The TMP43x are designed to be used with either discrete transistors or substrate transistors built into processor chips and ASICs. Either NPN- or PNP-type transistors can be used, as long as the base-emitter junction is used as the remote temperature sense. 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 and current excitation used by the TMP43x 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 TMP43x use 6 μA for ILOW and 120 μA for IHIGH. The TMP43x allow for different η-factor values; see η-Factor Correction Register.
The ideality factor (η) is a measured characteristic of a remote temperature sensor diode as compared to an ideal diode. The ideality factor for the TMP43x is trimmed to be 1.008. For transistors whose ideality factor does not match the TMP43x, Equation 6 can be used to calculate the temperature error. Note that for the equation to be used correctly, actual temperature (°C) must be converted to Kelvin (K).
For n = 1.004 and T(°C) = 100°C:
If a discrete transistor is used as the remote temperature sensor with the TMP43x, the best accuracy can be achieved by selecting the transistor according to the following criteria:
Based on these criteria, two recommended small-signal transistors are the 2N3904 (NPN) or 2N3906 (PNP).