8.4.13.1 Low-Side MOSFET Overcurrent Protection

The TPS543B20 utilizes ILIM pin to set the OCP level. The ILIM pin should be connected to AGND through the ILIM voltage setting resistor, RILIM. The ILIM terminal sources IILIM current, which is around 11.2 μA typically at room temperature, and the ILIM level is set to the OCP ILIM voltage VILIM as shown in Equation 2. In order to provide both good accuracy and cost effective solution, TPS543B20 supports temperature compensated MOSFET R_DS(on) sensing.

Equation 2.

Consider R_DS(on) variation vs VDD in calculation

Also, TPS543B20 performs both positive and fixed negative inductor current limiting.

The inductor current is monitored by the voltage between GND pin and SW pin during the OFF time. ILIM has 1200 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 device has cycle-by-cycle over-current limiting control. The inductor current is monitored during the OFF state and the controller maintains the OFF state during the period that the inductor current is larger than the overcurrent ILIM level. V_ILIM sets the Peak level of the inductor current. Thus, the load current at the overcurrent threshold, I_OCP, can be calculated as shown in Equation 3.

Equation 3.

where

• R_DS(on) is the on-resistance of the low-side MOSFET.

Equation 3 is valid for VDD ≥ 5 V. Use 1.58 mΩ for R_DS(on) in calculation, which is the pure on-resistance for current sense.

If an overcurrent event is detected in a given switching cycle, the device increments an overcurrent counter. When the device detects three consecutive overcurrent (either high-side or low-side) events, the converter responds, entering continuous restart hiccup. In continuous hiccup mode, the device implements a 7 soft-start cycle timeout, followed by a normal soft-start attempt. When the overcurrent fault clears, normal operation resumes; otherwise, the device detects overcurrent and the process repeats.