ASC7531A ETC2 [List of Unclassifed Manufacturers], ASC7531A Datasheet - Page 15

• Manufacturer Part Number: ASC7531A
• Description: LOW-VOLTAGE 1-WIRE DIGITAL TEMPERATURE SENSOR AND VOLTAGE MONITOR
• Manufacturer: ETC2 [List of Unclassifed Manufacturers]

Discrete Remote Diodes
When sensing temperatures other than the CPU or GPU
substrate, an NPN or PNP transistor may be used. Most
commonly used are the 2N3904 and 2N3906. These have
characteristics similar to the CPU substrate diode with non-
ideality around 1.0046. They are connected with base to
collector shorted as shown in Figure 18.
While it is important to minimize the distance to the remote
diode to reduce high-frequency noise pickup, they may be
located many feet away with proper shielding. Shielded,
twisted-pair
connected only at the aSC7531A end as close as possible to
the ground pin of the device.
As with the CPU substrate diode, the temperature reported
will be subject to the same errors due to non-ideality variation
and series resistance. However, the transistor’s die
temperature is usually not the temperature of interest and
care must be taken to minimize the thermal resistance and
physical distance between that temperature and the remote
diode. The offset and response time will need to be
characterized by the user.
CPU or ASIC Substrate Remote Diodes
A substrate diode is a parasitic PNP transistor that has its
collector tied to ground through the substrate and the base
(D-) and emitter (D+) brought out to pins. Connection to
these pins is shown in Figure 19. The non-ideality figures in
Table 6 include the effects of any package resistance and
represent the value seen from the CPU socket. The
temperature indicated will need to be compensated for the
departure from a non-ideality of 1.0046 and series resistance
of 0.6Ω .
© Andigilog, Inc. 2006
Figure 18 Discrete Remote Diode Connection
Figure 19 CPU Remote Diode Connection
2N3904
2N3906
CPU
cable
Substrate
is
recommended,
D+
D-
D -
aSC7531B
D +
D -
D +
aSC7531A
aSC7531A
with
the
shield
www.andigilog.com
- 15 -
Series Resistance
Any external series resistance in the connections from the
aSC7531 to the CPU pins should be accounted for in
interpreting the results of a measurement.
The
temperature is a result of measurement currents developing
offset voltages that add to the diode voltage. This is relatively
constant with temperature and may be corrected with a fixed
value in the offset register. To determine the temperature
impact of resistance is as follows:
where:
For example, a total series resistance of 10Ω would give an
offset of +6.75°C.
Board Layout Considerations
The distance between the remote sensor and the aSC7531
should be minimized. All wiring should be defended from high
frequency noise sources and a balanced differential layout
maintained on D+ and D-.
Any noise, both common-mode and differential, induced in
the remote diode interconnect may result in an offset in the
temperature reported. Circuit board layout should follow the
recommendation of Figure 20. Basically, use 10-mil lines and
spaces with grounds on each side of the differential pair.
Choose the ground plane closest to the CPU when using the
CPU’s remote diode.
Noise filtering is accomplished by using a bypass capacitor
placed as close as possible to the aSC7531 D+ and D- pins.
A 1.0nF ceramic capacitor is recommended, but up to 3.3nF
may be used. Additional filtering takes place within the
aSC7531.
T = scale of temperature vs. V
Δ
R = total series resistance of interconnect (both leads).
Δ
V
T
S
I
Figure 20 Recommended Remote Diode Circuit
D
10 mils
10 mils
R
= difference in the temperature reading from actual.
= difference in the two diode current levels (135µA).
impact
Δ
or
Δ
T
T
,
aSC7531A / aSC7531B
R
R
=
=
of
R
R
S
S
Board Interconnect
series
×
×
T
⎛
⎜ ⎜
⎝
V
200
×
135
Δ
μ
I
V
resistance
D
μ
A
/
°
C
BE
⎞
⎟ ⎟
⎠
(200µV/°C).
December
=
R
S
×
on
. 0
675
GND
D +
D -
GND
2006 - 70A05012
the
°
C
/
Ω
measured