The voltage on the EN pin controls the ON/OFF operation of the LMR544xx. A voltage of less than 0.95 V shuts down the device, while a voltage of greater than 1.36 V is required to start the converter. The EN pin is an input and cannot be left open or floating. The simplest way to enable the operation of the LMR544xx is to connect EN to VIN. This allows self-start-up of the LMR544xx when V_IN is within the operating range.

Many applications benefit from the employment of an enable divider, R_ENT and R_ENB (Figure 7-3) to establish a precision system UVLO level for the converter. A system UVLO can be used for supplies operating from utility power as well as battery power. It can be used for sequencing, ensuring reliable operation, or supplying protection, such as a battery discharge level. An external logic signal can also be used to drive the EN input for system sequencing and protection.

Figure 7-3 System UVLO by Enable Divider