7.4.2.4 Current Slope Compensation and Current Limit

Current-mode control requires addition of a compensation ramp to the sensed inductive (transformer or inductor) current for stability at duty cycles near and over 50%. The TPS23751 and TPS23752 have a maximum duty cycle limit of 80%, permitting the design of wide input-range converters with lower voltage stress on the output rectifiers. While the maximum duty cycle is 80%, converters may be designed that run at duty cycles well below this for a narrower, 36 V to 57 V PI range. The TPS23751 and TPS23752 provide fixed internal slope compensation which suffices for most applications.

The TPS23751 and TPS23752 provide internal, frequency independent, slope compensation (V_PK = 40 mV at D_MAX) starting from D_{SLOPE_ST} to the PWM comparator input for current-mode control-loop stability. This voltage is not applied to the current-limit comparator whose threshold is 0.25 V (V_CSMAX). If the provided slope is not sufficient, the effective slope may be increased by addition of R_S per Figure 32. The additional slope voltage is provided by (I_{CS_RAMP} × R_S). There is also a small dc offset caused by the I_CSDC (approximately 2.0 μA) current. The peak current limit does not have duty cycle dependency unless R_S is used which is easier designing the current limit to a fixed value. See the Current Slope Compensation section for more information.

The internal comparators monitoring CS are isolated from the CS pin by the blanking circuits while GATE is low, and for a short time (blanking period) just after GATE switches high. A 500 Ω (max) equivalent pulldown resistor on CS is applied while GATE is low.