Current-mode control requires addition of a compensation ramp to the sensed inductor (flyback transformer) current for stability at duty cycles near and over 50%. The TPS23753A has a maximum duty cycle limit of 78%, permitting the design of wide input-range flyback converters with a lower voltage stress on the output rectifiers. While the maximum duty cycle is 78%, converters may be designed that run at duty cycles well below this for a narrower, 36-V to 57-V range. The TPS23753A provides a fixed internal compensation ramp that suffices for most applications. R_S (see Figure 8-8) may be used if the internally provided slope compensation is not enough. It works with ramp current (I_PK = I_SL-EX, approximately 40 μA) that flows out of the CS pin when the MOSFET is on. The I_PK specification does not include the approximately 3-μA fixed current that flows out of the CS pin.

Most current-mode control papers and application notes define the slope values in terms of V_PP/T_S (peak ramp voltage / switching period); however, Electrical Characteristics: Controller Section Only specifies the slope peak (V_SLOPE) based on the maximum duty cycle. Assuming that the desired slope, V_SLOPE-D (in mV/period), is based on the full period, compute R_S per Equation 7 where V_SLOPE, D_MAX, and I_SL-EX are from Electrical Characteristics: Controller Section Only with voltages in mV, current in μA, and the duty cycle is unitless (for example, D_MAX = 0.78).

Equation 7.

Figure 8-8 Additional Slope Compensation

C_S may be required if the presence of R_S causes increased noise, due to adjacent signals like the gate drive, to appear at the C_S pin. The TPS23753A has an internal pulldown on C_S ( approximately 400 Ω maximum) while the MOSFET is OFF to reduce cycle-to-cycle carry-over voltage on C_S.