Apply a voltage less than 0.25V to the EN1 pin to put the LM644A2-Q1 into shutdown mode. In shutdown mode, the quiescent current drops to 0.5µA (typical). Above this voltage but below the lower EN threshold, VCC is active but switching on SW1 and SW2 remains inactive. After EN1 is above V_EN, the SW1 becomes active. EN2 controls switching on the second output SW2. In dual output configuration EN2 can be used to independently turn off the second output voltage, but does not control entering shutdown mode. In single-output multi-phase configuration EN1 on primaries and secondaries must be tied together. In single output configuration EN1 must not be used to disable the secondary devices for phase shedding. EN2 of the primary and secondaries must be tied together and can be used to shutdown the secondary phases. The very high efficiency of the device in PFM operation eliminates the need to phase shed in most designs as phase of the secondaries is controlled even under PFM operation.

The EN terminals cannot be left floating. The simplest way to enable the operation is to connect the EN pins to VIN. This action allows the self-start-up of the device when VIN drives the internal VCC above the 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 10% tolerance. Hysteresis is enough to prevent retriggering upon shutdown of the load (approximately 38%). The external logic output of another IC can also be used to drive the EN terminals, allowing system power sequencing.

Resistor values can be calculated using the following equations:

where

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