даташит
https://www.fairchildsemi.com/datasheets/FS/FSDM311.pdf
2 pin Vcc
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 Internal Block Diagram section). When VCC reaches the
UVLO upper threshold (9V), the internal start-up switch opens and device
power is supplied by the auxiliary transformer winding.
3 pin Vfb
Feedback. Inverting input to the PWM comparator with its normal input level
between 0.5V and 2.5V. It has a 0.4mA current source connected internally,
while a capacitor and opto-coupler are typically connected externally. A
feedback voltage of 4.5V triggers overload protection (OLP). There is a time
delay while charging the 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.
5 pin V str
Start-up. This pin connects directly to the rectified AC line voltage source. At
start-up, the internal switch supplies internal bias and charges an external
storage capacitor placed between the VCC pin and ground. Once the VCC
reaches 9V, the internal switch stops charging the capacitor.
схема
FSP550-80GLN
