The error amplifier output is an open collector in parallel with a current source. With a low output resistance, various impedance networks may be used on the compensation pin input for error amplifier feedback. The error amplifier output, COMP, is frequently used as a control port for secondary-side regulation by using an external secondary-side adjustable voltage regulator, such as a TL431, to send an error signal across the secondary-to-primary isolation boundary through an opto-isolator, in this configuration connect the COMP pin directly to the opto-isolator feedback. On the primary side, the inverting input to the UCx48x error amplifier, VFB, should be connected to GROUND. With VFB tied to GROUND, the error amplifier output, COMP, is forced to its high state and sources current, typically 0.8 mA. The opto-isolator must overcome the source current capability to control the COMP pin below the error amplifier output high level, VOH.
For primary-side regulation, configure the inverting input to the error amplifier, VFB, with a resistor divider to provide a signal that is proportional to the converter output voltage being regulated. Add the voltage loop compensation components between VFB and COMP. The internal noninverting input to the error amplifier is trimmed to 2.5 V. For best stability, keep VFB lead length as short as possible and minimize the stray capacitance on VFB.
The internal resistor divider on COMP is maintained at an R:2R ratio, the specific values of these internal resistors should not be critical in any application.