Analog Devices dBCool ADT7476, Руководство пользователя

Страница: 22 / 72

ADT7476
Rev. B | Page 22 of 72
FACTORS AFFECTING DIODE ACCURACY
Remote Sensing Diode
The ADT7476 is designed to work with either substrate
transistors built into processors or with discrete transistors.
Substrate transistors are generally PNP types with the collector
connected to the substrate. Discrete types can be either PNP or
NPN transistors connected as a diode (base-shorted to the
collector). If an NPN transistor is used, the collector and base
are connected to D+ and the emitter to D−. If a PNP transistor
is used, the collector and base are connected to D− and the
emitter is connected to D+.
To reduce error due to variations in both substrate and discrete
transistors, a number of factors should be taken into consideration:
•
The ideality factor, n
f
, of the transistor is a measure of the
deviation of the thermal diode from ideal behavior. The
ADT7476 is trimmed for an n
f
value of 1.008. Use the
following equation to calculate the error introduced at a
temperature T (°C) when using a transistor whose n
f
does
not equal 1.008. See the processor data sheet for the n
f
values.
Δ
T = ( n
f
− 1.008) × (273.15 K + T )
•
To factor this in, the user can write the ΔT value to the
offset register. The ADT7476 then automatically adds it to
or subtracts it from the temperature measurement.
•
Some CPU manufacturers specify the high and low current
levels of the substrate transistors. The high current level of
the ADT7476, I
HIGH
, is 180 μA and the low level current,
I
LOW
, is 11 μA. If the ADT7476 current levels do not match
the current levels specified by the CPU manufacturer, it
might be necessary to remove an offset. The data sheet for
the CPU advises whether this offset needs to be removed
and how to calculate it. This offset can be programmed to
the offset register. It is important to note that, if more than
one offset must be considered, the algebraic sum of these
offsets must be programmed to the offset register.
If a discrete transistor is used with the ADT7476, the best
accuracy is obtained by choosing devices according to the
following criteria:
•
Base-emitter voltage greater than 0.25 V at 11 μA, at the
highest operating temperature.
•
Base-emitter voltage less than 0.95 V at 180 μA, at the
lowest operating temperature.
•
Base resistance less than 100 Ω.
•
Small variation in h
FE
(approximately 50 to 150) that
indicates tight control of V
BE
characteristics.
Transistors, such as 2N3904, 2N3906, or equivalents in SOT-23
packages, are suitable devices to use.
Table 10. Twos Complement Temperature Data Format
Temperature Digital Output (10-Bit)
1
–128°C 1000 0000 00 (diode fault)
–50°C 1100 1110 00
–25°C 1110 0111 00
–10°C 1111 0110 00
0°C 0000 0000 00
10.25°C 0000 1010 01
25.5°C
0001 1001 10
50.75°C 0011 0010 11
75°C
0100 1011 00
100°C 0110 0100 00
125°C
0111 1101 00
127°C 0111 1111 00
1 Bold numbers denote 2 LSBs of measurement in Extended Resolution
Register 2 (0x77) with 0.25°C resolution.
Table 11. Extended Range Temperature Data Format
Temperature Digital Output (10-Bit) 1
–64°C
0000 0000 00 (diode fault)
–1°C 0011 1111 00
0°C
0100 0000 00
1°C 0100 0001 00
10°C
0100 1010 00
25°C 0101 1001 00
50°C
0111 0010 00
75°C 1000 1001 00
100°C
1010 0100 00
125°C 1011 1101 00
191°C
1111 1111 00
1
Bold numbers denote 2 LSBs of measurement in Extended Resolution
Register 2 (0x77) with 0.25°C resolution.
Nulling Out Temperature Errors
As CPUs run faster, it is more difficult to avoid high frequency
clocks when routing the D+/D– traces around a system board.
Even when recommended layout guidelines are followed, some
temperature errors can still be attributed to noise coupled onto
the D+/D– lines. Constant high frequency noise usually atten-
uates, or increases, temperature measurements by a linear,
constant value.
The ADT7476 has temperature offset registers at Register 0x70
and Register 0x72 for the Remote 1 and Remote 2 temperature
channels. By doing a one-time calibration of the system, the
user can determine the offset caused by system board noise and
null it out using the offset registers. The offset registers auto-
matically add a twos complement 8-bit reading to every temper-
ature measurement.
www.BDTIC.com/ADI