MAX6645 Ver la hoja de datos (PDF) - Maxim Integrated
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MAX6645 Datasheet PDF : 17 Pages
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Automatic PWM Fan-Speed Controllers with
Overtemperature Output
Table 3. Configuring the FAN_IN_ Inputs with TACHSET
TACHSET
VDD
FAN_IN1
FAN_IN2
GND
FAN_IN1
FAN_IN2
MAX6643
MAX6644
MAX6645
Tachometer
Tachometer
Tachometer
Tachometer
Tachometer
Tachometer
Do not connect
to GND
Current sense
Current sense
Do not connect
to GND
Current sense
Current sense
UNCONNECTED
FAN_IN1
FAN_IN2
Disables fan-
Disables fan-
failure detection failure detection
Locked rotor
Locked rotor
Locked rotor
Locked rotor
Table 4. Setting the Overtemperature
Thresholds (TOVERT) (MAX6643 and MAX6644)
OT2
OT1
TOVERT (°C)
L SUFFIX
0
0
60
0
High-Z
65
0
1
70
High-Z
0
75
High-Z
High-Z
80
High-Z
1
85
1
0
90
1
High-Z
95
1
1
100
High-Z = high impedance
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.
Table 5. Remote-Sensor Transistor
Manufacturers
MANUFACTURER
Central Semiconductor (USA)
Rohm Semiconductor (USA)
Samsung (Korea)
Siemens (Germany)
MODEL NO.
CMPT3906
SST3906
KST3906-TF
SMBT3906
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 nNOMINAL is used to measure the temperature of a
diode with a different ideality factor, n1. The measured
temperature TM can be corrected using:
TM = TACTUAL
⎛
⎝⎜
n1
nNOMINAL
⎞
⎠⎟
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:
( ) TACTUAL
= TM
⎛
⎝⎜
nNOMINAL
n1
⎞
⎠⎟
= TM
⎛ 1.01 ⎞
⎝⎜ 1.008⎠⎟
= TM 1.00198
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.
8 _______________________________________________________________________________________
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