JAJSE40A October 2017 – February 2018 UCC28780
The UCC28780 uses an external NTC resistor (RNTC) tied to the NTC pin to program a thermal shutdown temperature near the hotspot of the converter. The NTC shutdown threshold (VNTCTH) of 1 V with an internal 105-μA current source flowing through RNTC results in a 9.5-kΩ thermistor shutdown threshold. If the NTC resistance stays lower than 9.5 kΩ for more than three consecutive PWML pulses, an OTP fault event is triggered, and the 1-V threshold is increased to 2.25 V. The NTC resistance has to increase above 21.7 kΩ to reset the OTP fault. This threshold change provides a safe temperature hysteresis to help the hot-spot temperature cool down before the next VO restart attempt, reducing the thermal stress to the components. This pin can also be used as an electrical shutdown function by shorting this pin with a controlled switch to GND. With the pin shorted to GND, VVDD performs UVLO cycling, and there is at least three consecutive PWML pulses generated to check the state. The NTC pin can be left floating or tied to the REF pin if not used.