Because the controller employs fixed-point sampling for output voltage sensing, there are some transformer design constraints that must be observed. The minimum magnetizing volt-seconds during the on-time interval occurs at the minimum CS pin voltage, VCS(min), under light-load conditions. This minimum should be the case at all line voltages, because the controller compensates for line-dependent peak-current overshoot during turn-off delay. The choice of transformer turns ratio, transformer inductance (LPRI), and current sense resistance (RCS) must ensure that the corresponding reset volt-seconds during the flyback interval are sufficient that a valid output sample is available at the sample point, tOUT(smp). This constraint is summarized in Equation 15.
Additionally, the device requires a minimum on-time, tON(min) , to ensure enough time for the system input voltage (VIN) and switch current (ISW ) to be measured. To meet the minimum on-time requirement at maximum line, and minimum load, the ratio of current sense resistance (RCS) to transformer inductance (LPRI) must meet the constraint shown in Equation 16.