7.3.10 Current Sense and Overcurrent Protection

TPS53219A 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, TPS53219A 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 I_TRIP current, 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 6.

Equation 6.

The inductor current is monitored by the voltage between GND pin and SW pin so that SW pin should be connected to the drain terminal of the low-side MOSFET properly. I_TRIP has 4700-ppm/°C temperature slope to compensate the temperature dependency of the R_DS(on). The GND pin is used as the positive current-sensing node. The GND pin should be connected to the proper current sensing device, (for example, the source terminal of the low-side MOSFET.)

As the comparison is done 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 7.

Equation 7.

In an overcurrent condition, the current to the load exceeds the current to the output capacitor thus the output voltage tends to fall down. 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.

During the CCM, the negative current limit (NCL) protects the external FET from carrying too much current. The NCL detect threshold is set as the same absolute value as positive OCL but negative polarity.