© 2007 Fairchild Semiconductor
Page 5
of 17
Rev 1.1 April 2007
www.fairchildsemi.com
Table 1: Jumper JP1/JP2 Status and LED Current
Refer to the schematic in Figure 3. As the LED current returns back to the power supply, it develops a
voltage across the sense resistor(s). Once this voltage reaches the base-emitter junction voltage of the
NPN Q2, typically about 650mV, the transistor will conduct collector current. This current also flows
through the photodiode inside the optocoupler, IC2. The optocoupler will transfer this feedback
information optically across the primary-secondary boundary to the primary side of the supply
thereby maintaining the necessary isolation. Thermistor RTH1, is a negative temperature coefficient
device and compensates for changes in base-emitter voltage due to temperature change.
LED Current
JP1 Status
JP2 Status
350mA
Out
Out
700mA
In
Out
1A
In
In
