Solid State
The simplest semiconductor temperature sensor is a PN junction, such as a signal
diode or the base-emitter junction of a transistor. If the current through the
forward-biased silicon PN junction is held constant, the forward drop decreases
about 1.8mv per
using an external transistor, such as a 2N3904, as a temperature-sensing element.
The transistor can be a discrete part, or it can be embedded in the die of an IC to
measure the IC temperature. The MAX1617 has a serial SMBus output.
The LM335 (Figure 3.5) from National Semiconductor produces an output
voltage proportional to temperature. The LM135 produces 10mv per degree
Kelvin. At 0
LM335 operates with input current from 400
The National LM34 and LM35 sensors operate from supply voltages between
4V and 20 V, and produce a voltage output that directly corresponds to voltage.
The LM35 produces a voltage of 500mv at 50
every additional
and the LM35 for Centigrade. The outputs of the LM34/LM35 can be
connected directly to an ADC or to a comparator.
The National LM74 measures temperatures between 55
communicates with a microprocessor via the serial SPI/Microwire interface. The
LM74 output is a 13-bit signed value. The part contains a temperature sensor and
a sigma-delta converter. It is available in 3.3V or 5V versions and comes in an 8-
pin SMT package. The National LM75 is similar to the LM74, but uses the I
interface. The LM75 has a narrower operating temperature range: 55
C. The Maxim MAX1617 is an IC that measures temperatureC, the output is 2.73 V, and at 100 C the output is 3.73 V. Thema to 5 ma.C, with an additional 10mv forC increase. The LM34 is calibrated for Fahrenheit temperatures,C and þ150 C and2CC to
