Datasheet

LTC2995
10
2995f
Temperature Measurements
The LTC2995 continuously measures the sensor diode at
different test currents and generates a voltage proportional
to the absolute temperature of the sensor at the V
PTAT
pin.
The voltage at V
PTAT
is updated every 3.5ms.
The gain of V
PTAT
is calibrated to 4mV/K for the measure-
ment of the internal diode as well as for remote diodes
with an ideality factor of 1.004.
T
KELVIN
=
V
PTAT
4mV/K
(η=1.004)
If an external sensor with an ideality factor different from
1.004 is used, the gain of V
PTAT
will be scaled by the ratio
of the actual ideality factor (η
ACT
) to 1.004. In these cases,
the temperature of the external sensor can be calculated
from V
PAT
by:
T
KELVIN
=
V
PTAT
4mV/K
•
1.004
η
ACT
Temperature in degrees Celsius can be deduced from
degrees Kelvin by:
T
CELSIUS
= T
KELVIN
– 273.15
The three-state diode select pin (DS) determines whether
the temperature of the external or the internal diode is
measured and displayed at V
PTAT
as described in Table 1.
Table 1. Diode Selection
DIODE LOCATION DS PIN
Internal V
CC
External GND
Both Open
If the DS pin is left open, the LTC2995 measures both
diodes alternately and V
PTAT
changes every 30ms from the
voltage corresponding to the temperature of the internal
sensor to the voltage corresponding to the temperature
of the external sensor. If D
+
is tied to V
CC
, the LTC2995
measures the internal diode regardless of the state of
the DS pin.
APPLICATIONS INFORMATION
Choosing an External Sensor
The LTC2995 is factory calibrated for an ideality factor of
1.004, which is typical of the popular MMBT3904 NPN
transistor. Semiconductor purity and wafer level process-
ing intrinsically limit device-to-device variation, making
these devices interchangeable between manufacturers
with a temperature error of typically less than 0.5°C. Some
recommended sources are listed in Table 2:
Table 2 Recommended Transistors for Use As Temperature
Sensors
MANUFACTURER PART NUMBER PACKAGE
Fairchild
Semiconductor
MMBT3904 SOT-23
Central
Semiconductor
CMBT3904 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 are not recommended as
remote sensing devices as their ideality factor is typically
much higher than 1.004. Also MOS transistors are not
suitable as they don’t exhibit the required current to tem-
perature relationship. Furthermore gold doped transistors
(low beta), high frequency and high voltage transistors
should be avoided as remote sensing devices.
Connecting an External Sensor
The change in sensor voltage per °C is hundreds of
microvolts, so electrical noise must be kept to a mini-
mum. Bypass D
+
and D
–
with a 470pF capacitor close to
the LTC2995 to suppress external noise. Recommended
shielding and PCB trace considerations for best noise
immunity are illustrated in Figure 1.
Figure 1. Recommended PCB Layout
D
+
D
–
LTC2995
2995 F01
GND
470pF
GND SHIELD TRACE
NPN SENSOR