7.3.5 Overcurrent Protection

The LM5007 contains an intelligent current limit off-timer intended to reduce the foldback characteristic inherent with fixed off-time overcurrent protection (OCP) schemes. If the current in the buck switch exceeds 725 mA, the present cycle on-time is immediately terminated (cycle-by-cycle current limit). Following the termination of the cycle a non-resettable current limit off-timer is initiated. The duration of the off-time is a function of the external resistor (R_CL) and the FB voltage. When the FB voltage equals zero, the current limit off-time is internally preset to 17 µs. This condition occurs during a short-circuit condition when a maximum amount of off-time is required.

In case of output overload (not a complete short circuit), the current limit off-time is reduced as a function of the output voltage (measured at the FB pin). Scaling the off-time with smaller overloads reduces the amount of foldback and also reduces the initial start-up time. Calculate the current limit off-time for a given FB voltage and R_CL resistor using Equation 5.

Equation 5.

Select the current limit off-time such that it is less than the MOSFET off-time during normal steady-state switching operation. Applications utilizing low-resistance inductors and/or a low-voltage-drop freewheeling power diodes may require special evaluation at high line, short-circuited conditions. In this special case the preset 17-µs off-time (V_FB = 0 V) may be insufficient to provide inductor volt-seconds balance. Additional inductor resistance, output resistance or a larger voltage drop diode may be necessary to balance inductor volt-seconds and limit the short-circuit current.