7.3.6 Current Limit

The LM5163 manages overcurrent conditions with cycle-by-cycle current limiting of the peak inductor current. The current sensed in the high-side MOSFET is compared every switching cycle to the current limit threshold (0.75 A). To protect the converter from potential current runaway conditions, the LM5163 includes a foldback valley current limit feature, set at 0.6 A, that is enabled if a peak current limit is detected. As shown in Figure 11, if the peak current in the high-side MOSFET exceeds 0.75 A (typical), the present cycle is immediately terminated regardless of the programmed on-time (t_ON), the high-side MOSFET is turned off and the foldback valley current limit is activated. The low-side MOSFET remains on until the inductor current drops below this foldback valley current limit, after which the next on-pulse is initiated. This method folds back the switching frequency to prevent overheating and limits the average output current to less than 0.75 A to ensure proper short-circuit and heavy-load protection of the LM5163.

Figure 11. Current Limit Timing Diagram

Current is sensed after a leading-edge blanking time following the high-side MOSFET turnon transition. The propagation delay of the current limit comparator is 100 ns. During high step-down conditions when the on-time is less than 100 ns, a back-up peak current limit comparator in the low-side FET also set at 0.75 A enables the foldback valley current limit set at 0.6 A. This innovative current limit scheme enables ultra-low duty-cycle operation, permitting large step-down voltage conversions while ensuring robust protection of the converter.