The TPS51397A has overcurrent protection and undervoltage protection. The output overcurrent limit (OCL) is implemented using a cycle-by-cycle valley detect control circuit. The switch current is monitored during the OFF state by measuring the low-side FET drain-to-source voltage. This voltage is proportional to the switch current. To improve accuracy, the voltage sensing is temperature compensated.

During the on-time of the high-side FET switch, the switch current increases at a linear rate determined by Vin, Vout, the on-time, and the output inductor value. During the on-time of the low-side FET switch, this current decreases linearly. The average value of the switch current is the load current I_OUT. If the monitored current is above the OCL level, the converter maintains low-side FET on and delays the creation of a new pulse, even the voltage feedback loop requires one, until the current level becomes OCL level or lower. In subsequent switching cycles, the on-time is set to a fixed value and the current is monitored in the same manner.

There are some important considerations for this type of overcurrent protection. When the load current is higher than the overcurrent threshold by one half of the peak-to-peak inductor ripple current, the OCL is triggered and the output current is being limited, the output voltage tends to drop because the load demand is higher than what the converter can support. When the output voltage falls below 60% of the target voltage, the UVP comparator detects it, and the device is shut off after a wait time of 256 μs. This protection is a latched function. The fault latching can be reset by EN going low or VCC power cycling.

The TPS51397A also implements negative overcurrent protection, which can prevent inductor current runaway when IC works in OOA mode. When the inductor valley current hits the negative overcurrent threshold (NOCL = –3.2 A typical), the low-side FET turns off, then high-side FET turns on.