The UCC21759-Q1 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, etc. 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 UCC21759-Q1 encodes the voltage signal V_AIN to a PWM signal, passing through the basic 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 0.6V to 4.5V, 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 4.5V to 0.6V. This corresponds to the temperature coefficient of the negative temperature coefficient (NTC) resistor and thermal diode. When AIN is floating, the AIN voltage is 5V and the APWM operates at 400kHz with approximately 10% duty cycle. The duty cycle absolute error is ±1.5% at 0.6V and 2.5V, and is +1.5% / -2.5% at 4.5V across process and temperature. The in-system 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, etc. The internal current source I_AIN should be taken into account when designing the potential divider if sensing a high voltage.

Figure 7-7 Isolated Analog to PWM Signal