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MAX6642 Datasheet(PDF) 10 Page - Maxim Integrated Products |
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MAX6642 Datasheet(HTML) 10 Page - Maxim Integrated Products |
10 / 14 page where temperature is measured in Kelvin and nNOMIMAL for the MAX6642 is 1.008. As an example, assume you want to use the MAX6642 with a CPU that has an ideality factor of 1.002. If the diode has no series resistance, the measured data is related to the real temperature as follows: For a real temperature of +85°C (358.15K), the mea- sured temperature is +82.91°C (356.02K), an error of -2.13°C. Effect of Series Resistance Series resistance in a sense diode contributes addition- al errors. For nominal diode currents of 10µA and 100µA, the change in the measured voltage due to series resistance is: ∆VM = RS (100µA - 10µA) = 90µA RS Since +1°C corresponds to 198.6µV, series resistance contributes a temperature offset of: Assume that the diode being measured has a series resistance of 3 Ω. The series resistance contributes an offset of: The effects of the ideality factor and series resistance are additive. If the diode has an ideality factor of 1.002 and series resistance of 3 Ω, the total offset can be cal- culated by adding error due to series resistance with error due to ideality factor: 1.36°C - 2.13°C = -0.77°C for a diode temperature of +85°C. In this example, the effect of the series resistance and the ideality factor partially cancel each other. Discrete Remote Diodes When the remote-sensing diode is a discrete transistor, connect its collector and base together. Table 7 lists examples of discrete transistors that are appropriate for use with the MAX6642. The transistor must be a small-signal type with a rela- tively high forward voltage; otherwise, the A/D input voltage range can be violated. The forward voltage at the highest expected temperature must be greater than 0.25V at 10µA, and at the lowest expected tempera- ture, the forward voltage must be less than 0.95V at 100µA. Large power transistors must not be used. Also, ensure that the base resistance is less than 100 Ω. Tight specifications for forward current gain (50 < ß <150, for example) indicate that the manufacturer has good process controls and that the devices have consistent VBE characteristics. Manufacturers of discrete transistors do not normally specify or guarantee ideality factor. This is normally not a problem since good-quality discrete transistors tend to have ideality factors that fall within a relatively narrow range. We have observed variations in remote tempera- ture readings of less than ±2°C with a variety of dis- crete transistors. Still, it is good design practice to verify good consistency of temperature readings with several discrete transistors from any manufacturer under consideration. ADC Noise Filtering The integrating ADC used has good noise rejection for low-frequency signals such as 60Hz/120Hz power-sup- ply hum. In noisy environments, high-frequency noise reduction is needed for high-accuracy remote mea- surements. The noise can be reduced with careful PCB layout and proper external noise filtering. High-frequency EMI is best filtered at DXP with an external 2200pF capacitor. Larger capacitor values can be used for added filtering, but do not exceed 3300pF because excessive capacitance can introduce errors 3 0 453 1 36 Ω× ° Ω =+ ° .. C C 90 198 6 0 453 µ Ω µ ° = ° Ω V V C C . . TT n n T T ACTUAL M NOMINAL M M = ⎛ ⎝⎜ ⎞ ⎠⎟ = ⎛ ⎝⎜ ⎞ ⎠⎟ = 1 1 008 1 002 1 00599 . . ( . ) ±1°C, SMBus-Compatible Remote/Local Temperature Sensor with Overtemperature Alarm 10 ______________________________________________________________________________________ MANUFACTURER MODEL NO. Central Semiconductor (USA) CMPT3906 Rohm Semiconductor (USA) SST3906 Samsung (Korea) KST3906-TF Siemens (Germany) SMBT3906 Zetex (England) FMMT3906CT-ND Table 7. Remote-Sensor Transistor Manufacturers Note: Discrete transistors must be diode connected (base short- ed to collector). |
Similar Part No. - MAX6642_08 |
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Similar Description - MAX6642_08 |
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