Datasheet
either the measured value must drop below the thresh-
old minus the hysteresis value (4°C or 4 LSBs), or the
trip threshold must be set at least 4°C (or 4 LSBs) above
the current value.
Applications Information
Remote-Diode Selection
The MAX6698 directly measures the die temperature of
CPUs and other ICs that have on-chip temperature-
sensing diodes (see the Typical Application Circuit) or
it can measure the temperature of a discrete diode-
connected transistor.
Effect of Ideality Factor
The accuracy of the remote temperature measurements
depends on the ideality factor (n) of the remote “diode”
(actually a transistor). The MAX6698 is optimized for n
= 1.008. A thermal diode on the substrate of an IC is
normally a pnp with the base and emitter brought out
the collector (diode connection) grounded. DXP_ must
be connected to the anode (emitter) and DXN_ must be
connected to the cathode (base) of this pnp. If a sense
transistor with an ideality factor other than 1.008 is
used, the output data is different from the data
obtained with the optimum ideality factor. Fortunately,
the difference is predictable. Assume a remote-diode
sensor designed for a nominal ideality factor n
NOMINAL
is used to measure the temperature of a diode with a
different ideality factor n1. The measured temperature
T
M
can be corrected using:
where temperature is measured in Kelvin and
n
NOMIMAL
for the MAX6698 is 1.008. As an example,
assume you want to use the MAX6698 with a CPU that
has an ideality factor of 1.002. If the diode has no
series resistance, the measured data is related to the
real temperature as follows:
For a real temperature of +85°C (358.15K), the mea-
sured temperature is +82.87°C (356.02K), an error of
-2.13°C.
Series Resistance Cancellation
Some thermal diodes on high-power ICs can have
excessive series resistance, which can cause tempera-
ture measurement errors with conventional remote tem-
perature sensors. Channel 1 of the MAX6698 has a
series resistance cancellation feature (enabled by bit 3
of the configuration 1 register) that eliminates the effect
of diode series resistance. Set bit 3 to 1 if the series
resistance is large enough to affect the accuracy of
TT
n
n
TT
ACTUAL M
NOMINAL
MM
=×
⎛
⎝
⎜
⎞
⎠
⎟
=×
⎛
⎝
⎜
⎞
⎠
⎟
=
1
1 008
1 002
1 00599
.
.
(. )
TT
n
n
M ACTUAL
NOMINAL
=
⎛
⎝
⎜
⎞
⎠
⎟
1
MAX6698
______________________________________________________________________________________ 13
7-Channel Precision Remote-Diode, Thermistor,
and Local Temperature Monitor
BIT NAME
POR
STATE
FUNCTION
7(MSB) Reserved 0 —
6 Reserved 0 —
5
Mask Thermistor 3
OVERT
0 Thermistor 3 OVERT Mask Bit. Set to logic 1 to mask thermistor 3 OVERT.
4
Mask Thermistor 2
OVERT
0 Thermistor 2 OVERT Mask Bit. Set to logic 1 to mask thermistor 2 OVERT.
3
Mask Thermistor 1
OVERT
0 Thermistor 1 OVERT Mask Bit. Set to logic 1 to mask thermistor 1 OVERT.
2 Reserved 0 —
1 Reserved 0 —
0 Mask OVERT 10
Channel 1 Remote-Diode OVERT Mask Bit. Set to logic 1 to mask channel 1
OVERT.
Table 7. Configuration 3 Register