Datasheet

LTC2997
10
2997fa
Choosing a Sensor
The LTC2997 is factory calibrated for an ideality factor of
1.004, which is typical of the popular MMBT3904 NPN
transistor. Semiconductor purity and wafer-level processing
intrinsically limit device-to-device variation, making these
devices interchangeable between most manufacturers with
a temperature error of typically less than 0.5°C. Some
recommended sources are listed in Table 1:
Table 1. Recommended Transistors for Use as Temperature
Sensors.
MANUFACTURER PART NUMBER PACKAGE
Fairchild
Semiconductor
MMBT3904 SOT-23
Central Semiconductor CMPT3904 SOT-23
Diodes, Inc. MMBT3904 SOT-23
On Semiconductor MMBT3904LT1 SOT-23
NXP MMBT3904 SOT-23
Infineon MMBT3904 SOT-23
Rohm UMT3904 SC-70
Discrete two terminal diodes usually have ideality factors
significantly higher than 1.004 and are therefor not recom-
mended as remote sensing devices.
Protection
The LTC2997 can withstand up to ±4kV of electrostatic
discharge (ESD, human body). ESD beyond this voltage
can damage or degrade the device including lowering the
remote sensor measurement accuracy due to increased
leakage currents on D
+
and D
–
.
To protect the sensing inputs against larger ESD strikes,
external protection can be added using TVS diodes to
ground (Figure 4). Care must be taken to choose diodes
with low capacitance and low leakage currents in order
not to degrade the external sensor measurement accuracy
(see Typical Performance Characteristics).
APPLICATIONS INFORMATION
Ideality Factor Scaling
While an ideality factor value of 1.004 is typical of many
sensor devices, small deviations can yield significant tem-
perature errors. The ideality factor acts as a temperature
scaling factor. The temperature error for a 1% deviation
is 1% of the Kelvin temperature. Thus, at 25°C (298K) a
+1% accurate ideality factor error yields a +2.98 degree
error. At 85°C (358K) a +1% error yields a 3.58 degree
error. It is possible to scale the PTAT voltage if an external
sensor with an ideality factor other than 1.004 is used.
The scaling equation for the compensated PTAT voltage
is listed below.
LTC2997 Ideality Calibration Value:
η
CAL
= 1.004
Actual Remote Sensor Ideality Value:
η
ACT
Compensated PTAT Voltage:
V
PTAT _COMP
=
η
CAL
η
ACT
•V
PTAT _MEAS
Compensated Kelvin Temperature:
T
KELVIN_COMP
=
η
CAL
η
ACT
•T
KELVIN_MEAS
Compensated Celsius Temperature:
T
CELSIUS_COMP
=
η
CAL
η
ACT
•(T
KELVIN_MEAS
) – 273.15
Figure 4. Increasing ESD Robustness with TVS Diodes
D
+
D
–
LTC2997
MMBT3904
PESD5Z6.0
220pF
10
10
2997 F04
GND