8.1.4 Load Transient

A load-transient event can trigger the internal overcharge protection circuit on the LDO_IN pin. This condition prevents C_{LDO_IN} from overcharging when a heavy load is abruptly removed. The overvoltage protection circuit engages and prevents the switched capacitors from switching until the excess charge on C_{LDO_IN} is discharged into the load. This protection behavior occurs most often during heavy load-transient events on devices with higher output voltages. The value of the C_{LDO_IN} capacitor and the load current determine how long the overvoltage protection circuit remains engaged. Figure 21 shows the overvoltage protection circuit behavior after the load is removed without tripping the PG signal.

Figure 21. Overvoltage Protection Circuit Behavior for a 5.0-V Output Voltage Device During Load Transient

As illustrated in Figure 22, a load-transient event that exceeds the maximum output current can disable the output when the heavy load pulls down the V_{LDO_IN} voltage below the V_{UVLO_LDO_IN} falling threshold. If the application is prone to heavy load-transient events as illustrated in Figure 22, increase the C_{LDO_IN} capacitor value as necessary. However, as illustrated in Figure 20, too large of a C_{LDO_IN} leads to a longer startup time.

VAC = 120 VRMS at 60 Hz, FB, CLDO_IN = 10 µF, VLDO_OUT = 3.3 V, IOUT = 1 mA to 600 mA, current slew = 1 A/µs

Figure 22. Heavy Load-Transient Event Triggering a Restart

VAC = 120 VRMS at 60 Hz, FB, CLDO_IN = 56 µF, VLDO_OUT = 3.3 V, IOUT = 1 mA to 600 mA, current slew = 1 A/µs

Figure 23. Heavy Load-Transient Event Without Triggering a Restart