Datasheet
LTC6803-2/LTC6803-4
32
680324fa
APPLICATIONS INFORMATION
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 in this case. Probe loads up to about 1mA maximum
are supported in this configuration. Since V
REF
is shut
down during the LTC6803 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 (12µA).
Figure 13. Internal Protection Diodes
Expanding Probe Count
As shown Figure 16, a dual 4:1 multiplexer is used to ex-
pand the general purpose V
TEMP1
and V
TEMP2
ADC inputs
to accept 8 different probe signals. The channel is selected
by setting the general purpose digital outputs GPIO1 and
GPIO2 and the resultant signals are buffered by sections
of the LT6004 micropower dual operational amplifier. The
probe excitation circuitry will vary with probe type and is
not shown here.
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 17 shows the basic concept. In any of the sensor
configurations shown, a full-scale cold readout would be
an indication of a failed-open sensor connection to the
LTC6803.
Figure 14. Driving Thermistors Directly from V
REF
Figure 15. Buffering V
REF
for Higher Current Sensors
3
S12
2
C12
1
V
+
4
C11
5
S11
6
C10
Z
CLAMP
LTC6803-4
Z
CLAMP
Z
CLAMP
Z
CLAMP
Z
CLAMP
Z
CLAMP
7
S10
8
C9
9
S9
10
C8
11
S8
12
C7
13
S7
14
C6
15
S6
16
C5
17
S5
18
C4
19
S4
20
C3
21
S3
22
C2
23
S2
24
C1
25
S1
26
27
C0
NOTE: NOT SHOWN ARE PN DIODES TO ALL OTHER PINS FROM PIN 27
V
–
44
CSBI
43
SDO
42
SDI
41
SCKI
40
A3
39
A2
38
A1
37
A0
32
V
REG
V
REF
V
TEMP2
V
TEMP1
31
30
29
36
GPIO2
35
GPIO1
34
WDTB
33
TOS
680324 F13
V
REG
V
REF
V
TEMP2
V
TEMP1
NC
V
–
LTC6803-4
1µF
680324 F14
1µF
100k
NTC
100k 100k
100k
NTC
V
REG
V
REF
V
TEMP2
V
TEMP1
NC
V
–
LTC6803-4
680324 F15
10k
NTC
10k 10k
10k
NTC
–
+
LT6000