Rainbow Electronics MAX6645 - Manual (Page 8)

MAX6643/MAX6644/MAX6645
systems, or for an over-temperature condition (by con-
necting OT to FULLSPD).
FULLSPD Input
The MAX6643_B_ features a FULLSPD input. Pulling
FULLSPD high forces PWM_OUT to 100% duty cycle.
The FULLSPD input allows a microcontroller to force
the fan to full speed when necessary. By connecting
FANFAIL to an inverter, the MAX6643_B_ can force
other fans to 100% in multifan systems, or for an over-
temperature condition (by connecting OT inverter to
FULLSPD).
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 100
Ω. 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 discrete 2N3904 and 2N3906 transistors. It is also
near the ideality factors of many widely available CPUs,
GPUs, and FPGAs. However, any time a sense transis-
tor with a different ideality factor is used, the output data
is different. Fortunately, the difference is predictable.
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
T OVERT (°C)
H SUFFIX
0 0 60 80
0 High-Z 65 85
0 1 70 90
High-Z 0 75 95
High-Z High-Z 80 100
High-Z 1 85 105
1 0 90 110
1 High-Z 95 115
1 1 100 120
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_A_ Tachometer Tachometer
Do not connect
to GND
Do not connect
to GND
Disables fan-
failure detection
Disables fan-
failure detection
MAX6644_A_ Tachometer Tachometer Current sense Current sense Locked rotor Locked rotor
MAX6645_A_ Tachometer Tachometer Current sense Current sense Locked rotor Locked rotor
MAX6643_B_ Tachometer Tachometer
Do not connect
to GND
Do not connect
to GND
Disables fan-
failure detection
Disables fan-
failure detection
MAX6644_B_ Tachometer Tachometer Current sense Current sense Locked rotor Locked rotor
MAX6645_B_ Tachometer Tachometer Current sense Current sense Locked rotor Locked rotor
High-Z = High impedance