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MAX6644LBAAEE Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MAX6644LBAAEE Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 17 page 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 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: 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 TOVERT (°C) L SUFFIX 00 60 0 High-Z 65 01 70 High-Z 0 75 High-Z High-Z 80 High-Z 1 85 10 90 1 High-Z 95 1 1 100 Table 4. Setting the Overtemperature Thresholds (TOVERT) (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 |
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