LTC6804-1/LTC6804-2
63
680412fc
For more information www.linear.com/LTC6804-1
applicaTions inForMaTion
ADCV command with DCP = 0. In this table, OFF implies
that a discharge is forced off during that period even if
the corresponding DCC[x] bit is high in the configuration
register. ON implies that if the discharge is turned on, it
will stay on during that period. Refer to Figure 3 for the
timing of the ADCV command.
POWER DISSIPATION AND THERMAL SHUTDOWN
The internal MOSFETs connected to the pins S1 through
S12 pins can be used to discharge battery cells. An exter
-
nal resistor should be used to limit the power dissipated
by the MOSFETs. The maximum power dissipation in the
MOSFETs is limited by the amount of heat that can be tol
-
erated by the LTC6804. Excessive heat results in elevated
die temperatures. Little or no degradation will be observed
in the measurement accuracy for die temperatures up to
125°C. Damage may occur above 150°C, therefore the
recommended maximum die temperature is 125°C. To
protect the LTC6804 from damage due to overheating a
thermal shutdown circuit is included. Overheating of the
device can occur when dissipating significant power in the
cell discharge switches. The thermal shutdown circuit is
enabled whenever the device is not in sleep mode (see
LTC6804 Core State Descriptions). If the temperature de
-
tected on the device goes above approximately 150°C the
configuration registers will be reset to default states turn-
ing off all discharge switches. When a thermal shutdown
has occurred, the THSD bit in the status register group
B
will go
high. The bit is cleared after a read operation of
the status register group B. The bit can also be set using
the CLRSTAT command. Since thermal shutdown inter
-
rupts normal operation, the internal temperature monitor
should be used to determine when the device temperature
is approaching unacceptable levels.
M
ETHOD TO VERIFY BALANCING CIRCUITRY
The functionality of the discharge circuitry is best verified
by cell measurements. Figure 37 shows an example using
the LTC6804 battery monitor IC. The resistor between the
battery and the source of the discharge MOSFET causes
cell voltage measurements to decrease. The amount of
measurement change depends on the resistor values and
the MOSFET on resistance.
The following algorithm could be used in conjunction
with Figure 37:
1. Measure all cells with no discharging (all S outputs
off) and read and store the results.
2. Turn on S1 and S7
3. Measure C1-C0, C7-C6
4. Turn off S1 and S7
5. Turn on S2 and S8
6. Measure C2-C1, C8-C7
7. Turn off S2 and S8
…
14. Turn on S6 and S12
15. Measure C6-C5, C12-C11
16. Turn off S6 and S12
17. Read the voltage register group to get the results of
steps 2 thru 16.
18. Compare new readings with old readings. Each cell
voltage reading should have decreased by a fixed
percentage set by R
B1
and R
B2
(Figure 37). The exact
amount of decrease depends on the resistor values
and MOSFET characteristics.
Improved PEC Calculation
The PEC allows the user to have confidence that the serial
data read from the LTC6804 is valid and has not been cor
-
rupted by any external noise source. This is a critical feature
for reliable communication and the LTC6804
requires that
a PEC be calculated for all data being read from and written
to the LTC6804. For this reason it is important to have an
efficient method for calculating the PEC. The code below
demonstrates a simple implementation of a lookup table
derived PEC calculation method. There are two functions,
the first function init_PEC15_Table() should only be called
once when the microcontroller starts and will initialize a
PEC15 table array called pec15Table[]. This table will be
used in all future PEC calculations. The pec15 table can
also be hard coded into the microcontroller rather than
running the init_PEC15_Table() function at startup. The
pec15() function calculates the PEC and will return the
correct 15 bit PEC for byte arrays of any given length.
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