Datasheet

LTC2996
8
2996f
operaTion
Overview
The LTC2996 provides a buffered voltage proportional to
the absolute temperature of either an internal or a remote
diode (V
PTAT
) and compares this voltage to thresholds that
can be set by external resistor dividers from the on-board
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.
Diode Temperature Sensor
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
D
and diode current I
D
can be solved for absolute
Temperature in degrees Kelvin T:






T =
q
η • k
•
V
D
ln
I
D
I
S
where I
S
is a process dependent factor on the order of
10
–13
A, η is the diode ideality factor, k is the Boltzmann
constant and q is the electron charge. This equation shows
a relationship between temperature and voltage dependent
on the process depended variable I
S
. Measuring the same
diode (with the same value I
S
) at two different currents
(I
D1
and I
D2
) yields an expression independent of I
S
:
ln
I
D2
I
D1






T =
q
η • k
•
V
D2
– V
D1
Series Resistance Cancellation
Resistance in series with the remote diode causes a positive
temperature error by increasing the measured voltage at
each test current. The composite voltage equals:






V
D
+ V
ERROR
= η
kT
q
• ln
I
D
I
S
+ R
S
• I
D
The LTC2996 removes this error term from the sensor
signal by subtracting a cancellation voltage V
CANCEL
. A
resistance extraction circuit uses one additional current
measurement to determine the series resistance in the
measurement path. Once the correct value of the resistor is
determined, V
CANCEL
equals V
ERROR
. Now the temperature
to voltage converter input signal is free from errors due
to series resistance.
LTC2996 cancels series resistances up to several hundred
ohms (see Typical Performance Characteristics curves).
Higher series resistances cause the cancelation voltage
to saturate.