Datasheet

LTC2997
9
2997fa
Power Up and UVLO
The basic LTC2997 application using an external NPN
transistor is shown in Figure 2.
APPLICATIONS INFORMATION
of up to 100 to an error smaller than 1°C (see Typical
Performance Characteristics). The LTC2997 continuously
measures the sensor diode at different test currents and
updates V
PTAT
every 3.5ms (typical).
Input Noise Filtering
The change in sensor voltage per °C is hundreds of mi-
crovolts, so electrical noise must be kept to a minimum.
Bypass D+ and D– with a 470pF capacitor close to the
LTC2997 to suppress external noise. Bypass capacitors
greater 1nF cause settling time errors of the different
measurement currents. See Typical Performance Char-
acteristics. Long wires connecting external sensors add
series resistance, mutual capacitance between D
+
and
D
–
, and cause leakage currents. A 10m CAT6 cable has
~500pF of mutual capacitance and adds negligible series
resistance and leakage currents. Recommended shielding
and PCB trace considerations for best noise immunity are
illustrated in Figure 3.
The V
CC
pin must exceed the undervoltage threshold of
1.9V (typical) for normal operation. For V
CC
below UVLO
the LTC2997 enters power-on reset and V
PTAT
is pulled low.
Temperature Measurements
Before each conversion a voltage comparator connected
to D
+
automatically sets the LTC2997 into external or in-
ternal mode. Tying D
+
to V
CC
enables internal mode and
V
PTAT
represents the die temperature. The V
PTAT
gain, K
T
,
is 4mV/K. The temperature in Kelvin is easily calculated:
T
KELVIN
=
V
PTAT
K
T
For V
D
+
more than 300mV below V
CC
(typical) the LTC2997
assumes that an external sensor is connected and will start
sending sensing currents to the remote sensor diode. The
anode of the external sensor must be connected to pin D
+
.
The cathode should be connected to D
–
for best external
noise immunity. For single wire measurements the sen-
sor cathode is connected to remote GND and D
–
must be
connected to local GND (see Figure 7). Small ground DC
voltages (<±200mV) between the two cathode potentials
do not impact the measurement accuracy. AC voltages at
odd multiples of 6kHz (±20%) cause temperature measure-
ment errors (see Typical Performance Characteristics).
The LTC2997 is calibrated to yield a V
PTAT
gain of 4mV/K
for a remote diode with an ideality factor of 1.004. A
built-in algorithm cancels errors due to series resistance
Figure 2. Basic Application Circuit
Output Noise Filtering
The V
PTAT
output typically exhibits 1mV RMS (0.25°C RMS)
noise. For applications which require lower noise digital
or analog averaging can be applied to the output. Choose
the averaging time according to the following equation:
t
AVG
=
0.01
5
°C/ Hz
T
NOISE
⎛
⎝
⎜
⎞
⎠
⎟
2
[]
where t
AVG
is the averaging time and T
NOISE
the desired
temperature noise in °C RMS. For example, if the desired
noise performance is 0.015°C RMS, set the averaging time
to one second. See Typical Performance Characteristics.
D
+
D
–
LTC2997
2997 F03
GND
470pF
GND SHIELD TRACE
NPN SENSOR
Figure 3. Recommended PCB Layout
V
REF
2.5V TO 5.5V
D
+
D
–
LTC2997
MMBT3904
470pF
2997 F02
V
CC
V
PTAT
GND
4mV/K
1.8V
0.1µF