
- 13 - 
 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 V
DD
-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 
μ
A or less when there is no 
activity on the SMBus and 100 
μ
A 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: 
Δ
V
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
 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