The LM5009 contains an intelligent current limit off timer. If the current in the buck switch exceeds 0.31 A, then the present cycle is immediately terminated and a non-resettable off timer is initiated. The length of off-time is controlled by an external resistor (R_CL) and the FB voltage. When FB = 0 V, a maximum off-time is required and the time is preset to 35 µs. This condition occurs when the output is shorted and during the initial part of start-up. This amount of time ensures safe short-circuit operation up to the maximum input voltage of 95 V. In cases of overload where the FB voltage is above 0 V (not a short-circuit) the current limit off-time is less than 35 µs. Reducing the off-time during less severe overloads reduces the amount of foldback, recovery time, and start-up time. The off-time is calculated from Equation 5:

Equation 5.

The current limit sensing circuit is blanked for the first 50 ns to 70 ns of each on-time so it is not falsely tripped by the current surge that occurs at turn-on. The current surge is required by the recirculating diode (D1) for its turn-off recovery.