To further protect the external circuitry during a prolonged overcurrent condition, the LM25088-2 provides a current limit timer to disable the switching regulator and provide a delay before restarting (hiccup mode). The number of current limit events required to trigger the restart mode is programmed by an external capacitor at the RES pin. During each PWM cycle, as shown in Figure 7-8, the LM25088 either sinks current from or sources current into the RES capacitor. If the emulated current ramp exceeds the 1.2-V current limit threshold, the present PWM cycle is terminated and the LM25088 sources 50 µA into the RES pin capacitor during the next PWM clock cycle. If a current limit event is not detected in a given PWM cycle, the LM25088 disables the 50-µA source current and sinks 27 µA from the RES pin capacitor during the next cycle. In an overload condition, the LM25088 protects the converter with cycle-by-cycle current limiting until the voltage at RES pin reaches 1.2 V. When RES reaches 1.2 V, a hiccup mode sequence is initiated as follows:

• The SS capacitor is fully discharged.
• The RES capacitor is discharged with 1.2 µA.
• Once the RES capacitor reaches 0.2 V, a normal soft-start sequence begins. This provides a time delay before restart.
• If the overload condition persists after restart, the cycle repeats.
• If the overload condition no longer exists after restart, the RES pin is held at ground by the 27-µA discharge current source and normal operation resumes.

The overload protection timer is very versatile and can be configured for the following modes of protection:

1. Cycle-by-Cycle only: The hiccup mode can be completely disabled by connecting the RES pin to GND. In this configuration, the cycle-by-cycle protection limits the output current indefinitely and no hiccup sequence occurs.
2. Delayed Hiccup: Connecting a capacitor to the RES pin provides a programmed number of cycle-by-cycle current limit events before initiating a hiccup mode restart, as previously described. The advantage of this configuration is that a short term overload does not cause a hiccup mode restart, but during extended overload conditions, the average dissipation of the power converter is very low.
3. Externally Controlled Hiccup: The RES pin can also be used as an input. By externally driving the pin to a level greater than the 1.2-V hiccup threshold, the controller is forced into the delayed restart sequence. For example, the external trigger for a delayed restart sequence can come from an overtemperature protection or an output overvoltage sensor.

Figure 7-7 Current Limit Restart Circuit

Figure 7-8 Current Limit Restart Timing Diagram