SLUSDB2A August 2018 – December 2021 UCC28951
The cycle-by-cycle current limit provides peak current limiting on the primary side of the converter when the load current exceeds its predetermined threshold. For peak current mode control, a certain leading edge blanking time is needed to prevent the controller from false tripping due to switching noise. An internal 30-ns filter at the CS input is provided. The total propagation delay TCS from CS pin to outputs is 100 ns. An external RC filter is still needed if the power stage requires more blanking time. The 2.0-V ±3% cycle-by-cycle current limit threshold is optimized for efficient current transformer based sensing. The duration when a converter operates at cycle-by-cycle current limit depends on the value of soft-start capacitor and how severe the overcurrent condition is. This is achieved by the internal discharge current IDS Equation 16 and Equation 17 at SS pin.
The soft-start capacitor value also determines the so-called hiccup mode off-time duration. The behavior of the converter during different modes of operation, along with related soft-start capacitor charge and discharge currents are shown in Figure 7-15.
The largest discharge current of 20 µA is when the duty cycle is close to zero. This current sets the shortest operation time during the cycle-by-cycle current limit and is defined in Equation 18 and Equation 19
Thus, if the soft-start capacitor CSS = 100 nF is selected, then the TCL(on) time is 5 ms.
With the same soft-start capacitor value at 100 nF, the off-time before the restart is 122 ms. If the overcurrent condition occurs before the soft-start capacitor voltage reaches the 3.7-V threshold during start-up, the controller limits the current but the soft-start capacitor continues to be charged. As soon as the 3.7-V threshold is reached, the soft-start voltage is quickly pulled up to the 4.65-V threshold by an internal 1-kΩ RDS(on) switch and the cycle-by-cycle current limit duration timing starts by discharging the soft-start capacitor. Depending on specific design requirements, the user can override this default behavior by applying external charge or discharge currents to the soft-start capacitor. The whole cycle-by-cycle current limit and hiccup operation is shown in Figure 7-15. In this example, the cycle-by-cycle current limit lasts about 5 ms followed by 122 ms of off-time.
Similarly to the overcurrent condition, the hiccup mode with the restart can be disabled by the user if a pullup resistor of 261 k Ω is connected between the SS and VREF pins. The controller remains in the latch-off mode if an overcurrent condition occurs. In this case, calculate an external soft-start capacitor value with the additional pullup current taken into account. The latch-off mode can be reset externally if the soft-start capacitor is forcibly discharged below 0.55 V or the VDD voltage is lowered below the UVLO threshold.