ZHCSGB8C May 2017 – October 2019 TMP464
PRODUCTION DATA.
The TMP464 device is designed to be used with either discrete transistors or substrate transistors built into processor chips, field programmable gate arrays (FPGAs), and application-specific integrated circuits (ASICs) ; see Figure 21. Either NPN or PNP transistors can be used, as long as the base-emitter junction is used as the remote temperature sensor. NPN transistors must be diode-connected. PNP transistors can either be transistor- or diode-connected (see Figure 21).
Errors in remote temperature sensor readings are typically the consequence of the ideality factor (η-factor) and current excitation used by the TMP464 device versus the manufacturer-specified operating current for a given transistor. Some manufacturers specify a high-level and low-level current for the temperature-sensing substrate transistors. The TMP464 uses 7.5 μA (typical) for ILOW and 120 μA (typical) for IHIGH.
The ideality factor (η-factor) is a measured characteristic of a remote temperature sensor diode as compared to an ideal diode. The TMP464 allows for different η-factor values; see the η-Factor Correction Register section.
The η-factor for the TMP464 device is trimmed to 1.008. For transistors that have an ideality factor that does not match the TMP464 device, Equation 4 can be used to calculate the temperature error.
NOTE
For Equation 4 to be used correctly, the actual temperature (°C) must be converted to Kelvin (K).
where
In Equation 4, the degree of delta is the same for °C and K.
For η = 1.004 and T(°C) = 100°C:
If a discrete transistor is used as the remote temperature sensor with the TMP464 device, then select the transistor according to the following criteria for best accuracy:
Based on these criteria, TI recommends using a MMBT3904 (NPN) or a MMBT3906 (PNP) transistor.