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N
meas
N
ref
–R
× C × ln
meas
V
ref1
V
CC
–R
× C × ln
ref
V
ref1
V
CC
N
meas
N
ref
R
meas
R
ref
R
= R
×
meas
ref
N
meas
N
ref
=
=
V
C
V
or V
CC
REF0
V
REF1
Phase I:
Charge
Phase II:
Discharge
Phase III:
Charge
t
ref
Phase IV
Discharge
t
meas
t
R
meas
R
ref
Comparator_D Operation
The resources used to calculate the temperature sensed by Rmeas are:
•
Two digital I/O pins charge and discharge the capacitor.
•
I/O is set to output high (V
CC
) to charge capacitor, reset to discharge.
•
I/O is switched to high-impedance input with CDPDx set when not in use.
•
One output charges and discharges the capacitor through Rref.
•
One output discharges capacitor through Rmeas.
•
The + terminal is connected to the positive terminal of the capacitor.
•
The – terminal is connected to a reference level; for example, 0.25 × V
CC
.
•
The output filter should be used to minimize switching noise.
•
CDOUT is used to gate a timer capturing capacitor discharge time.
More than one resistive element can be measured. Additional elements are connected to CD0 with
available I/O pins and switched to high impedance when not being measured.
The thermistor measurement is based on a ratiometric conversion principle. The ratio of two capacitor
discharge times is calculated as shown in
Figure 17-7. Timing for Temperature Measurement Systems
The V
CC
voltage and the capacitor value should remain constant during the conversion but are not critical,
because they cancel in the ratio:
468
Comparator_D
SLAU272C – May 2011 – Revised November 2013
Copyright © 2011–2013, Texas Instruments Incorporated