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ZHCSIQ7B August 2018 – October 2024 UCC28950 , UCC28951

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PW|24
- MPDS363A

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6.3.14 Cycle-by-Cycle Current Limit Current Protection and Hiccup Mode

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 30ns filter at the CS input is provided. The total propagation delay TCS from CS pin to outputs is 100ns. An external RC filter is still needed if the power stage requires more blanking time. The 2.0V ±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 I_DS Equation 16 and Equation 17 at SS pin.

Equation 16.

I_{D S (l e a d e r)} = (- 25 \times (1 - D) + 5) µ A

Equation 17.

I_{D S (f o l l o w e r)} = (- 25 \times (1 - D)) µ A

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 6-15.

UCC28950 UCC28951 Timing
Diagram of Soft-Start Voltage VSS

Figure 6-15 Timing Diagram of Soft-Start Voltage V_SS

The largest discharge current of 20µA 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

Equation 18.

T_{C L (o n_l e a d e r)} = \frac{C_{S S} \times (4.65 V - 3.7 V)}{20 µ A}

Equation 19.

T_{C L (o n_f o l l o w e r)} = \frac{C_{S S} \times (4.65 V - 3.7 V)}{25 µ A}

Thus, if the soft-start capacitor C_SS = 100nF is selected, then the T_CL(on) time is 5ms.

To calculate the hiccup off time T_CL(off) before the restart, use Equation 20 or Equation 21.

Equation 20.

T_{C L (o f f_l e a d e r)} = \frac{C_{S S} \times (3.6 V - 0.55 V)}{2.5 µ A}

Equation 21.

T_{C L (o f f_f o l l o w e r)} = \frac{C_{S S} \times (3.6 V - 0.55 V)}{4.9 µ A}

With the same soft-start capacitor value at 100nF, the off-time before the restart is 122ms. If the overcurrent condition occurs before the soft-start capacitor voltage reaches the 3.7V threshold during start-up, the controller limits the current but the soft-start capacitor continues to be charged. As soon as the 3.7V threshold is reached, the soft-start voltage is quickly pulled up to the 4.65V threshold by an internal 1kΩ R_DS(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 6-15. In this example, the cycle-by-cycle current limit lasts about 5ms followed by 122ms 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 261kΩ 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 0.55V or the V_DD voltage is lowered below the UVLO threshold.