SNAS207B May 2004 – January 2024 LM64
PRODUCTION DATA
The LM64 can be applied to remote diode sensing in the same way as other integrated-circuit temperature sensors. It can be soldered to a printed-circuit board, and because the path of best thermal conductivity is between the die and the pins, its temperature will effectively be that of the printed-circuit board lands and traces soldered to its pins. This presumes that the ambient air temperature is nearly the same as the surface temperature of the printed-circuit board. If the air temperature is much higher or lower than the surface temperature, the actual temperature of the LM64 die will be an intermediate temperature between the surface and air temperatures. Again, the primary thermal conduction path is through the leads, so the circuit board surface temperature will contribute to the die temperature much more than the air temperature.
To measure the temperature external to the die use a remote diode. This diode can be located on the die of the target IC, such as a CPU processor chip, allowing measurement of the IC’s temperature, independent of the LM64’s temperature. The LM64 has been optimized for use with a MMBT3904 diode-connected transistor.
A discrete diode can also be used to sense the temperature of external objects or ambient air. Remember that a discrete diode’s temperature will be affected, and often dominated by, the temperature of its leads.
Most silicon diodes do not lend themselves well to this application. It is recommended that a diode-connected MMBT3904 transistor be used. The base of the transistor is connected to the collector and becomes the anode. The emitter is the cathode.