8.3.2 Enable and Start-up

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 the External UVLO section). 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 10) fully enables the device, allowing it to enter start-up mode and beginning the soft-start period. When the EN input is brought below V_EN-OUT-H (V_EN-H in Figure 10) by V_EN-OUT-HYS (V_EN-HYS in Figure 10), 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 10. The EN input may 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 the table.

The LMR34206-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 11 along with typical timings. The rise time of the output voltage is about 4 ms.

Figure 10. Precision Enable Behavior

Figure 11. Typical Start-up Behavior
V_IN = 12V, V_OUT = 3.3 V, I_OUT = 0.6 A