Single transistor provides short-circuit protection

In certain dc/dc-converter applications, on-chip, cycle-by-cycle current limit could be insufficient protection to prevent a Failure during a short circuit. A nonsynchronous boost converter provides a direct path from the input to the short circuit through the inductor and the C Atch diode. Regardless of Current-limit Protection in the IC, when a short circuit exists in the load, extremely high currents that FL ow through the load path can damage the catch diode, the inductor, and the IC. In a SEPIC (single-ended, primary-inductance-converter) circuit, the coupling capacitor breaks this path. Thus, when a short circuit exists in the load, no-direct path exists for current-to-flow from input to output. However, if the required minimum on-time is less than the application-specific duty cycle, the inductor and, thus, the SWI Tch current can rapidly increase, causing IC failure, input-supply overload, or both. Even in certain buck-regulator applications, duty-cycle limitations sometimes keep the switch on too long to maintain control during a output short-circuit condition, especially at very high input voltage W ith extremely high-frequency ICs. A single-transistor Approach protects the SEPIC circuit from short-circuit fault conditions by pulling down the V_{c pin (the output of the error amplifier) when the inductor current starts to run away Dur ing an overload or short circuit in the load (Figure 1).}

Pulling the V_{C pin low forces the IC to stop switching, skipping minimum on-time switch cycles, and allowing the Curre NT in each inductor to ramp down. During A short circuit, the peak current in L1, which decreases because of the limited number of switch cycles, and th E Peak Current in L2 sum-to-equal the peak current in the switch, which are less than the 1.5A limit of the Lt1961em s8e. Figure 2 shows the short-circuit input and output currents at different input voltages. Figure 3 shows the maximum load current versus input voltage. The average current in L 2 are equal to the load current and area maximum of the under all load conditions. If the sense resistor sees, it knows, a overload condition have occurred and tells the transistor to protect th E circuit.}

Single transistor provides short-circuit protection