5.1 Current Regulator OTP with NTC Sensing

One of the concerns with this first version of the design is that there is no way to directly measure the temperature of the current regulator without losing another feature that was implemented. However, for future versions this simple circuit can implement Over-Temperature Protection (OTP) without using an I/O pin on the MCU.

Rather than directly measuring the temperature at the current regulator, instead use an external MCP6566 analog comparator with a temperature sensor, as shown in Figure 5-1. This comparator is open-drain, rather than push-pull. Since the output is usually high-Z, the comparator can sit on the NTC thermistor line for the heat sink. If the current regulator reaches a high temperature and activates the comparator, then the comparator will pull the line to ground. Since NTCs reduce their resistance with temperature, the microcontroller will detect this state as a very high temperature on the main heat sink and activate a shutdown of the system.

Figure 5-1. NTC Measurement Circuit with OTP