SLUSBL5A February 2015 – June 2019 UCC28730
Timing information at the VS pin and current information at the CS pin allow accurate regulation of the secondary average current. The control law dictates that as power is increased in CV regulation and approaching CC regulation the primary-peak current will be at IPP(max). Referring to Figure 17 below, the primary-peak current, turns ratio, secondary demagnetization time (tDM), and switching period (tSW) determine the secondary average output current. Ignoring leakage inductance effects, the average output current is given by Equation 6.
When the average output current reaches the CC regulation reference in the current control block, the controller operates in frequency modulation mode to control the output current, IOCC, at any output voltage down to or below the minimum operating voltage target, VOCC (as seen in Figure 18), as long as the auxiliary winding can keep VDD voltage above the UVLO turn-off threshold. When VO falls so low that VDD cannot be sustained above UVLO, the device shuts down.