4.3 Current Limit

The MIC26400 uses the R_DS(ON) of the internal low-side power MOSFET to sense over-current conditions. This method will avoid adding cost, board space, and power losses taken by a discrete current sense resistor. The low-side MOSFET is used because it displays much lower parasitic oscillations during switching than the high-side MOSFET.

In each switching cycle of the MIC26400 converter, the inductor current is sensed by monitoring the low-side MOSFET in the OFF period. If the inductor current is greater than the current limit threshold, then the MIC26400 turns off the high-side MOSFET and a soft-start sequence is triggered. This mode of operation is called “hiccup mode” and its purpose is to protect the downstream load in case of a hard short. The current limit threshold has a foldback characteristic related to the feedback voltage, as shown in Figure 4-3.