LTC2991I(RevA) Просмотр технического описания (PDF) - Linear Technology
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LTC2991I Datasheet PDF : 32 Pages
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LTC2991
APPLICATIONS INFORMATION
The basic LTC2991 application circuit is shown in Figure 1.
5V
3.3V
2.5V
RSENSE
1.8V
3.3V I/O
2.5V I/O
1.8V CORE
2-WIRE
I2C INTERFACE
TAMBIENT
VCC V1
V2 V3 V4
SDA
V5
SCL
LTC2991
ADR0
V6
ADR1
ADR2
V7
GND
V8
PWM
2991 F01
FPGA
FPGA
TEMPERATURE
BOARD
TEMPERATURE
TO FAN
Figure 1.
Power Up
The VCC 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.
Temperature 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.
GND SHIELD
TRACE
470pF
NPN SENSOR
LTC2991
V1
VCC
V2
ADR2
V3
ADR1
0.1μF
V4
ADR0
V5
PWM
V6
SCL
V7
SDA
V8
GND
2991 F01
Figure 2. Recommended PCB Layout
The diode equation:
VBE
=
η
t
k
tT
q
t
⎛
Mn ⎝⎜
IC
IS
⎞
⎠⎟
(1)
can be solved for T, where T is Kelvin degrees, IS 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 = VBE t q
(2)
η
t
k
t
In
⎛
⎝⎜
IC
IS
⎞
⎠⎟
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 www.DataSheet.net/ 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:
ηCAL = 1.004
Actual sensor ideality value:
ηACT
2991fa
11
Datasheet pdf - http://www.DataSheet4U.co.kr/
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