LTC2990I(RevD) データシートの表示（PDF） - Linear Technology
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LTC2990I Datasheet PDF : 24 Pages
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LTC2990
Applications Information
The change in sensor voltage per degree temperature
change is 275µV/°C, so environmental noise must be kept
to a minimum. Recommended shielding and PCB trace
considerations are illustrated in Figure 2.
The diode equation:
VBE
=
η
•
k
•
q
T
•
ln
IC
IS
(1)
can be solved for T, where T is Kelvin degrees, IS is a
process dependent factor on the order of 1E-13, η is the
diode ideality factor, k is Boltzmann’s constant and q is
the electron charge.
T
=
η
VBE •q
•k •InIICS
(2)
The LTC2990 makes differential measurements of diode
voltage to calculate temperature. Proprietary techniques
allow for cancellation of error due to series resistance.
GND SHIELD
TRACE
470pF
NPN SENSOR
0.1µF
LTC2990
V1
VCC
V2
ADR1
V3
ADR0
V4
SCL
GND SDA
2990 F02
Figure 2. Recommended PCB Layout
Ideality Factor Scaling
The LTC2990 is factory calibrated for an ideality factor of
1.004, which is typical of the popular MMBT3904 NPN
transistor. The semiconductor purity and wafer-level pro-
cessing limits device-to-device variation, making these
devices interchangeable (typically <0.5°C) for no additional
cost. Several manufacturers supply suitable transistors,
some recommended sources are listed in Table 1. Discrete
2-terminal diodes are not recommended as temperature
sensors. While an ideality factor value of 1.004 is typical of
target sensors, small deviations can yield significant tem-
perature errors. Contact LTC Marketing for parts trimmed
to ideality factors other than 1.004. The ideality factor of
Table 1. Recommended Transistors to Be Used as Temperature
Sensors
MANUFACTURER
PART NUMBER
PACKAGE
Fairchild Semiconductor
MMBT3904
FMMT3904
SOT-23
SOT-23
Central Semiconductor
CMPT3904
CET3904E
SOT-23
SOT-883L
Diodes, Inc.
MMBT3904
SOT-23
On Semiconductor
MMBT3904LT1
SOT-23
NXP
MMBT3904
SOT-23
Infineon
MMBT3904
SOT-23
Rohm
UMT3904
SC-70
the diode sensor can be considered a temperature scaling
factor. The temperature error for a 1% accurate ideality
factor error is 1% of the Kelvin temperature. Thus, at 25°C,
or 298K, a +1% accurate ideality factor error yields a +2.98
degree error. At 85°C or 358K, a +1% error yields a 3.6
degree error. It is possible to scale the measured Kelvin
or Celsius temperature measured using the LTC2990 with
a sensor ideality factor other than 1.004, to the correct
value. The scaling Equations (3) and (4) are simple, and
can be implemented with sufficient precision using 16-bit
fixed-point math in a microprocessor or microcontroller.
Factory Ideality Calibration Value:
ηCAL = 1.004
Actual Sensor Ideality Value:
ηACT
Compensated Kelvin Temperature:
TK _ COMP
=
ηCAL
ηACT
• TK _ MEAS
(3)
Compensated Celsius Temperature
( ) TC _ COMP
=
ηCAL
ηACT
•
TC _MEAS + 273
– 273
(4)
A 16-bit unsigned number is capable of representing the
ratio ηCAL/ηACT in a range of 0.00003 to 1.99997, by
multiplying the fractional ratio by 215. The range of scal-
ing encompasses every conceivable target sensor value.
The ideality factor scaling granularity yields a worst-case
temperature error of 0.01° at 125°C. Multiplying this 16‑bit
For more information www.linear.com/LTC2990
2990fd
9
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