Datasheet
MAX6643/MAX6644/MAX6645
Applications Information
Figures 3–6 show various configurations.
Remote-Diode Considerations
When using an external thermal diode, temperature
accuracy depends upon having a good-quality, diode-
connected, small-signal transistor. Accuracy has been
experimentally verified for a variety of discrete small-
signal transistors, some of which are listed in Table 5.
The MAX6643/MAX6644/MAX6645 can also directly
measure the die temperature of CPUs and other ICs
with on-board temperature-sensing diodes.
The transistor must be a small-signal type with a rela-
tively high forward voltage. This ensures that the input
voltage is within the ADC input voltage range. The for-
ward voltage must be greater than 0.25V at 10µA at the
highest expected temperature. The forward voltage
must be less than 0.95V at 100µA at the lowest expect-
ed temperature. The base resistance has to be less
than 100Ω. Tight specification of forward-current gain
(+50 to +150, for example) indicates that the manufac-
turer has good process control and that the devices
have consistent characteristics.
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 MAX6643/MAX6644/MAX6645
are optimized for n = 1.01, which is typical of many dis-
crete 2N3904 and 2N3906 transistors. It is also near the
ideality factors of many widely available CPUs, GPUs, and
FPGAs. However, any time a sense transistor with a differ-
ent ideality factor is used, the output data is different.
Fortunately, the difference is predictable. Assume a
remote-diode sensor designed for a nominal ideality fac-
tor n
NOMINAL
is used to measure the temperature of a
diode with a different ideality factor, n
1
. The measured
temperature T
M
can be corrected using:
where temperature is measured in Kelvin.
As mentioned above, the nominal ideality factor of the
MAX6643/MAX6644/MAX6645 is 1.01. As an example,
assume the MAX6643/MAX6644/MAX6645 are config-
ured with a CPU that has an ideality factor of 1.008. If
the diode has no series resistance, the measured data
is related to the real temperature as follows:
For a real temperature of +60°C (333.15K), the mea-
sured temperature is 59.33°C (332.49K), which is an
error of -0.66°C.
TT
n
n
T
1.01
1.008
T
ACTUAL M
NOMINAL
1
MM
=
⎛
⎝
⎜
⎞
⎠
⎟
=
⎛
⎝
⎜
⎞
⎠
⎟
=
()
.1 00198
TT
n
n
M ACTUAL
1
NOMINAL
=
⎛
⎝
⎜
⎞
⎠
⎟
Automatic PWM Fan-Speed Controllers with
Overtemperature Output
8 _______________________________________________________________________________________
MANUFACTURER MODEL NO.
Central Semiconductor (USA) CMPT3906
Rohm Semiconductor (USA) SST3906
Samsung (Korea) KST3906-TF
Siemens (Germany) SMBT3906
Table 5. Remote-Sensor Transistor
Manufacturers
OT2 OT1
T
OVERT
(°C)
L SUFFIX
0060
0 High-Z 65
0170
High-Z 0 75
High-Z High-Z 80
High-Z 1 85
1090
1 High-Z 95
1 1 100
Table 4. Setting the Overtemperature
Thresholds (T
OVERT
) (MAX6643 and MAX6644)
Table 3. Configuring the FAN_IN_ Inputs with TACHSET
VDD GND UNCONNECTED
TACHSET
FAN_IN1 FAN_IN2 FAN_IN1 FAN_IN2 FAN_IN1 FAN_IN2
MAX6643 Tachometer Tachometer
Do not connect
to GND
Do not connect
to GND
Disables fan-
failure detection
Disables fan-
failure detection
MAX6644 Tachometer Tachometer Current sense Current sense Locked rotor Locked rotor
MAX6645 Tachometer Tachometer Current sense Current sense Locked rotor Locked rotor
High-Z = high impedance