A capacitor is considered fully charged when the output voltage reaches the supply voltage (VS) level. At the same time, the capacitor stops drawing current and the resistive load begins sinking current. Figure 1-1 shows that, during start-up, current takes the orange path until the capacitor is fully charged, and then the current takes the green path. On a waveform, when the capacitor is approximately charged to VS, the current stabilizes to the level the resistive load is expected to draw, I = VS/R_LOAD.

Figure 1-1 Concept Diagram

Based on the following points, the total charge time of the capacitive load also increases with capacitance and temperature:

• Ohm’s Law, V = IR, the change in voltage is dependent upon current and resistance. As the junction temperature increases, the on-resistance of the switch also increases, further limiting current flow.
• RC (resistor - capacitor) time constant, τ = RC, where charging time is equal to the resistance times capacitance.