The UCC21750 features an isolated analog to PWM signal function from AIN to APWM pin, which allows the isolated temperature sensing, high voltage dc bus voltage sensing, and so forth. An internal current source I_AIN in AIN pin is implemented in the device to bias an external thermal diode or temperature sensing resistor. The UCC21750 encodes the voltage signal V_AIN to a PWM signal, passing through the reinforced isolation barrier, and output to APWM pin on the input side. The PWM signal can either be transferred directly to DSP/MCU to calculate the duty cycle, or filtered by a simple RC filter as an analog signal. The AIN voltage input range is from 0.6 V to 4.5 V, and the corresponding duty cycle of the APWM output ranges from 88% to 10%. The duty cycle increases linearly from 10% to 88% while the AIN voltage decreases from 4.5 V to 0.6 V. This action corresponds to the temperature coefficient of the negative temperature coefficient (NTC) resistor and thermal diode. When AIN is floating, the AIN voltage is 5 V and the APWM operates at 400 kHz with approximately 10% duty cycle. The accuracy of the duty cycle is ±3% across temperature without one time calibration. The accuracy can be improved using calibration. The accuracy of the internal current source I_AIN is ±3% across temperature.

The isolated analog to PWM signal feature can also support other analog signal sensing, such as the high voltage dc bus voltage, and so forth. The internal current source I_AIN must be taken into account when designing the potential divider if sensing a high voltage.

Figure 8-7 Isolated Analog to PWM Signal