AN835 Datasheet, PDF (2/4 Pages) Silicon Laboratories – COMBINING THREE VERY LOW SUPPLY CURRENT ANALOG ICS INTO A USB-POWERED THERMOSTAT

AN835
2.1. Circuit Implementation
The temperature sensor chosen for this design is an industry-standard LM35 temperature sensor, available from
many sources. This temperature sensor generates an output voltage proportional to the ambient temperature in
degree Celsius and exhibits a scale factor of +10mV/ºC. This particular temperature sensor provides a linear
output response with a maximum output voltage of 1500mV corresponding to a 150ºC ambient temperature.
However, given that the hottest temperature achieved on earth is approximately 60ºC, this design will focus on
temperatures up to this ambient level. The transfer curve for the sensor is shown in Figure 2. In addition, a
Si9942DY N-channel MOSFET was used to switch an AP121P 5V/200mA fan ON and OFF.
Figure 2. LM35 Temperature Sensor Transfer Curve
The TS6001 voltage reference provides a 2.5V power supply for the TS1001 op-amp. The TS9001-1 comparator,
TS6001 voltage reference, LM35 temperature sensor, and the fan are powered with a common 5V power supply,
supplied by the USB connector. In this particular design, the TS1001 op-amp is configured in a non-inverting
scheme where the output of the temperature sensor is connected to the non-inverting input of the op-amp. When
the temperature rises to 26.4ºC, the LM35 sensor output is 264mV and is connected to to the non-inverting input of
the TS1001. With a gain of 4.75, the TS1001 generates a 1.254V output voltage. The output of the TS1001 is
connected to the non-inverting input of the TS9001-1 that then switches to a HIGH state at its output, turns on the
MOSFET which, in turn, turns on the fan.
2
Rev. 1.0