7.4.1 Enable Pin / Shutdown Mode

Connect the EN pin to a voltage source greater than 1.8V to enable operation of the LMR12015/20. Apply a voltage less than 0.4V to put the part into shutdown mode. In shutdown mode the quiescent current drops to typically 70 nA. Switch leakage adds another 40 nA from the input supply. For proper operation, the LMR12015/20 EN pin should never be left floating, and the voltage should never exceed V_IN + 0.3 V.

The simplest way to enable the operation of the LMR12015/20 is to connect the EN pin to AVIN which allows self start-up of the LMR12015/20 when the input voltage is applied.

When the rise time of V_IN is longer than the soft-start time of the LMR12015/20 this method may result in an overshoot in output voltage. In such applications, the EN pin voltage can be controlled by a separate logic signal, or tied to a resistor divider, which reaches 1.8V after V_IN is fully established (see Figure 25). This will minimize the potential for output voltage overshoot during a slow V_IN ramp condition. Use the lowest value of V_IN , seen in your application when calculating the resistor network, to ensure that the 1.8-V minimum EN threshold is reached.

Figure 25. Resistor Divider on EN

Equation 7.