Previous generations of remote junction temperature sensors were operated by controlling the emitter current of the sensing transistor. However, examination of the physics of a transistor shows that VBE is actually a function of the collector current. If beta is independent of the collector current, then VBE can be calculated from the emitter current. In earlier generations of processors that contained PNP transistors connected to these temperature sensors, controlling the emitter current provided acceptable temperature measurement results. At 90-nm process geometry and below, however, the beta factor continues to decrease and the premise that it is independent of collector current becomes less certain.
To manage this increasing temperature measurement error, the TMP43x control the collector current instead of the emitter current. The TMP43x automatically detect and choose the correct range depending on the beta factor of the external transistor. This auto-ranging is performed at the beginning of each temperature conversion in order to correct for any changes in the beta factor as a result of temperature variation. The device can operate a PNP transistor with a beta factor as low as 0.1. See Beta Compensation Configuration Register for further information.