Internal to the module is a 2-MΩ pullup resistor connected from V_IN to Enable. For applications not requiring precision undervoltage lockout (UVLO), the Enable input can be left open circuit and the internal resistor will always enable the module. In such case, the internal UVLO occurs typically at 4.3 V (V_IN rising).

In applications with separate supervisory circuits, Enable can be directly interfaced to a logic source. In the case of sequencing supplies, the divider is connected to a rail that becomes active earlier in the power-up cycle than the LMZ12010 output rail.

Enable provides a precise 1.274-V threshold to allow direct logic drive or connection to a voltage divider from a higher enable voltage such as V_IN. Additionally there is 13 μA (typical) of switched offset current, allowing programmable hysteresis.

The function of the enable divider is to allow the designer to choose an input voltage below which the circuit will be disabled. This implements the feature of a programmable UVLO. The two resistors must be chosen based on the following ratio:

The LMZ12010 typical application shows 12.7 kΩ for R_ENB and 42.2 kΩ for R_ENT, resulting in a rising UVLO of 5.51 V. Note that this divider presents 4.62 V to the EN input when V_IN is raised to 20 V. This upper voltage must always be checked, making sure that it never exceeds the absolute maximum 5.5-V limit for Enable. A 5.1-V Zener clamp can be applied in cases where the upper voltage would exceed the range of operation for the EN input. The Zener clamp is not required if the target application prohibits the maximum Enable input voltage from being exceeded.

Additional enable voltage hysteresis can be added with the inclusion of R_ENH. It is possible to select values for R_ENT and R_ENB such that R_ENH is a value of zero allowing it to be omitted from the design.

Rising threshold can be calculated as follows:

Whereas the falling threshold level can be calculated using:

Figure 8-2 Enable Input Detail