After the EN pin is tied to high voltage, the TPS61299x/xA begins to startup.

For the high input current limit is 250mA, 500 mA, 1.2A and 1.9A version, at the beginning, when output voltage is lower than 0.5V, device limits the output power for the short protection. As output voltage is higher than 0.5V, the device operates at the boundary of Discontinuous Conduction Mode (DCM) and Continuous Conduction Mode (CCM), and the inductor peak current is limited to around 350mA during this stage. After the output voltage reaches close to 1.8V, the TPS61299x/xA starts to detect the output voltage configuration of the VSEL pins, then latches the configuration. The version detection time depends on the resistance at VSEL pin, the higher resistance, the longer version detection time. Eg. for 5V normal version, the TPS61299x/xA needs approximately 170μs for version detection. After version detection, TPS61299x/xA continues switching and output ramps up further. The internal soft-start time is approximately 1.3ms, and the output soft start time varies with the different output capacitance, load condition, and configuration conditions. The TPS61299x/xA limits the inductor average current lower than 500mA, (input current limit to 250mA for 250mA version) when output voltage is lower than 2.5V. In this way, the soft start function reduces the inrush current during startup. For the low input current limit 5mA, 25mA, 50mA and 100mA version, the device limits the input current limit to 25mA during the soft start. The device works at DCM during start up.

For A version device, when EN is low, the high side MOSFET is fully on so the Vout is directly connected with Vin. When EN is pulled to high, Vout ramps from Vin and afterwards the start up process is the same with non-A version device.

Figure 7-3 Soft-Start Timing