ADT7475 Analog Devices, Inc., ADT7475 Datasheet - Page 17

• Manufacturer Part Number: ADT7475
• Description: Dbcool Remote Thermal Monitor And Fan Controller
• Manufacturer: Analog Devices, Inc.

FACTORS AFFECTING DIODE ACCURACY
Remote Sensing Diode
The ADT7475 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 the error due to variations in both substrate and
discrete transistors, a number of factors should be taken into
consideration:
•
•
If a discrete transistor is used with the ADT7475, the best
accuracy is obtained by choosing devices according to the
following criteria:
•
•
•
•
Transistors, such as 2N3904, 2N3906, or equivalents in SOT-23
packages, are suitable devices to use.
The ideality factor, n
deviation of the thermal diode from ideal behavior. The
ADT7475 is trimmed for an n
following equation to calculate the error introduced at a
temperature, T (°C), when using a transistor whose n
does not equal 1.008. See the processor data sheet for the
n
To factor this in, the user can write the ΔT value to the
offset register. The ADT7475 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 ADT7475, I
I
the current levels specified by the CPU manufacturer, it
might be necessary to remove an offset. The CPU’s data
sheet advises whether this offset needs to be removed and
how to calculate it. This offset can be programmed to the
offset register. If more than one offset must be considered,
the algebraic sum of these offsets must be programmed to
the offset register.
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
indicates tight control of V
LOW
f
values.
, is 11 μA. If the ADT7475 current levels do not match
ΔT = (n
f
− 1.008) × (273.15 k + T)
HIGH
, is 180 μA and the low level current,
FE
f
, of the transistor is a measure of the
(approximately 50 to 150) that
BE
characteristics.
f
value of 1.008. Use the
f
Rev. B | Page 17 of 68
Table 7. 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 8. 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
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 attributable to noise coupled
onto the D+/D– lines. Constant high frequency noise usually
attenuates, or increases, temperature measurements by a linear,
constant value.
The ADT7475 has two temperature offset registers, 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 automatically add a twos complement 8-bit reading to
every temperature measurement.
Bold numbers denote 2 LSBs of measurement in Extended Resolution
Bold numbers denote 2 LSBs of measurement in Extended Resolution
Register 2 (0x77) with 0.25°C resolution.
Register 2 (0x77) with 0.25°C resolution.
Digital Output (10-Bit)
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
Digital Output (10-Bit)
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
1
1
ADT7475