Datasheet

LTC2990
9
2990fc
APPLICATIONS INFORMATION
Figure 2. Recommended PCB Layout
V1
V2
V3
V4
V
CC
ADR1
ADR0
SCL
SDA
LTC2990
2990 F02
GND SHIELD
TRACE
NPN SENSOR
470pF
0.1μF
GND
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:
V
BE
=η•
k•T
q
•ln
I
C
I
S
⎛
⎝
⎜
⎞
⎠
⎟
(1)
can be solved for T, where T is Kelvin degrees, I
S
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 =
V
BE
•q
η•k •In
I
C
I
S
⎛
⎝
⎜
⎞
⎠
⎟
(2)
The LTC2990 makes differential measurements of diode
voltage to calculate temperature. Proprietary techniques
allow for cancellation of error due to series resistance.
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 suffi cient precision using 16-bit
fi xed-point math in a microprocessor or microcontroller.
Factory Ideality Calibration Value:
η
CAL
= 1.004
Actual Sensor Ideality Value:
η
ACT
Compensated Kelvin Temperature:
T
K_COMP
=
η
CAL
η
ACT
•T
K_MEAS
(3)
Compensated Celsius Temperature
T
C_COMP
=
η
CAL
η
ACT
• T
C_MEAS
+273
()
⎡
⎣
⎢
⎤
⎦
⎥
– 273
(4)
A 16-bit unsigned number is capable of representing the
ratio η
CAL
/η
ACT
in a range of 0.00003 to 1.99997, by
multiplying the fractional ratio by 2
15
. 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
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 signifi cant 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
Infi neon MMBT3904 SOT-23
Rohm UMT3904 SC-70