TPS53355 has cycle-by-cycle overcurrent limiting control. The inductor current is monitored during the OFF state and the controller maintains the OFF state during the period in that the inductor current is larger than the overcurrent trip level. In order to provide both good accuracy and cost effective solution, TPS53355 supports temperature compensated MOSFET R_DS(on) sensing. The TRIP pin should be connected to GND through the trip voltage setting resistor, R_TRIP. The TRIP terminal sources current (I_TRIP) which is 10 μA typically at room temperature, and the trip level is set to the OCL trip voltage V_TRIP as shown in Equation 1.

Equation 1.

The inductor current is monitored by the LL pin. The GND pin is used as the positive current sensing node and the LL pin is used as the negative current sense node. The trip current, I_TRIP has 4700ppm/°C temperature slope to compensate the temperature dependency of the R_DS(on).

As the comparison is made during the OFF state, V_TRIP sets the valley level of the inductor current. Thus, the load current at the overcurrent threshold, I_OCP, can be calculated as shown in Equation 2.

Equation 2.

In an overcurrent or short circuit condition, the current to the load exceeds the current to the output capacitor thus the output voltage tends to decrease. Eventually, it crosses the undervoltage protection threshold and shuts down. After a hiccup delay (16 ms with 0.7 ms sort-start), the controller restarts. If the overcurrent condition remains, the procedure is repeated and the device enters hiccup mode.

Hiccup time calculation:

where

• n = 8, 9, 10, or 11 depending on soft start time selection