Datasheet

LTC6802-1
30
68021fa
V
REG
V
REF
V
TEMP2
V
TEMP1
NC
V
−
LTC6802-1
10k 10k
10k
NTC
10k
NTC
–
+
LT6000
68021 F15
APPLICATIONS INFORMATION
Cell-Voltage Filtering
The LTC6802-1 employs a sampling system to perform
its analog-to-digital conversions and provides a conver-
sion result that is essentially an average over the 0.5ms
conversion window, provided there isn’t noise aliasing with
respect to the delta-sigma modulator rate of 512kHz. This
indicates that a lowpass filter with useful attenuation at
500kHz may be beneficial. Since the delta-sigma integra-
tion bandwidth is about 1kHz, the filter corner need not
be lower than this to assure accurate conversions.
Series resistors of 100 may be inserted in the input
paths without introducing meaningful measurement
error, provided only external discharge switch FETs are
being used. Shunt capacitors may be added from the cell
inputs to V
–
, creating RC filtering as shown in Figure 13.
Note that this filtering is not compatible with use of the
internal discharge switches to carry current since this
would induce settling errors at the time of conversion as
any activated switches temporarily open to provide Kelvin
mode cell sensing. As a discharge switch opens, cell wiring
resistance will also form a small voltage step (recovery
of the small IR drop), so keeping the frequency cutoff of
the filter relatively high will allow adequate settling prior
to the actual conversion. A guard time of about 60µs is
provided in the ADC timing, so a 16kHz LP is optimal and
offers about 30dB of noise rejection.
No resistor should be placed in series with the V
–
pin.
Because the supply current flows from the V
–
pin, any
resistance on this pin could generate a significant conver-
sion error for CELL1.
The V
+
pin is powered from the top cell potential of the
monitored cell group. A decoupling network of 20 /100nF
is recommended.
READING EXTERNAL TEMPERATURE PROBES
Using Dedicated Inputs
The LTC6802-1 includes two channels of ADC input, V
TEMP1
and V
TEMP2
, that are intended to monitor thermistors
(tempco about –4%/°C generally) or diodes (–2.2mV/°C
typical) located within the cell array. Sensors can be
powered directly from V
REF
as shown in Figure 14 (up to
60µA total).
For sensors that require higher drive currents, a buffer op
amp may be used as shown in Figure 15. Power for the
sensor is actually sourced indirectly from the V
REG
pin
Figure 15. Buffering V
REF
for Higher-Current Sensors
Figure 14. Driving Thermistors Directly from V
REF
Figure 13. Adding RC Filtering to the Cell Inputs
(One Cell Connection Shown)
C(n)
S(n)
C(n – 1)
100
100
100nF
7.5V
100nF
68021 F13
100k 100k
100k
NTC
100k
NTC
1µF
1µF
V
REG
V
REF
V
TEMP2
V
TEMP1
NC
V
−
LTC6802-1
68021 F14