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# [FAQ] How to design for over temperature protection with a thermistor and a comparator

Is there a solution for over temperature protection using a comparator with external hysteresis? My automotive design requires the comparator output to toggle high when a temperature of 100C is reached. I would like the toggle low temperature to be 80C to act as some temperature recovery. I would like to use an open-drain/open-collector comparator.

•  The first important component selection is the thermistor. For this example, we will use the TMP63-Q1 PTC thermistor.

Thermistor with positive temperature coefficient (PTC): https://www.ti.com/lit/ds/snis215c/snis215c.pdf?ts=1609446217819&ref_url=https%253A%252F%252Fwww.google.com%252F

Using the configuration shown above for Rbias = 100k, TH = 100C, TL = 80C, VBIAS = 3.3V, we will use the thermistor voltage (VTHR) as the non-inverting input to our comparator.

@TH, Rtmp63 ≈ 156k from figure above, Vtemp = 2.01, VH = 2.01V

@TL, Rtmp63 ≈ 140k from figure above, Vtemp = 1.925, VL = 1.925V

Input the VH and VL values into excel tool from this e2e thread or follow non-inverting comparator with hysteresis cookbook.

All devices can operate off of a single 3.3V supply. For simulation, a DC sweep using “V1” as the thermistor voltage and sweeping its voltage from 1.8V to 2.2V illustrates how this circuit has the correct switching thresholds.

Please see further explanation in this cookbook: non-inverting comparator with hysteresis cookbook

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