Datasheet

23Maxim Integrated
±1°C Accurate 8-Channel Temperature Sensor
MAX6581
Applications Information
Remote-Diode Selection
The MAX6581 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.
Discrete Remote Diodes
When the remote-sensing diode is a discrete transistor,
its collector and base must be connected together. Table
13 lists examples of discrete transistors that are appro-
priate for use with the MAX6581. The transistor must be
a small-signal type with a relatively high forward voltage;
otherwise, the A/D input-voltage range can be violated.
The forward voltage at the highest expected temperature
must be greater than 0.25V at 10FA, and at the low-
est expected temperature the forward voltage must be
less than 0.95V at 100FA. Large power transistors must
not be used. Also, ensure that the base resistance is
less than 100I. Tight specifications for forward-current
gain (e.g., 50 < A < 150) indicate that the manufacturer
has good process controls and that the devices have
consistent V
BE
characteristics. Manufacturers of discrete
transistors do not normally specify or guarantee ideality
factor. This normally is not a problem since good-quality
discrete transistors tend to have ideality factors that fall
within a relatively narrow range. Variations in remote
temperature readings of less than Q2NC with a variety of
discrete transistors have been observed. However, it is
good design practice to verify good consistency of tem-
perature readings with several discrete transistors from
any supplier under consideration.
Unused Diode Channels
If one or more of the remote-diode channels is not need-
ed, disconnect the DXP_ and DXN_ inputs for that chan-
nel, or connect the DXP_ to the corresponding DXN_.
The status register indicates a diode “fault” for this
channel and the channel is ignored during the tempera-
ture-measurement sequence. It is also good practice to
mask any unused channels immediately upon power-up
by setting the appropriate bits in the ALERT Mask and
OVERT Mask registers. This prevents unused channels
from causing ALERT or OVERT to assert.
Table 15. Remote Sensors Transistor Suppliers (for Channels 1–7)
Note: Discrete transistors must be diode connected (base shorted to collector).
SUPPLIER
MODEL NO.
PNP NPN
Central Semiconductor Corp. (USA)
CMPT3906
2N3906
CMPT3904
2N3904
Fairchild Semiconductor (USA)
MMBT3906
2N3906
2N3904
Infineon (Germany) SMBT3906 —
ON Semiconductor (USA)
MMBT3906
2N3906
2N3904
ROHM Semiconductor (USA) SST3906 SST3904
Samsung (Korea) KST3906-TF KST3904-TF
Siemens (Germany) SMBT3906 SMBT3904
Zetex (England) FMMT3906CT-ND FMMT3904CT-ND