Applications Information
Remote-Diode selection
The MAX6687/MAX6688 are optimized to measure the
die temperature of CPUs and other ICs that have on-
chip temperature-sensing diodes. These on-chip
diodes are substrate PNPs with their collectors ground-
ed. Connect the base of the PNP to DXN and the emit-
ter to DXP. When using a discrete, diode-connected
NPN or PNP as a sensing diode, use a good-quality
small-signal device. Examples are listed in Table 2.
Tight specifications for forward current gain indicate
the manufacturer has good process controls and that
the devices have consistent V
BE
characteristics.
Always use a transistor for the sensing junction; diodes
do not work.
The MAX6687/MAX6688 are optimized for use with
thermal-sensing transistors with an ideality factor of
1.008. Different ideality factors cause predictable, usu-
ally small deviations in trip temperature thresholds.
Noise-Filtering Capacitors
A quality ceramic capacitor must be connected across
the DXP/DXN inputs to maintain temperature threshold
accuracy by filtering out noise. The capacitor should be
located physically close to the DXP/DXN pins and
should typically have a value of 2200pF. Larger capaci-
tor values can cause temperature measurement errors.
A 50% increase from the recommended capacitor
value can cause up to ±1°C error.
Sensing Circuit Board and
Ambient Temperature
Temperature switches like the MAX6687/MAX6688 that
sense their own die (local) temperatures must be
mounted on or close to the object whose temperature
they are intended to measure. The MAX6687/MAX6688
can accurately measure the temperature of a circuit
board to which they are soldered because the package
leads provide a good thermal path between the circuit
board and their own die. If the MAX6687/MAX6688 are
intended to be triggered by the temperature of a heat-
generating component on the circuit board, they should
be mounted as close as possible to that component
and should share supply and ground traces (if they are
not noisy) with that component where possible. The
thermal path between the plastic package and the die
is not as good as the path through the package leads,
so the MAX6687/MAX6688 are less sensitive to the sur-
rounding air temperature than they are to the tempera-
ture of their package leads, but they can be
successfully used to respond to the ambient tempera-
ture if the circuit board is designed to track the ambient
temperature.
Chip Information
TRANSISTOR COUNT: 7765
PROCESS: BiCMOS
MAX6687/MAX6688
Local/Remote Temperature Switches in a
µMAX Package
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