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 the following:
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 valley current is above the OCL level, the converter maintains a 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, which can cause the output voltage to fall. When the FB voltage falls below the UVP threshold voltage, the UVP comparator detects it and the device shuts down after the UVP delay time (typically 256 µs) and restarts after the hiccup wait time (typically 13 ms).
When the overcurrent condition is removed, the output voltage returns to the regulated value.
The TPS564247 is a FCCM mode part. In this mode, the device has negative inductor current at light loading. The device has NOC (negative overcurrent) protection to avoid too large negative current. NOC protection detects the valley of inductor current. When the valley value of inductor current exceeds the NOC threshold, the IC turns off the low side then turns on the high side. When the NOC condition is removed, the device returns to normal switching.
Because the TPS564247 is a FCCM mode port, if the inductance is so small that the device trigger NOC, it will cause output voltage to be higher than target value. The minimum inductance is identified as Equation 2.