SLUSE25 September 2020 – MONTH TPS563207
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 Iout. If the monitored current is above the OCL level, the converter keeps the low-side FET on and delays the creation of a new set 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. 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 can be higher than the current available from the converter. This can cause the output voltage to fall. When the FB voltage falls below the UVP threshold voltage, the UVP comparator detects it. The device will then shut down after the UVP delay time (typically 24 µs) and re-start after the hiccup time (typically 18.3 ms).
When the overcurrent condition is removed, the output voltage returns to the regulated value.
The TPS563207 also implements the negative overcurrent protection which can prevent inductor current run away. When the inductor valley current hits the negative overcurrent threshold, the low-side FET will turn off, then high-side FET will turn on.