Current-mode control requires the addition of a compensation ramp to the sensed inductive (transformer or inductor) current for stability at duty cycles near and over 50%. The TPS23754 device has a maximum duty cycle limit of 78%, permitting the design of wide input-range flyback and active clamp 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 PI range. The TPS23754 device provides a fixed internal compensation ramp that suffices for most applications.
The TPS23754 device provides internal, frequency independent, slope compensation (150 mV, VSLOPE) to the PWM comparator input for current-mode control-loop stability. This voltage is not applied to the current limit comparator whose threshold is 0.55 V (VCSMAX). If the provided slope is not sufficient, the effective slope may be increased by addition of RS per Figure 31. The additional slope voltage is provided by (ISL-EX × RS). There is also a small DC offset caused by the about 2.5-μA pin current. The peak current limit does not have duty cycle dependency unless RS is used. This makes it easier to design the current limit to a fixed value. See Current Slope Compensation for more information.
The internal comparators monitoring CS are isolated from the IC pin by the blanking circuits while GATE is low, and for a short time (blanking period) just after GATE switches high. A 440 Ω (maximum) equivalent pulldown on CS is applied while GATE is low.