Datasheet

LTC2991
11
2991fd
For more information www.linear.com/LTC2991
The basic LTC2991 application circuit is shown in Figure 1.
applicaTions inForMaTion
Figure 1.
Power Up
The V
CC
pin must exceed the undervoltage (UV) thresh-
old of 2.5V to keep the LTC2991 out of power-on reset.
Power-on reset will clear all of the data registers and the
control registers.
T
emperature Measurements
The LTC2991 can measure internal temperature and up
to four external diode or transistor sensors. During tem
-
perature conversion, current is sourced through the V1,
V3, V5 or the V7 pin to forward bias the remote sensing
diode. The change in sensor voltage per degree temperature
change is hundreds of µV/°C, so environmental noise must
be kept to a minimum. Recommended shielding and PCB
trace considerations are illustrated in Figure 2.
The diode equation:
V
kT
q
I
I
BE
C
S
=






η•
•
•ln
(1)
can be solved for T, where T is Kelvin degrees, I
S
is a
process dependent factor on the order of 1E-13, η is the
diode ideality factor, k is Boltzmann’s constant and q is
the electron charge.
T
Vq
kIn
I
I
BE
C
S
=






•
••η
(2)
The LTC2991 makes differential measurements of diode
voltage to calculate temperature. Proprietary techniques
allow for cancellation of error due to series resistance.
Ideality Factor Scaling
The LTC2991 is calibrated to yield the correct temperature
for a remote diode with an ideality factor of 1.004. While
this value is typical of target sensors, small deviations can
yield significant temperature errors. The ideality factor of
the diode sensor can be considered a temperature scaling
factor. The temperature error for a 1% accurate ideality fac
-
tor error is 1% of the Kelvin temperature. Thus, at 25°C, or
298°K, a +1% accurate ideality factor error yields a +2.98
degree error
. At 85°C, or 358°K, a +1% error yields a 3.6
degree error
. It is possible to scale the measured Kelvin
or Celsius temperature measured using the LTC2991 with
a sensor ideality factor other than 1.004, to the correct
value. The scaling Equations (3) and (4), are simple, and
can be implemented with sufficient precision using 16-bit
fixed point math in a microprocessor or microcontroller.
Factory ideality calibration value:
η
CAL
= 1.004
Actual sensor ideality value:
η
ACT
Figure 2. Recommended PCB Layout
V
CC
2-WIRE
I
2
C INTERFACE
V1
LTC2991
T
AMBIENT
R
SENSE
3.3V
5V
1.8V
2.5V
GND
SDA
SCL
ADR0
ADR1
ADR2
3.3V I/O
2.5V I/O
1.8V CORE
FPGA
FPGA
TEMPERATURE
BOARD
TEMPERATURE
V3 V4
V5
V6
V7
V8
PWM TO FAN
V2
2991 F01
V1
V2
V3
V4
V5
V6
V7
V8
V
CC
ADR2
ADR1
ADR0
PWM
SCL
SDA
GND
LTC2991
2991 F01
GND SHIELD
TRACE
NPN SENSOR
470pF
0.1µF