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MAX6640 Datasheet(PDF) 17 Page - Maxim Integrated Products |
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MAX6640 Datasheet(HTML) 17 Page - Maxim Integrated Products |
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17 / 21 page 
4-Wire Fans Some fans have an additional, fourth terminal that accepts a logic-level PWM speed-control signal as shown in Figure 10. These fans require no external power circuitry and combine the low noise of linear drive with the high efficiency of PWM power-supply drive. Higher PWM frequencies are recommended when using 4-wire fans. Quick-Start Guide for 8000RPM 4-Pole (2 Pulses per Revolution) Fan in Automatic RPM Mode Using the Circuit of Figure 7 1) Write 02h to register 11h to set the PWM output to drive the n-channel MOSFET. 2) Write 4Bh to register 22h to set the minimum RPM to 3200. 3) Write 5Eh to register 24h to set the pulses per revo- lution to 2 and to set the maximum RPM speed to 8000RPM. 4) Write 19h to register 28h to set the fan-start temper- ature to +25°C. 5) Write 6Ah to register 10h to start automatic RPM mode. Remote-Diode Considerations Temperature accuracy depends upon having a good- quality, diode-connected, small-signal transistor. Accuracy has been experimentally verified for all the devices listed in Table 11. The MAX6640 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 A/D 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. 2-Channel Temperature Monitor with Dual Automatic PWM Fan-Speed Controller ______________________________________________________________________________________ 17 VCC PWM1 4.7k Ω 4.7k Ω TACH1 3V TO 5.5V 3V TO 5.5V TACH OUTPUT VFAN (5V OR 12V) Figure 10. 4-Wire Fan with PWM Speed-Control Input VCC PWM1 4.7k Ω 4.7k Ω TACH1 3V TO 5.5V TACH OUTPUT VFAN (12V OR 5V) 5V Figure 8. High-Side PWM Drive with “Keep-Alive” Supply VCC PWM1 TACH1 4.7k Ω 3V TO 5V 4.7k Ω 100k Ω 91k Ω 33k Ω 100k Ω 3.3V 2N3904 2.2 µF 10 µF TACH OUTPUT VFAN (5V OR 12V) TACH OUTPUT Figure 9. High-Side Linear Drive Circuit |
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