The enable threshold is set by configuring the R_{BOT_EN} and R_{TOP_EN} resistors in order to turn on the switch at the desired input voltage as described in Section 9.3.1. For this design, the goal is to turn on the switch when VIN reaches 4.5 V. First we set R_{TOP_EN} = 100 kΩ with a 0.1% tolerance resistor, and then use Equation 16 to calculate the nominal R_{BOT_EN}. A 16.2-kΩ 0.1% tolerance resistor is found to best satisfy the equation.

Equation 16.

where

• V_IHEN(TYP) = 0.63 V
• R_{TOP_EN} = 100 kΩ
• VIN_{EN_RISE} = 4.5 V

Additionally, it should be ensured the worst case minimum and maximum turn-on voltages are acceptable. The minimum turn-on voltage would ideally be above 4.5 V (the minimum operating voltage). However, in this case that is not possible to achieve, and it is acceptable to allow the minimum turn-on voltage to be lower than 4.5 V (such as 4.2 V). Note however that the device will not be fully operational until at least 4.5 V is reached. This is okay in this case since the VIN voltage will quickly rise to above 4.5 V which puts the device in a fully operational state. The eFuse maximum turn-on voltage must be less than the minimum final VIN value (which is 4.9 V as determined by the 2% tolerance on the 5-V rail). The maximum turn-on voltage can be calculated using Equation 17. It is determined that VIN_{EN_RISE(MAX)} = 4.89 V which is under 4.9 V.

Equation 17.

where

• V_IHEN(MAX) = 0.68 V
• R_{TOP_EN} = 100 kΩ
• R_{BOT_EN} = 16.2 kΩ
• R_tolerance = 0.1% = 0.001

An alternative method to ensure the selected R_{BOT_EN} value is acceptable for both the minimum and maximum enable thresholds is to select minimum and maximum values for VIN_{EN_RISE} and VIN_{EN_FALL} and ensure Equation 18 and Equation 19 are satisfied.

Equation 18.

Equation 19.

To summarize, using Equation 1 and Equation 2 with R_{TOP_EN} = 100 kΩ and R_{BOT_EN} = 16.2 kΩ, shows the eFuse will nominally turn on at 4.52 V and turn off at 3.73 V. The turn-off voltage is different due to the enable pin hysteresis. Taking into account the maximum and minimum EN pin thresholds and resistor tolerances the switch will turn on between 4.30 V and 4.89 V and turn off between 3.58 V and 4.10 V. To change the turn-off levels requires changing the turn-on levels. The turn-off level will act as an under voltage protection (UVP) feature to protect the downstream circuitry from receiving a sustained voltage under 3.58 V (which could potentially put the circuit in an undefined state).

Additionally, as the turn-on voltage minimum is 4.30 V, this is greater than 75% of the final VIN value (4.30 V > 4.9 V × 0.75 = 3.68 V). Therefore, there is no EN and slew rate related requirements as indicated in the electrical characteristics footnote (see Section 7.5). If the device was enabled under 3.68 V (not advised; this is less than the recommended operating VIN voltage of 4.5 V), the output voltage slew rate must be less than the input voltage slew rate or a false overcurrent trigger may occur.