There is two stage to control the output voltage for this PSU. The first part will utilize a 10bits DAC (MCP4716) with a precise reference voltage of 2.048V from ISIL21070. With help from ESP8266 SoC’s I2C, we can easily adjust the output of DAC from 0V to 2.048V.
The second stage will use another LM6142 to work as 10X gain amplifier. If DAC output 0.5V to input of op-amp, the op-amp will eventually amplify the signal and output 5V to the SET pin of LT3080. LT3080 is a linear regulator and its typical dropout voltage is around 1.2V to 1.35V, which literally means Vin of LT3080 must higher 1.35V than Vset. For example, if Vin is 8V, the saturated output of Vout is around 6.65V, even though Vset is adjusted higher than 6.65V.
Now, combining clues from current limiter and voltage controller, we can run full simulation for our PSU.
- Load : Initially 0.2A, ramping to 0.8A
- Vin : 5V (Voltage provide to PSU)
- Vdac : 0.3V (Output voltage 3V)
- Vlim : 0.3V (limit maximum output current 0.3A)
From the simulation we can see when the load consuming lower than 0.3A, it allows PSU output 3V to the load. When the load consuming more current after 78ms, current limiter circuit will cut off Vset and make the output voltage drop to 0V. Thus, simulation above basically proven the PSU control circuit is working and now we shall move to assemble all the real parts.