7.3.6 Overcurrent and Undervoltage Protection

The TPS56339 is protected from overcurrent conditions by cycle-by-cycle current limiting on both the peak and valley of the inductor current.

During the on time of the high-side MOSFET switch, the inductor current flow through high-side FET and increases at a linear rate determined by V_IN, V_OUT, the on-time and the output inductor value. The high-side switch current is sensed when the high-side is turned on after a set blanking time and then compared with the high-side MOSFET current limit every switching cycle. If the cross-limit event detected after the minimum On-time, the high-side MOSFET is turned off immediately and the high-side MOSFET current is limited by a clamped maximum peak current threshold I_{HS_LIMIT} which is constant.

The current going through low-side MOSFET is also sensed and monitored. When the low-side MOSFET turns on, the inductor current begins to ramp down. The low-side MOSFET is not turned OFF at the end of a switching cycle if its current is above the low-side current limit I_{LS_LIMIT}. The low-side MOSFET is kept ON for the next cycle so that inductor current keeps ramping down, until the inductor current ramps below the low-side current limit I_{LS_LIMIT} and the subsequent switching cycle comes, the low-side MOSFET is turned OFF, and the high-side MOSFET is turned on after a dead time.

There are some important considerations for this type of overcurrent protection. The load current is higher than the overcurrent threshold by one half of the peak-to-peak inductor ripple current. Also, when the current is being limited, the output voltage tends to fall as the demanded load current may be higher than the current available from the converter. When the V_FB voltage falls below the UVP threshold voltage, the UVP comparator detects it. The device will shut down after the UVP delay time (typically 120 μs) and re-start after the hiccup time (typically 38 ms). The hiccup mode helps to reduce the device power dissipation under severe overcurrent conditions.

When the over current condition is removed, the output voltage returns to the regulated value.