The Enable (EN) pin is bidirectional, and has two functions:

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

Figure 8-6 Enable Functional Block Diagram

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 resistor to limit the current flowing into the EN pin (see Section 10.1).

When power is first applied to the VIN pin, the device pulls the EN pin low until it has loaded the default register settings from nonvolatile memory and read the state of the VSEL, FSEL 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 8.3.18).

In stand-alone (non-stacked) applications, you can disable the active pulldown of the EN pin if you set SINGLE = 1 in the CONTROL3 register. 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.

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 the DC/DC converter in the device. (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 only 20µA (typical).

The rising threshold voltage of the EN pin is 1.0V and the falling threshold voltage is 0.9V. The tolerance of the threshold voltages is ±30mV, which means that you can use the EN pin to implement precise turn-on and turn-off behavior.