LTC2995
APPLICATIONS INFORMATION
Temperature Measurements
°hoosing an External Sensor
The LTC2995 continuously measures the sensor diode at
differenttestcurrentsandgeneratesavoltageproportional
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
withatemperatureerroroftypicallylessthan0.5°C.Some
recommended sources are listed in Table 2:
to the absolute temperature of the sensor at the V
pin.
PTAT
The voltage at V
is updated every 3.5ms.
PTAT
The gain of V
is calibrated to 4mV/K for the measure-
PTAT
ment of the internal diode as well as for remote diodes
with an ideality factor of 1.004.
Table ꢀ Recommended Transistors for Use As Temperature
Sensors
V
PTAT
TKELVIN
=
(η = 1.004)
MANUFA°TURER
PART NUMBER
PA°KAGE
4mV/K
Fairchild
Semiconductor
MMBT3904
SOT-23
If an external sensor with an ideality factor different from
1.004 is used, the gain of V
will be scaled by the ratio
Central
Semiconductor
CMBT3904
SOT-23
PTAT
oftheactualidealityfactor(η )to1.004. Inthesecases,
ACT
Diodes Inc.
On Semiconductor
NXP
MMBT3904
MMBT3904LT1
MMBT3904
MMBT3904
UMT3904
SOT-23
SOT-23
SOT-23
SOT-23
SC-70
the temperature of the external sensor can be calculated
from V
by:
PAT
V
1.004
PTAT
Infineon
TKELVIN
=
•
4mV/K
η
Rohm
ACT
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-
peraturerelationship. Furthermoregolddopedtransistors
(low beta), high frequency and high voltage transistors
should be avoided as remote sensing devices.
Temperature in degrees Celsius can be deduced from
degrees Kelvin by:
T
= T
– 273.15
KELVIN
CELSIUS
The three-state diode select pin (DS) determines whether
the temperature of the external or the internal diode is
measured and displayed at V
as described in Table 1.
PTAT
Table ±. Diode Selection
°onnecting an External Sensor
DꢁODE LO°ATꢁON
Internal
DS PꢁN
The change in sensor voltage per °C is hundreds of
microvolts, so electrical noise must be kept to a mini-
V
CC
External
GND
+
–
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.
Both
Open
If the DS pin is left open, the LTC2995 measures both
diodesalternatelyandV
changesevery30msfromthe
PTAT
GND SHIELD TRACE
voltage corresponding to the temperature of the internal
LTC2995
+
sensor to the voltage corresponding to the temperature
D
470pF
+
–
D
of the external sensor. If D is tied to V , the LTC2995
CC
GND
measures the internal diode regardless of the state of
2995 F01
NPN SENSOR
the DS pin.
Figure ±. Recommended P°B Layout
2995f
10
Linear Systems [ Linear Systems ]
