Apply a voltage less than 0.4 V to the EN pin to put the device into shutdown mode. In shutdown mode, the quiescent current drops to 0.66 µA (typical). Above this voltage but below the LM6x4xx-Q1 lower EN threshold, VCC is active but the SW node remains inactive. Once EN is above V_EN, the chip operates normally as long as input voltage is above the minimum operating voltage.

The EN terminal cannot be left floating. The simplest way to enable the operation is to connect the EN pin to VIN. This action allows the self-start-up of the device when VIN drives the internal VCC above its UVLO level. However, many applications benefit from employing an enable divider string, which establishes a precision input undervoltage lockout (UVLO). The precision UVLO can be used for the following:

• Sequencing
• Preventing the device from retriggering when used with long input cables
• Reducing the occurrence of deep discharge of a battery power source

Note that EN thresholds are accurate. The rising enable threshold has 8.1% tolerance. Hysteresis is enough to prevent retriggering upon shutdown of the load (approximately 25%). The external logic output of another IC can also be used to drive the EN terminal, allowing system power sequencing.

Resistor values can be calculated using Equation 2:

where

• V_ON = V_IN turn-on voltage
• V_OFF = V_IN turn-off voltage