LTC2996
operaTion
Overview
diode (with the same value I ) at two different currents
S
(I and I ) yields an expression independent of I :
D1
D2
S
The LTC2996 provides a buffered voltage proportional to
the absolute temperature of either an internal or a remote
q
η • k
V
– V
D2
D1
T =
•
diode(V
)andcomparesthisvoltagetothresholdsthat
PTAT
I
D2
can be set by external resistor dividers from the on-board
ln
I
D1
reference (V ).
REF
Remote temperature measurements usually use a diode
connected transistor as a temperature sensor, allowing
the remote sensor to be a discrete NPN (ex. MMBT3904)
or an embedded device in a microprocessor or FPGA.
Series Resistance Cancellation
Resistanceinserieswiththeremotediodecausesapositive
temperature error by increasing the measured voltage at
each test current. The composite voltage equals:
Diode Temperature Sensor
I
kT
= η • ln
q
D
Temperature measurements are conducted by measuring
the voltage of either an internal or an external diode with
multiple test currents. The relationship between diode
voltage V and diode current I can be solved for absolute
V + V
+ R •I
S D
D
ERROR
I
S
The LTC2996 removes this error term from the sensor
signal by subtracting a cancellation voltage V . A
D
D
Temperature in degrees Kelvin T:
CANCEL
resistance extraction circuit uses one additional current
measurement to determine the series resistance in the
measurementpath.Oncethecorrectvalueoftheresistoris
q
η • k
V
D
T =
•
I
D
ln
I
determined,V
equalsV
.Nowthetemperature
CANCEL
ERROR
S
to voltage converter input signal is free from errors due
to series resistance.
where I is a process dependent factor on the order of
S
–13
10 A, η is the diode ideality factor, k is the Boltzmann
LTC2996cancelsseriesresistancesuptoseveralhundred
ohms (see Typical Performance Characteristics curves).
Higher series resistances cause the cancelation voltage
to saturate.
constantandqistheelectroncharge.Thisequationshows
arelationshipbetweentemperatureandvoltagedependent
on the process depended variable I . Measuring the same
S
2996f
8
Linear [ Linear ]
