Datasheet

LTC6802-2
27
68022fa
V
REG
V
REF
V
TEMP2
V
TEMP1
NC
V
−
LTC6802-2
10k 10k
10k
NTC
10k
NTC
–
+
LT6000
68022 F12
applicaTions inForMaTion
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.
R
EADING
EXTERNAL TEMPERATURE PROBES
Using Dedicated Inputs
The LTC6802-2 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 11 (up to
60µA total).
For sensors that require higher drive currents, a buffer op
amp may be used as shown in Figure 12. Power for the
sensor is actually sourced indirectly from the V
REG
pin
in this case. Probe loads up to about 1mA maximum are
supported in this configuration. Since V
REF
is shutdown
during the LTC6802-2 idle and shutdown modes, the
thermistor drive is also shut off and thus power dissipa-
tion minimized. Since V
REG
remains always on, the buffer
op amp (LT6000 shown) is selected for its ultralow power
consumption (10µA).
Expanding Probe Count
The LTC6802-2 provides general purpose I/O pins, GPIO1
and GPIO2, that may be used to control multiplexing of
several temperature probes. Using just one of the GPIO
pins, the sensor count can double to four as shown in
Figure 13. Using both GPIO pins, up to eight sensor inputs
can be supported.
Figure 12. Buffering V
REF
for Higher Current Sensors
Figure 11. Driving Thermistors Directly from V
REF
100k 100k
100k
NTC
100k
NTC
1µF
1µF
V
REG
V
REF
V
TEMP2
V
TEMP1
NC
V
−
LTC6802-2
68022 F11
Using Diodes to Monitor Temperatures
in Multiple Locations
Another method of multiple sensor support is possible
without the use of any GPIO pins. If the sensors are PN
diodes and several used in parallel, then the hottest diode
will produce the lowest forward voltage and effectively
establish the input signal to the V
TEMP
input(s). The hottest
diode will therefore dominate the readout from the V
TEMP
inputs that the diodes are connected to. In this scenario,
the specific location or distribution of heat is not known,
but such information may not be important in practice.
Figure 14 shows the basic concept.
In any of the sensor configurations shown, a full-scale
cold readout would be an indication of a failed-open sen-
sor connection to the LTC6802-2.
Figure 13. Expanding Sensor Count with Multiplexing
LTC6802-2
100k
NTC
100k
NTC
100k
NTC
100k
NTC
1µF
100k
100k
100k
SN74LVC1G3157
OR SIMILAR DEVICE
GPIO1
V
REG
V
REF
V
TEMP2
V
TEMP1
NC
V
−
68022 F13