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© Andigilog, Inc. 2006
www.andigilog.com
August 2006 - 70A04010
aSC7511
limits, RTHERM and LTHERM, will change as soon as a
reading is greater than the limit and will reset as soon as
a reading goes below the limit minus the programmable
hysteresis value.
This hysteresis value applies to both remote and local
sensors. The scenario is described in Figure 8.
Figure 8. THERM Limits and Responses
The THERM limits default to 85ºC with 10ºC hysteresis.
The hysteresis value may be set from 0ºC to any positive
value up to 127ºC.
All limit values must take into account whether
measurements are being made in normal or extended
range. In extended range, there is a 64ºC offset in
reported values and limits must be adjusted accordingly.
THERM2 Option
The ALERT pin and associated alarm bits may be re-
assigned as a second THERM alarm. This alarm will
work exactly like THERM . The ALERT mask will have no
effect and there is only a high limit. The THERM
hysteresis value is applied to this alarm.
Sensor Open Detect
A protective circuit monitors the D+ pin for a voltage level
that would indicate the path to the remote diode is open.
If the voltage exceeds a typical value of VDD-1V, bit 2 of
the status register and the ALERT flag are set and
the ALERT pin is driven low.
This will require the master to service the ALERT in order
to reset the condition. If the remote diode is not being
used, it is recommended that D+ and D- be shorted to
prevent setting the open alarm.
In the event that a remote diode open-circuit has caused
an ALERT condition to occur and that condition is
restored to normal, when the host sends an Alert
Response and reads the Status Register the alert pin
may not clear. To ensure that the ALERT pin clears,
perform a read to internal register 42h.
Standby Mode
The aSC7511 may be placed in a minimum-power
standby mode by writing a 1 to bit 6 of the configuration
register. No measurements are made but the SMBus
interface will respond when addressed.
Any measurement in progress when standby is selected
will be terminated and no new values will be written into
the temperature registers.
While in this mode, a one-shot measurement of both
channels may be commanded by the user by writing any
data value to the one-shot register, 0Fh. All alarm
comparisons will continue to be made and reported. The
alarm values may be changed while in standby and
current measured temperatures will be compared and
alarms generated if an out-of-limit condition exists.
Operating current will be 10 μA or less when there is no
activity on the SMBus and 100 μA or less when clock and
data are active.
Applications Information
Remote Diodes
The aSC7511 is designed to work with a variety of
remote sensors in the form of the substrate thermal diode
of a CPU or graphics controller or a diode-connected
transistor. Actual diodes are not suited for these
measurements.
There is some variation in the performance of these
diodes, described in terms of its departure from the ideal
diode equation. This factor is called diode non-
ideality,
.
nf
The equation relating diode temperature to a change in
thermal diode voltage with two driving currents is:
ΔV
BE
= (nf )
KT
q
ln( N )
where:
nf = diode non-ideality factor, (nominal 1.008).
K = Boltzman’s constant, (1.38 x 10
-23).
T = diode junction temperature in Kelvins.
q = electron charge (1.6 x 10
-19 Coulombs).
N = ratio of the two driving currents (16).
The aSC7511 is designed and trimmed for an expected
value of 1.008, based on the typical value for the Intel
Pentium™ III and AMD Athlon™. There is also a
tolerance on the value provided. The values for other
CPUs and the 2N3904 may have different nominal values
and tolerances. Consult the CPU or GPU manufacturer’s
data sheet for the
factor. Table 8 gives a
representative sample of what one may expect in the
nf
nf
Temperature
Conversion
Ext. THERM Limit
THERM Hysteresis
Status Bit-1, EXTHERM
THERM Pin 4
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