# NTC Thermistor selection for high temperature where beta value needs to be > 22000

We would really appreciate some help with finding a thermistor (and accompanying resistors) in order to do some current limiting in high-ambient temperatures. The circuit comes from the LT3741 datasheet (below).

Options A, B, C and D show different methods of use, some which would allow better linearization of the R-T curve, but it's still not exactly what we want.

The problem is that most NTC thermistors have the largest resistance differential at a temperature lower than our application. This means that we can't get a large enough range of resistance to be useful.

Vref is 2V, and CTRL2 should be 1.5V at 80°C, falling to ~0.075V at 100°C.

For example, if R2 was 120k, then the desired resistances of the thermistor would be 360k at 70 degrees C and 4.6k (or lower) at 100 degrees C. I can't seem to find any thermistors which such a great resistance differential at that range. By my calculations this is about a 28000 beta value. Most thermistors don't go above 5000.

Do I need an op-amp to increase the voltage change?

I would welcome any advice on how to manipulate a standard thermistor such that we get a large resistance differential at these temperatures, or are there known thermistors which operate around this range for the resistance change we require?

• Do you need to use this over a wide range of temperatures? (Unicurve device?) Or can you use one that is calibrated only near your needed temperature? (I think you can ask manufacturers to provide single-point calibration at temperatures other than 25$^\circ$C if that's what you mostly care about.) Have you considered unicurve devices? – jonk Apr 5 '17 at 17:07
• And of course you've Googled "high temperature thermistor", right? – WhatRoughBeast Apr 5 '17 at 20:21
• I have- but I don't find any clear guides on how to pick an ntc. What parameter should I be looking for, and is there a formula I can use to calculate this given two temperature points? For example the link on digi key, what makes that Vishay thermistor appropriate? Would like to use common parts if possible. Only need it to reduce voltage at temps > 80C. Don't really care how high or accurate it is below that. – Chris Apr 5 '17 at 21:25
• I want to achieve 1/20th of 1.5V at the bottom end so that the current is limited to 1/20th of the value on CTRL1 pin. This is 0.075V. I want this limit to be about 100 degrees, i.e. max limit at 100 degrees C. – Chris Apr 6 '17 at 8:48