7.3.3 Current Protection

There are three kinds of current protection in TPS563249: High-side FET source current limit, low-side FET source current limit, and low-side FET sink current limit.

The output over-current 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 low-side FET switch, the inductor current flow through low-side FET and decreases linearly. The average value of the inductor current is the load current I_OUT. If the monitored current is above the low-side FET source current limit level, the converter maintains low-side FET on and delays the creation of a new set pulse, even the voltage feedback loop requires one, until the current cross the low-side FET source current limit level. 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 over-current protection. The load current is higher than the over-current 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. This may cause the output voltage to fall. When the VFB voltage falls below the UVP threshold voltage, the UVP comparator detects it. And then, the device will shut down after the UVP delay time (typically 0.36 ms) and re-start after the hiccup time (typically 25 ms).

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

During the on time of the high-side FET 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 switch current is compared with high-side FET source current limit after a short blanking time. If the cross-limit event detected before the one shot timer expires, the high-side FET is turn off immediately, and is not allowed on in the following 1 µS period.

TPS563249 works in Forced Continuous Conduction Mode (FCCM). To support light load operation, the current flowing through low-side FET is allowed to be negative, which means the current flow from drain to source of low-side FET. This negative current is compared with low-side FET sink current limit to prevent device from being over-current damaged. Once the sink current cross limit, the low-side FET is turn off immediately. Both high-side FET and low-side FET will keep off until the VFB voltage falls below reference voltage.