The internal LDO has a lower EN threshold than the
output of the regulator. The internal LDO regulates the VCC voltage at 3.3V, typically
when:
- The EN pin voltage is above 1.1V
(maximum).
- The EN pin voltage is below the
precision enable threshold for the output voltage.
The precision enable circuitry is ON after VCC is above the UVLO. The internal power
MOSFETs of the SW node remain off unless the voltage on the EN terminal goes above the
precision enable threshold. The LM644A2-Q1 also employs UVLO protection. If the VCC voltage
is below the UVLO level, the output of the regulator is turned off.