http://www.alldatasheet.com/datasheet-pdf/pdf/215013/FAIRCHILD/FSDM311A.html Про 2 ножку сказано: Про 3 ножку сказано:

Positive supply voltage input. Although connected to an auxiliary transformer winding, current is supplied from pin 5 (Vstr) via an internal switch during start-up (see the Internal Block Diagram in Figure 2). It is not until VCC reaches the UVLO upper threshold (9V) that the internal start-up switch opens and device power is supplied via the auxiliary transformer winding.

Feedback. Inverting input to the PWM comparator with its normal input level lies between 0.5V and 2.5V. It has a 0.4mA current source connected internally, while a capacitor and optocoupler are typically connected externally. A feedback voltage of 4.5V triggers overload protection (OLP). There is a time delay while charging external capacitor CFB from 3V to 4.5V using an internal 5μA current source. This time delay prevents false triggering under transient conditions, but allows the protection mechanism to operate under true overload conditions.