Named the pre-charge phase, some boost devices use the saturation characteristic of the high-side MOSFET to charge the output capacitor continuously before Vout reaches Vin. Taking TPS61022 as an example, the device has an internal switch on the high-side MOSFET. This switch allows the MOSFET to change the body diode direction and block inrush current when Vin > Vout.

Figure 2-9 Switchable Body Diode on TPS61022

When entering pre-charge phase, the low-side switch shuts off while the high-side MOSFET enters saturation region. The source switches to SW during this process. A saturated MOSFET functions as a current source whose current the gate voltage controls. Therefore, the output capacitor charges with limited current. Figure 2-10 depicts the device behavior of TPS61022 during start-up.

Figure 2-10 TPS61022 Start-Up Waveforms

After the output voltage reaches the input voltage, TPS61022 enters boost start-up mode. The device starts switching and the output voltage ramps up until reaching the target.