As shown in the Optocoupler Feedback Signal Input and Bias section, the FBreplica decreases while Iopto increases. When Iopto reaches the value of IFB, the FB pin voltage starts to drop because there is no current flow through the PMOS. FB pin pulled low will impact the system transient response, due to the extra delay introduced by charging the parasitic capacitor of the optocoupler to pull up the FB pin voltage. A FB pin voltage clamp circuit is used to prevent this scenario. When FB pin voltage drops below the FB pin clamp voltage threshold, an extra current source is turned on to clamp the FB voltage. The clamp strength is IFBclamp. The FB pin clamp circuit improves the system transient performance from light load to heavy load.