Datasheet

LTC2996
9
2996f
Temperature Measurements
Before each conversion, a voltage comparator connected
to D
+
automatically sets the LTC2996 into external or
internal mode. Tying D
+
to V
CC
enables internal mode,
where V
PTAT
represents the die temperature. For V
D
+
more
than 300mV below V
CC
(typical), the LTC2996 assumes
that an external sensor is connected.
The LTC2996 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
PTAT
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
Choosing an External Sensor
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
with a temperature error of typically less than 0.5°C. Some
recommended sources are listed in Table 2:
applicaTions inForMaTion
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 anode of the external sensor must be connected to
pin D
+
. The cathode should be connected to D
–
for best
external noise immunity.
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.
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).
D
+
D
–
LTC2996
2996 F01
GND
470pF
GND SHIELD TRACE
NPN SENSOR