A typical supercap charge profile is shown in Figure 2-1.

Figure 2-1 Supercap Charging Profile

With a fully discharged supercap, the charging circuit initially sends current directly to ground. The charging circuit operates at a much-reduced, slowly increasing current due to the converter die temperature reaching thermal regulation. Alternatively, the charging circuit may turn on and off due to a die temperature fault at changing duty cycle. Eventually, the super capacitor voltage, and therefore the charging circuit’s operating efficiency, increases so the capacitor charges at the desired constant (fast or max) charge current, I_CHG, until it reaches and remains at constant voltage (CV) regulation voltage, V_REG. Having CV regulation allows for total utilization of the supercap's capacity.

The charge time in CC mode can be estimated rearranging i = C × dv/dt to get dt = C × (V_REG-V1)/I_CHG. Charge time can be difficult to predict if the initial, reduced charge current is unknown or slowly increases out of thermal regulation up to voltage V1, as shown above.