AN835

2

Rev. 1.0

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.