adt7476a Analog Devices, Inc., adt7476a Datasheet - Page 19

• Manufacturer Part Number: adt7476a
• Description: Dbcool Remote Thermal Controller And Voltage Monitor
• Manufacturer: Analog Devices, Inc.

Table 10. Twos Complement Temperature Data Format
Temperature
–128°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 11. Extended Range, Temperature Data Format
Temperature
–64°C
–1°C
0°C
+1°C
+10°C
+25°C
+50°C
+75°C
+100°C
+125°C
+191°C
1
Remote Temperature Measurement
The ADT7476A can measure the temperature of two remote
diode sensors or diode-connected transistors connected to
Pin 17 and Pin 18, or Pin 15 and Pin 16.
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. As a result, this
technique is unsuitable for mass production. The technique
used in the ADT7476A is to measure the change in V
the device is operated at two different currents.
Bold numbers denote 2 LSB of measurement in Extended Resolution
Register 2 (0x77) with 0.25°C resolution.
Bold numbers denote 2 LSB of measurement in Extended Resolution
Register 2 (0x77) with 0.25°C resolution.
BE
varies from device to device, and individual
1000 0000 00 (diode fault)
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
0000 0000 00 (diode fault)
0011 1111 00
0100 0000 00
0100 0001 00
0100 1010 00
0101 1001 00
0111 0010 00
1000 1001 00
1010 0100 00
1011 1101 00
1111 1111 00
Digital Output (10-Bit)
Digital Output (10-Bit)
1
1
BE
when
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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 25 shows the input signal conditioning used to measure
the output of a remote temperature sensor. This figure shows
the external sensor as a substrate transistor, which is provided
on some microprocessors for temperature monitoring. It could
also be a discrete transistor such as a 2N3904/2N3906.
If a discrete transistor is used, the collector is not grounded and
is linked to the base. If a PNP transistor is used, the base is
connected to the D– input and the emitter to the D+ input. If an
NPN transistor is used, the emitter is connected to the D– input
and the base to the D+ input. Figure 26 and Figure 27 show how
to connect the ADT7476A to an NPN or PNP transistor for
temperature measurement. To prevent ground noise from
interfering with the measurement, the more negative terminal
of the sensor is not referenced to ground, but is biased above
ground by an internal diode at the D– input.
TRANSISTOR
Figure 25. Signal Conditioning for Remote Diode Temperature Sensors
SENSING
REMOTE
CPU
ΔV
Figure 26. Measuring Temperature Using an NPN Transistor
Figure 27. Measuring Temperature Using a PNP Transistor
BE
= KT/q × 1n(N)
THERMDA
THERMDC
2N3904
2N3906
NPN
PNP
D+
D–
I
DIODE
BIAS
N
×
I
I
BIAS
LOW-PASS FILTER
f
V
C
DD
= 65kHz
ADT7476A
ADT7476A
D+
D–
D+
D–
ADT7476A
V
V
OUT+
OUT–
TO ADC