7.3.6 Current Limit

The LM5008 contains an intelligent current limit off-timer. If the current in the buck switch exceeds 0.5 A the present cycle is immediately terminated, and a non-resetable off-timer is initiated. The length of off-time is controlled by an external resistor (R_CL) and the FB voltage (see Figure 4). 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 zero volts (not a short circuit), the current limit off-time will be less than 35 µs. Reducing the off-time during less severe overloads reduces the amount of foldback, recovery time, and the start-up time. The off-time is calculated from Equation 5.

Equation 5.

The current limit sensing circuit is blanked for the first 50-70 ns of each on-time so it is not falsely tripped by the current surge which occurs at turnon. The current surge is required by the re-circulating diode (D1) for its turnoff recovery.