SLUSDW9A June 2020 – June 2020 TPS51215A
In order to provide both cost effective solution and good accuracy, TPS51215A supports MOSFET RDS(on) sensing. For RDS(on) sensing scheme, TRIP pin should be connected to GND through the trip voltage setting resistor, RTRIP. In this scheme, TRIP terminal sources 10µA of ITRIP current (at TJ = 25°C) and the trip level is set to 1/8 of the voltage across the RTRIP. The inductor current is monitored by the voltage between the GND pin and the SW pin so that the SW pin is connected to the drain terminal of the low-side MOSFET. ITRIP has a 4700ppm/°C temperature slope to compensate the temperature dependency of the RDS(on). GND is used as the positive current sensing node so that GND should be connected to the sense resistor or the source terminal of the low-side MOSFET.
TPS51215A has cycle-by-cycle overcurrent limiting protection. The inductor current is monitored during the off-state and the controller maintains the off-state when the inductor current is larger than the overcurrent trip level. The overcurrent trip level, VOCTRIP, is determined by Equation 4.
Because the comparison is made during the off-state, VOCTRIP sets the valley level of the inductor current. The load current OCL level, IOCL, can be calculated by considering the inductor ripple current.
Overcurrent limiting using RDS(on) sensing is shown in Equation 5.
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.