LTC2996
applicaTions inForMaTion
Temperature Measurements
Table 2. Recommended Transistors for Use as Temperature
Sensors
Before each conversion, a voltage comparator connected
MANUFACTURER
PART NUMBER
PACKAGE
+
to D automatically sets the LTC2996 into external or
Fairchild
Semiconductor
MMBT3904
SOT-23
+
internal mode. Tying D to V enables internal mode,
CC
+
whereV
representsthedietemperature.ForV more
PTAT
D
Central
Semiconductor
CMBT3904
SOT-23
than 300mV below V (typical), the LTC2996 assumes
CC
that an external sensor is connected.
Diodes Inc.
On Semiconductor
NXP
MMBT3904
MMBT3904LT1
MMBT3904
MMBT3904
UMT3904
SOT-23
SOT-23
SOT-23
SOT-23
SC-70
The LTC2996 continuously measures the sensor diode at
differenttestcurrentsandgeneratesavoltageproportional
Infineon
to the absolute temperature of the sensor at the V
pin.
PTAT
Rohm
The voltage at V
is updated every 3.5ms.
PTAT
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. Furthermore,golddopedtransistors
(low beta), high frequency and high voltage transistors
should be avoided as remote sensing devices.
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.
V
PTAT
4mV/K
TKELVIN
=
(η = 1.004)
If an external sensor with an ideality factor different from
Connecting an External Sensor
1.004 is used, the gain of V
will be scaled by the ratio
PTAT
of the actual ideality factor (η ) to 1.004. In these cases
ACT
The anode of the external sensor must be connected to
the temperature of the external sensor can be calculated
+
–
pin D . The cathode should be connected to D for best
external noise immunity.
from V
by:
PTAT
V
1.004
η
ACT
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 LTC2996 to suppress external noise. Recommended
shielding and PCB trace considerations for best noise
immunity are illustrated in Figure 1.
PTAT
TKELVIN
=
•
4mV/K
+
–
Temperature in degrees Celsius can be deduced from
degrees Kelvin by:
T
= T
– 273.15
KELVIN
CELSIUS
GND SHIELD TRACE
Choosing an External Sensor
LTC2996
+
The LTC2996 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:
D
D
470pF
–
GND
2996 F01
NPN SENSOR
Figure 1. Recommended PCB Layout
+
Leakage currents at D affect the precision of the remote
temperature measurements. 100nA leakage current leads
to an additional error of 2°C (see Typical Performance
Characteristics).
2996f
9
Linear [ Linear ]
