Remote Temperature Sensor Transistor Selection Guide
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1 Introduction
Remote temperature sensors measure the temperature of an external transistor by measuring the base-emitter voltage (VBE) of the BJT, which depends on temperature. One method of determining a transistor base-emitter voltage is the two current method, using the difference of two voltage measurements to calculate temperature, as shown in Figure 1-1. However, thermal diode signals can be sensitive to noise, series resistance, and variation in transistors’ η-factors and betas can introduce additional errors if not accommodated.
Figure 1-1 ΔVBE Measurement with Two
Currents1.1 Series Resistance
Any resistance in PCB traces will cause a voltage drop between the actual VBE at the transistor and the measured VBE at the temperature sensor, which results in a temperature offset. Using three different current level measurements shown in Figure 1-2, a certain amount of series resistance can be canceled out when solving for temperature.
Figure 1-2 ΔVBE Measurement
with Three Currents1.2 Variation of η-Factor
Remote temperature sensors are typically calibrated for transistors with an η-factor of 1.008, but transistors may have η-factors that differ from this. If the value is known, an η-factor correction register can be used to correct the error resulting from a deviation of the calibrated η-factor. A combination of offset and η-factor correction settings can correct for both η-factor errors and other system errors that are present. The Remote Temperature Sensor Calibration Tool can calculate the optimal offset and η-factor correction settings for a given data set, and these register settings can be used with devices such as the TMP468, which includes an independent register for each remote input.
