7.3.7 Current Limit

The LM25119 contains a current limit monitoring scheme to protect the regulator from possible overcurrent conditions. When set correctly, the emulated current signal is proportional to the buck switch current with a scale factor determined by the current limit sense resistor, R_S, and current sense amplifier gain. The emulated signal is applied to the current limit comparator. If the emulated ramp signal exceeds 1.2 V, the present cycle is terminated (cycle-by-cycle current limiting). Shown in Figure 10 is the current limit comparator and a simplified current measurement schematic. In applications with small output inductance and high input voltage, the switch current may overshoot due to the propagation delay of the current limit comparator. If an overshoot must occur, the sample-and-hold circuit detects the excess recirculating current before the buck switch is turned on again. If the sample-and-hold DC level exceeds the internal current limit threshold, the buck switch is disabled and skip pulses until the current has decayed below the current limit threshold. This approach prevents current runaway conditions due to propagation delays or inductor saturation because the inductor current is forced to decay to a controlled level following any current overshoot.

Figure 10. Current Limit and Ramp Circuit