The LMR33640 incorporates both peak and valley inductor current limit to provide protection to the device from overloads and short circuits and limit the maximum output current. Valley current limit prevents inductor current run-away during short circuits on the output, while both peak and valley limits work together to limit the maximum output current of the converter. Cycle-by-cycle current limit is used for overloads while hiccup mode is used for sustained short circuits. Finally, a zero current detector is used on the low-side power MOSFET to implement DEM at light loads (see the Glossary). The typical value of this current limit is found under I_ZC in Section 7.5.

When the device is overloaded, the valley of the inductor current may not reach below I_LIMIT, (see Section 7.5) before the next clock cycle. When this occurs, the valley current limit control skips that cycle, causing the switching frequency to drop. Further overload causes the switching frequency to continue to drop and the inductor ripple current to increase. When the peak of the inductor current reaches the high-side current limit, I_SC (see Section 7.5), the switch duty cycle is reduced and the output voltage falls out of regulation. This represents the maximum output current from the converter and is given approximately by Equation 1.

Equation 1.

If during current limit, the voltage on the FB input falls below about 0.4 V due to a short circuit, the device enters into hiccup mode. In this mode, the device stops switching for t_HC (see the System Characteristics section), or about 94 ms, and then goes through a normal re-start with soft start. If the short-circuit condition remains, the device runs in current limit for about 20 ms (typical) and then shuts down again. This cycle repeats, as shown in Figure 8-5, as long as the short-circuit condition persists. This mode of operation reduces the temperature rise of the device during a hard short on the output. The output current is greatly reduced during hiccup mode (see Section 7.8). Once the output short is removed and the hiccup delay is passed, the output voltage recovers normally as shown in Figure 8-6.

Figure 8-7 shows the overall output voltage versus the output current characteristic.

Figure 8-5 Inductor Current Burst in Short-Circuit Mode

Figure 8-6 Short-Circuit Transient and Recovery V_IN = 12 V, V_OUT = 5 V

Figure 8-7 Output Voltage versus Output Current in Current Limit