Implementing the TL431 feedback loopImplementing the TL431 feedback loop Christophe BASSO MOTOROLA SPS, Toulouse Center Avenue Eisenhower, BP1029, 31023 TOULOUSE Cedex France 33 (0)5 61 19 90 12; e-mail: R38010@email.sps.mot.com This paper details the numerous ways to implement a feedback network with an optocoupler and a TL431 when implementing shunt regulators. Depending on the configuration of the devices and the method used to measure the open-loop response, some unusual results may appear. The shunt regulator The optocoupler is used alone and delivers some current to a shunt regulator such as implemented by the MC3337X series or the MC44608. In these devices, the duty-cycle DC is adjusted by injecting a current into a feedback pin (FB). When the current is low or zero, the duty-cycle is pushed to the max (74% for the M……