ADT7473 Analog Devices, Inc., ADT7473 Datasheet - Page 16

• Manufacturer Part Number: ADT7473
• Description: Dbcool Remote Thermal Monitor And Fan Controller
• Manufacturer: Analog Devices, Inc.

ADT7473
Local Temperature Measurement
The ADT7473 contains an on-chip band gap temperature
sensor whose output is digitized by the on-chip 10-bit ADC.
The 8-bit MSB temperature data is stored in the local tempera-
ture register (0x26). Because both positive and negative
temperatures can e measured, the temperature data is stored
Offset 64 format or twos complement format, as shown in
Table 8 and Table 9. Theoretically, the temperature sensor and
ADC can measure temperatures from −63°C to +127°C (or
−63°C to +191°C in the extended temperature range) with a
resolution of +0.25°C. However, this exceeds the operating
temperature range of the device, so local temperature
measurements outside the ADT7473 o
range are not possible.
Table 8. Twos Complement Temperature Data Format
Temperature
–128°C
–63°C
–50°C
–25°C
–10°C
0°C
10.25°C
25.5°C
50.75°C
75°C
100°C
125°C
127°C
1
Table 9. Extended Range, Temperature Data Format
Temperature
–64°C
–63°C
–1°C
0°C
1°C
10°C
25°C
50°C
75°C
100°C
125°C
191°C
1
Bold numbers denote 2 LSBs of measurement in Extended Resolution
Register 2 (0x77) with 0.25°C resolution.
Bold numbers denote 2 LSBs of measurement in Extended Resolution
Register 2 (0x77) with 0.25°C resolution.
b
Digital Output (10-Bit)
1000 0000 00 (diode fault)
1100 0001 00
1100 1110 00
1110 0111 00
1111 0110 00
0000 0000 00
0000 1010 01
0001 1001 10
0011 0010 11
0100 1011 00
0110 0100 00
0111 1101 00
0111 1111 00
Digital Output (10-Bit)
0000 0000 00 (diode fault)
0000 0001 00
0011 1111 00
0100 0000
0100 0001 00
0100 1010 00
0101 1001 00
0111 0010 00
1000 1001 00
1010 0100 00
1011 1101 00
1111 1111 00
00
perating temperature
1
1
Rev. A | Page 16 of 76
in
Remote Temperature Measurement
The ADT7473 can measure the temperature of two remote
diode sensors or diode-connected transistors connected to
Pin 10 and Pin 11, or Pin 12 and Pin 13.
The forward voltage of a diode or diode-connected transistor
operated at a constant current exhibits a negative temperature
coefficient of about −2 mV/°C. Unfortunately, the absolute
value of V
calibration is required to null this out, so the technique is
unsuitable for mass production. The technique used in the
ADT7473 is to measure the change in V
operated at three different currents. This is given by
where:
K is Boltzmann’s constant.
q is the charge on the carrier.
T is the absolute temperature in Kelvin.
N is the ratio of the two currents.
Figure 21 shows the input signal conditioning used to measure
the output of a remote temperature s
the external sensor as a substrate transistor, provided for
temperature monitoring on some microprocessors. It could als
be a discrete transistor such as a 2N3904/2N3906.
TRANS
If a
shou
ba
to t e D+ input. If an NPN transistor is used, the emitter is
conn
inpu
ADT
mea
the
not
inte
se
SE
RE
Figure 21. S
h
discrete transistor is used, the collector is not grounded and
measurement, the more negative terminal of the sensor is
rnal diode at the D– input.
referenced to ground, but is biased above ground by an
NSING
surement. To prevent ground noise from interfering with
ISTOR
t. Figure 22 and Figure 23 show how to connect the
MOTE
ld be linked to the base. If a PNP transistor is used, the
is connected to the D– input and the emitter is conn
ected to the D– input and the base is connected to the D+
∆
7473 to an NPN or PNP transistor for temperature
V
BE
D+
D–
BE
=
varies from device to device and individual
ignal Conditioning for Remote Diode Temperatur
I
KT
N2
/ ×
×
q
I
N1
1
n
×
(
I
N
)
I
BIAS
f
V
C
DD
= 65kHz
LPF
ensor. This figure shows
BE
when the devic
V
V
e Sensors
OUT+
OUT–
ected
e is
TO ADC
o