Author Topic: Help - Monitoring a NTC thermistor  (Read 468 times)

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Offline HastTopic starter

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Help - Monitoring a NTC thermistor
« on: November 14, 2019, 01:46:47 pm »
Hi

So I am doing a battery powered (Li-Po) product that is going to be medical device classified.
Medical products must be “single fault safe”.
I will us a charging circuit from TI, BQ24040 to charge the Li-Po.
It monitors the temperature via a NTC thermistor on the TS pin. “The TS feature is implemented using an internal 50μA current source to bias the thermistor.”
The risk I see is that the NTC breaks and is open. If it is open/floating the BQ24040 enters a “Termination and Timer Disable Mode”.
This is not by itself any risk unless the temperature is between 0-10 degrees C, between 0-10 degrees C the Li-Po shall be charged with half the normal charging current. This is handled by the charging IC.

So to solve this I am thinking that I need to monitor the NTC resistor somehow and if it breaks then I will prevent charging and signal the MCU that there is a HW fault.
My first thought was to use a comparator over a shunt resistor after the NTC but then I realized that this do not solve the problem as then the test house will only say that what happened if the shunt resistor breaks and then I am back where I started.
I then saw in a datasheet for a current sensing IC, INA285, that I could use two in a “current differencing” circuit (see attached pic).
This could probably work if the shunt resistors used do not introduce a to big of a fault when detecting if it is to warm/hot (the NTC resistance go down when it gets hot). But also it increases the BOM-cost.

So now I am looking for if there is another way to monitor if the NTC thermistor is broken or not?
Another constraint is that it cannot be the MCU (software) that do any of the safety monitoring as if the software has anything to do with safety then the software must be classed as class B/C and I do not want that. The software must be class A.

Any good ideas?

Best regards
Andreas
 

Offline T3sl4co1l

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Re: Help - Monitoring a NTC thermistor
« Reply #1 on: November 14, 2019, 01:52:48 pm »
Just measure the voltage across the thermistor.  It is its own shunt resistor, after all.

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
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