8.3.6 Switch Current Limit / Short Circuit Protection

The TPS6284x integrates a current limit on the high-side as well as on the low-side MOSFETs to protect the device against overload or short circuit conditions. The current in the switches is monitored cycle-by-cycle. If the high-side MOSFET current limit (I_LIMF) trips, the high-side MOSFET is turned off and the low-side MOSFET is turned on to ramp the inductor current down. Once the inductor current decreases below the low-side current limit (I_LIMF), the low-side MOSFET turns off and the high-side MOSFET turns on again.

During soft start, the current limit is reduced to I_{LIMF_SS}. After soft start has finished, the current limit value increases to the normal value I_LIMF.

Due to internal propagation delay, the actual inductor current can exceed the static current limit during that time. The dynamic current limit can be calculated as follows:

Equation 1.

where

• I_LIMF is the static current limit, specified in Electrical Characteristics
• L is the inductance
• V_L is the voltage across the inductor (V_IN - V_OUT)
• t_{I_LIM_DELAY} is the internal propagation delay

In forced PWM mode, a negative current limit (I_LIMN) is enabled to prevent excessive current flowing backwards to the input. When the inductor current reaches I_LIMN, the low-side MOSFET turns off and the high-side MOSFET turns on and kept on until TON time expires.