Hi team,
When the FB under sampling feedback, the optocoupler secondary is directly connected to the comp pin of the chip 28C44. Is the comp pin voltage provided by the Vref pin or the internal current source of the chip?
Base on the primary side of the optocoupler only controls the impedance change of the secondary side of the optocoupler, but the voltage source is Vref=5V. The current provided by Vref generates a voltage drop on the secondary side transistor of the optocoupler. This voltage drop is sampled by the comp pin.
There should be a mA level current source at the output of the error amplifier, which is relied upon to charge the COMP pin capacitor at the beginning of soft start. However, it is not clear whether the current source drawn from the comp pin during sampling and adjustment relies on this current source.
but the datasheet block diagram does not have such current source.
This below picture is from 3842:
I personally think that in power supply solutions such as 3842 and 28C44, the current of the optocoupler secondary (secondary side) should be provided by this internal current source. However, some chips directly use the Vref pin to connect a current-limiting resistor and the optocoupler secondary to the ground in series, and then connect the comp pin between the current-limiting resistor and the optocoupler secondary to sample. The current is provided by Vref.
So check you help to check the current source of UCC28C44?
