The TPS54538 incorporates both peak and valley inductor current limits to provide protection to the device from overloads and short circuits and limit the maximum output current. Valley current limit prevents inductor current runaway during short circuits on the output, while both peak and valley limits work together to limit the maximum output current of the converter. Hiccup mode is also incorporated for sustained short circuits.

The high-side switch current is sensed when turned on after a set blanking time (t_{ON_MIN}), the peak current of high-side switch is limited by the peak current threshold, I_{HS_LIMIT}. The current going through low-side switch is also sensed and monitored. When the low-side switch turns on, the inductor current begins to ramp down.

As the device is overloaded, a point is reached where the valley of the inductor current cannot reach below I_{LS_LIMIT} before the next clock cycle, then the low-side switch is kept on until the inductor current ramps below the valley current threshold, I_{LS_LIMIT}, then the low-side switch is turned off and the high-side switch is turned on after a dead time. When this action occurs, the valley current limit control skips that cycle, causing the switching frequency to drop. Further overload causes the switching frequency to continue to drop, but the output voltage remains in regulation. As the overload is increased, both the inductor current ripple and peak current increase until the high-side current limit, I_{HS_LIMIT}, is reached. When this limit is tripped, the switch duty cycle is reduced and the output voltage falls out of regulation. This action represents the maximum output current from the converter and is given approximately by Equation 12. The output voltage and switching frequency continue to drop as the device moves deeper into overload while the output current remains at approximately I_OMAX. There is another situation, if the inductor ripple current is large, the high-side current limit can be tripped before the low-side limit is reached. In this case, Equation 13 gives the approximate maximum output current.

Furthermore, if a severe overload or short circuit causes the FB voltage to fall below the V_UVP threshold, 65% of the V_REF, and triggering current limit, and the condition occurs for more than the hiccup on time (typical 256μs), the converter enters hiccup mode. In this mode, the device stops switching for hiccup off time, 10.5 × t_SS, and then goes to a normal restart with soft-start time. If the overload or short-circuit condition remains, the device runs in current limit and then shuts down again. This cycle repeats as long as the overload or short-circuit condition persists. This mode of operation reduces the temperature rise of the device during a sustained overload or short circuit condition on the output. Once the output short is removed, the output voltage recovers normally to the regulated value.

For FCCM version, the inductor current is allowed to go negative. When this current exceed the LS negative current limit I_{LS_NEG}, the LS switch is turned off and HS switch is turned on immediately, which is used to protect the LS switch from excessive negative current.