
on an otherwise open C pin, the 100µA current source
may not be enough to move the open C pin 200mV with
a single STOWAD command. If the STOWAD command
is repeated several times, the large external capacitor
will discharge enough to create a 200mV change in cell
readings. To detect an open connection with larger then
0.1µF capacitance still on the pin, one must repeat step 3
a number of times before proceeding to step 4.
The algorithm above determines if the Cn pin is open
based on measurements of the n + 1 cell. For example,
in a 12-cell system, the algorithm finds opens on Pins C1
through C11 by looking at the measurements of cells B2
through B12. Therefore the algorithm can not be used to
determine if the topmost C pin is open. Fortunately, an open
wire from the battery to the top C pin usually means the V
+
pin is also floating. When this happens, the readings for
the top battery cell will always be 0V, indicating a failure.
If the top C pin is open yet V
+
is still connected, then the
best way to detect an open connection to the top C pin
is by comparing the sum of all cell measurements using
the STCVAD command to an auxiliary measurement of the
sum of all the cells, using a method similar to that shown
in Figure 15. A significantly lower result for the calculated
sum of all 12 cells suggests an open connection to the top
C pin, provided it was already determined that no other
C pin is open.
Figure 3. Open Connection with RC Filtering
operaTion
MUX
C4
C3
C2
B4
B3
C1
V
–
100µA
68022 F02
LTC6802-2
+
+
+
+
MUX
C4
C3
C
F4
C
F3
C2
C1
V
–
100µA
B4
B3
68022 F03
LTC6802-2
+
+
+
+
+
B4 measurement when C3 is open. So, the best method to
detect an open wire at input C3 is to look for an increase
in the measurement of the cell connected between inputs
C3 and C4 (cell B4).
Thus the following algorithm can be used to detect an
open connection to cell pin Cn:
1.
I
ssue a STCVAD command (ADC convert without 100µA
current sources).
2. I
ssue a RDCV command and store all cell measurements
into array CELLA(n).
3. Issue
a STOWAD command (ADC convert with 100µA
current sources).