JAJSEA7B September 2017 – February 2020 TMP461-SP
PRODUCTION DATA.
The TMP461-SP device is designed to be used with either discrete transistors or substrate transistors built into processor chips and application-specific integrated circuits (ASICs). Either NPN or PNP 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 18).
Errors in remote temperature sensor readings are typically the consequence of the ideality factor and current excitation used by the TMP461-SP 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 TMP461-SP device uses 7.5 μA for ILOW and 120 μA for IHIGH.
The ideality factor (η) is a measured characteristic of a remote temperature sensor diode as compared to an ideal diode. The TMP461-SP allows for different η-factor values; see the η-Factor Correction Register section.
The ideality factor for the TMP461-SP device is trimmed to be 1.008. For transistors that have an ideality factor that does not match the TMP461-SP, 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 TMP461-SP, the best accuracy can be achieved by selecting the transistor according to the following criteria:
Based on this criteria, two recommended small-signal transistors are the 2N3904 (NPN) or 2N3906 (PNP).