The Enable (EN) pin is bidirectional and has two functions. See Figure 7-10:

• As an input, it enables and disables the DC/DC converter in the device
• In a stacked configuration, it is an output and provides a SYSTEM_READY signal to other devices.

Because there is an internal open-drain transistor connected to the EN pin, do not drive this pin directly from a low-impedance source. Instead, use a > 15-kOhm resistor to limit the current flowing into the EN pin.

When power is first applied to the VIN pin, the device pulls the EN pin low until it loads the default register settings from nonvolatile memory and reads the state of the VSETx and SYNC_OUT pins. The device also pulls EN low if a fault, such as thermal shutdown or overvoltage lockout, occurs. In a stacked configuration all devices share a common enable signal, which means that the DC/DC converters in the stack cannot start to switch until all devices in the stack have completed the initialization. Similarly, a fault in one or more devices in the stack disables all converters in the stack (see Section 7.3.17).

In standalone (non-stacked) applications, set SINGLE = 1 in the CONTROL3 register to disable the active pulldown of the EN pin. Fault conditions have no effect on the EN pin when SINGLE = 1. (Note that the EN pin is always pulled down during device initialization.) In stacked applications, make sure that SINGLE = 0. Setting SINGLE = 1 also disables the SYNC_OUT pin.

When the internal SYSTEM_READY signal is low (that is, initialization is complete and there are no fault conditions), the internal open-drain transistor is high impedance and the EN pin functions like a standard input: a high level on the EN pin enables the DC/DC converter in the device and a low level disables it. The I²C interface is enabled as soon as the device has completed the initialization and is not affected by the state of the internal ENABLE or SYSTEM_READY signals.

A low level on the EN pin forces the device into shutdown. During shutdown, the MOSFETs in the power stage are off, the internal control circuitry is disabled, and the device consumes less than 20 µA (typical). Do not leave the EN pin floating.

The Precise Enable input provides a user-programmable undervoltage lockout by adding a resistor divider to the input of the EN pin. The Precise Enable input also allows the user to drive the pin by a slowly changing voltage and enables the use of an external RC network to achieve a precise power-up delay. See Achieving a clean startup by using a DC/DC converter with a precise enable-pin threshold analog design journal for more details.