8.2.2.2 Enable Divider, R_ENT, R_ENB and R_ENH Selection

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 may 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 LMZ13608 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. See Figure 48.

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 LMZ13608 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. 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 Abs Max 5.5-V limit for Enable. A 5.1-V Zener clamp can be applied in cases where the upper voltage would exceed the EN input's range of operation. 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 48. Enable Input Detail