In the customer’s application circuit testing, the key parameters are as follows: the working current is 0.1mA, and the upper limit of the loading power is 0.5mW. After evaluation, the product’s functionality is achieved without technical barriers under this application condition.
The customer has reported the following abnormal phenomena:
NTC voltage abnormality: After continuous power on for 1 hour, the voltage across the NTC thermistor is only 0.2V;
Fixed resistor voltage abnormality: Using the ADC module built-in in the customer’s MCU, the voltage of the 47k Ω fixed voltage divider resistor is monitored. After being powered on for 1 hour, the detected value is 4.8V;
NTC resistance normality: Test the NTC body resistance under power-off state, and the parameters are within the nominal value range.
In response to the above phenomenon, the customer raised a technical question: Why is there a significant voltage difference between NTC and fixed resistor under the loading conditions of 0.1mA current and 0.5mW power? According to the principle of circuit voltage division, it is expected that the voltage values of the two should be close to 2-3V.
Further analysis shows that if the voltage across the NTC is 0.2V when powered on, according to Ohm’s law, its dynamic resistance value can be calculated to be much lower than the nominal value, and at this time, the NTC body should produce a significant temperature rise. However, according to the power calculation formula P=I2R, under a current condition of 0.1mA, even if the NTC resistance drops to its minimum value, its power consumption is still insufficient to cause severe heating. This contradiction suggests the possibility of the following technical issues:
NTC characteristic deviation: The B-value parameter or temperature coefficient of NTC may deviate from the nominal curve;
Circuit layout issue: PCB layout may cause local overheating or poor heat dissipation of NTC;
Measurement error: There is a deviation in the sampling accuracy or calibration of the ADC, or there is a systematic error in the testing method.
It is recommended to prioritize checking the B-value parameters of NTC, PCB heat dissipation design, and ADC sampling accuracy to locate the root cause of the problem.
