The optocoupler collector voltage
maintained at approximate constant voltage (~5.6 V) when FB sourcing current is
less than IFB. Due to this, no extra pole introduced due to the optocoupler
parasitic capacitor
The FB clamp circuit is to
provide extra 82uA current to the FB pin when needed to prevent FB voltage drop
and to maintain good transient performance
The Optocoupler current is
reflected as the FBreplica in the IC for control purpose
Voltage at the RFB is given by
FBreplica = (IFB-Ioptocoupler) *RFB where RFB typical value is 100 kohm and IFB
typical values are 82 uA (for 402, 404 devices) and 164 uA (403, 403A devices).
If the current through
optocoupler is zero during the heavy output load, FB pin voltage will be clamped
to internal whereas FBreplica is clamped to 6 V.