Start-up and shutdown are controlled by the EN input. This input features precision thresholds, allowing the use of an external voltage divider to provide an adjustable input UVLO (see Section 9.2.2.9). Applying a voltage of ≥ V_EN-VCC-H causes the device to enter standby mode, powering the internal VCC, but not producing an output voltage. Increasing the EN voltage to V_EN-OUT-H (V_EN-H in Figure 8-4) fully enables the device, allowing it to enter start-up mode and starting the soft-start period. When the EN input is brought below V_EN-OUT-H (V_EN-H in Figure 8-4) by V_EN-OUT-HYS (V_EN-HYS in Figure 8-4), the regulator stops running and enters standby mode. Further decrease in the EN voltage to below V_EN-VCC-L completely shuts down the device. This behavior is shown in Figure 8-4. The EN input can be connected directly to VIN if this feature is not needed. This input must not be allowed to float. The values for the various EN thresholds can be found in Section 7.5.

The LMR34215-Q1 utilizes a reference-based soft start that prevents output voltage overshoots and large inrush currents as the regulator is starting up. A typical start-up waveform is shown in Figure 8-5 along with typical timings. The rise time of the output voltage is about 4 ms.

Figure 8-4 Precision Enable Behavior

Figure 8-5 Typical Start-up Behavior V_IN = 12 V, V_OUT = 5 V, I_OUT = 1.5 A