LTC6803-1/LTC6803-3
29
680313fa
APPLICATIONS INFORMATION
discharge resistor to the cell, and the gate terminal to the
S output pin, as illustrated in Figure 12.
Figure 12. External Discharge FET Connection (One Cell Shown)
POWER DISSIPATION AND THERMAL SHUTDOWN
The MOSFETs connected to the pins S1 through S12 can be
used to discharge battery cells. An external 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 tolerated by the LTC6803.
Excessive heat results in elevated die temperatures. The
electrical characteristics for the LTC6803 I-grade are
guaranteed for die temperatures up to 85°C. Little or no
degradation will be observed in the measurement accuracy
for die temperatures up to 105°C. Damage may occur
above 150°C, therefore the recommended maximum die
temperature is 125°C.
To protect the LTC6803 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 problem is exacerbated when
the thermal conductivity of the system is poor.
The thermal shutdown circuit is enabled whenever the
device is not in standby mode (see Modes of Operation).
It will also be enabled when any current mode input or
output is sinking or sourcing current. If the temperature
detected on the device goes above approximately 145°C,
the configuration registers will be reset to default states,
turning off all discharge switches and disabling ADC
conversions. When a thermal shutdown has occurred, the
THSD bit in the temperature register group will go high.
The bit is cleared by performing a read of the temperature
registers (RDTMP command).
Since thermal shutdown interrupts normal operation, the
internal temperature monitor should be used to determine
when the device temperature is approaching unacceptable
levels.
USING THE LTC6803 WITH LESS THAN 12 CELLS
If the LTC6803 is powered by the stacked cells, the minimum
number of cells is governed by the supply voltage require-
ments of the LTC6803. The sum of the cell voltages must be
10V to guarantee that all electrical specifications are met.
Figure 13 shows an example of the LTC6803 when used to
monitor seven cells. The lowest C inputs connect to the-
seven cells and the upper C inputs connect to C12. Other
configurations, e.g., 9 cells, would be configured in the
same way: the lowest C inputs connected to the battery
cells and the unused C inputs connected to C12. The unused
inputs will result in a reading of 0V for those channels.
The ADC can also be commanded to measure a stack of
10 or 12 cells, depending on the state of the CELL10 bit
in the control register. The ADC can also be commanded
to measure any individual cell voltage.
FAULT PROTECTION
Care should always be taken when using high energy
sources such as batteries. There are numerous ways
that systems can be misconfigured when considering
the assembly and service procedures that might affect a
battery system during its useful lifespan. Table 15 shows
the various situations that should be considered when plan-
ning protection circuitry. The first five scenarios are to be
anticipated during production and appropriate protection
is included within the LTC6803-1/LTC6803-3 device itself.
BATTERY INTERCONNECTION INTEGRITY
The FMEA scenarios involving a break in the stack of battery
cells are potentially the most damaging. In the case where
the battery stack has a discontinuity between groupings
of cells monitored by LTC6803 ICs, any load will force a
large reverse potential on the daisy-chain connection. This
+
680313 F12
C (n)
C (n – 1)
S (n)
3.3k
33Ω
1W
Si2351DS
