3 Derivation

The math behind how much capacitance is needed to support the backup power is described in this section. The BQ25856-Q1 converter turns off once the capacitor voltage falls below 2.5V on the SRN pin. The amount of usable energy that can be stored in a capacitor is calculated by using Equation 4:

The total energy required by the circuit is set by the system voltage, the needed current, and the how long system needs to stay working can be calculated by using Equation 5:

Now, the two equations can be combined to calculate the total capacitance or charge voltage needed by using Equation 6:

Note, these equations are only an estimation of the total energy required by the system. Switching losses, conduction and the equivalent series resistance (ESR) of the capacitor losses from the converter reduces the usable power from the backup capacitor.

For backup energies of 10J or less, electrolytic capacitors can be used. For higher energy demands, supercapacitors work better because of the much greater capacitance.