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MAX6639 Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MAX6639 Datasheet(HTML) 8 Page - Maxim Integrated Products |
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8 / 22 page  Three additional temperature bits provide resolution down to 0.125 °C and are in the channel 1 extended temperature (05h) and channel 2 extended tempera- ture (06h) registers. All values below 0°C clip to 00h. The MAX6639 employs a register lock mechanism to avoid getting temperature results from the temperature register and the extended temperature register sam- pled at two different time points. Reading the extended register stops the MAX6639 from updating the temper- ature register for at least 0.25s, unless there is a tem- perature register read before the scheduled update. This allows enough time to read the main register before it is updated, thereby preventing reading the temperature register data from one conversion and the extended temperature register data from a different conversion. The MAX6639 measures the temperature at a fixed rate of 4Hz immediately after it is powered on. Setting bit 7 of the configuration register (04h) shuts down the tem- perature measurement cycle. OT Output When a measured temperature exceeds the corre- sponding OT temperature threshold and OT is not masked, the associated OT status register bit sets and the OT output asserts. If OT for the respective channel is masked, the OT status register sets, but the OT out- put does not assert. To deassert the OT output and the associated status register bit, either the measured tem- perature must fall at least 5 °C below the trip threshold or the trip threshold must be increased to at least 5 °C above the current measured temperature. THERM When a measured temperature exceeds the corre- sponding THERM temperature threshold and THERM is not masked, the associated THERM status register bit is set and the THERM output asserts. If THERM for the respective channel is masked, the THERM status regis- ter is set, but the THERM output does not assert. To deassert the THERM output and the associated status register bit, either the measured temperature must fall at least 5 °C below the trip threshold or the trip thresh- old must be increased to at least 5 °C above the current measured temperature. Asserting THERM internally or externally forces both PWM outputs to 100% duty cycle when bit 6 in address 13h (fan 1) or bit 6 in address 17h (fan 2) is set. ALERT The ALERT output asserts to indicate that a measured temperature exceeds the ALERT trip threshold for that temperature channel. The status bit and the ALERT out- put clear by reading the ALERT status register. If the ALERT status bit is cleared, but the temperature still exceeds the ALERT temperature threshold, ALERT reasserts on the next conversion, and the status bit sets again. A successful alert response protocol clears ALERT but does not affect the ALERT status bit. TACH1 and TACH2 Inputs To measure the fan speed, the MAX6639 has two tachometers. Each tachometer has an accurate internal clock to count the time elapsed in one revolution. Therefore, it is counting the time between two tachome- ter pulses for a fan with four poles. When the PWM sig- nal is used to directly modulate the fan’s power supply, the PWM frequency is normally in the 20Hz to 100Hz range. In this case, the time required for one revolution may be longer than the PWM on-time. For this reason, the PWM pulses are periodically stretched to allow tachometer measurement over a full revolution. Turn off pulse stretching by setting bit 5 of register 13h or regis- ter 17h when using a 4-wire fan. The tachometer count is inversely proportional to the fan’s RPM. The tachometer count data is stored in regis- ter 20h (for TACH1) and register 21h (for TACH2). Reading a value of 255 from the TACH count register means the fan’s RPM is zero or too slow for the range. Reading a value of zero in the TACH count register means the fan’s RPM is higher than the range selected. Table 2 shows the fan’s available RPM ranges. Use reg- isters 10h or 14h to select the appropriate RPM range for the fan being used. 2-Channel Temperature Monitor with Dual, Automatic, PWM Fan-Speed Controller 8 _______________________________________________________________________________________ TEMP ( °C) TEMP ( °C) DIGITAL OUTPUT 241 +241 1111 0001 240 +240 1111 0000 126 +126 0111 1110 25 +25 0001 1001 1.50 1 0000 0001 0.00 0 0000 0000 Table 2. Temperature Data Byte Format FAN RPM RANGE INTERNAL CLOCK FREQUENCY (kHz) 2000 1 4000 2 8000 4 16,000 8 Table 3. Tachometer Setting |
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