After the EN pin is tied to high voltage, the TPS61299x/xA begins to start up. Refer to Figure 7-3 to see the total process. The input current limit might be marginally distinct from defined value during soft start process, which should be taken into special account when doing the system design.

When output voltage is lower than 0.5V, TPS61299x/xA works in short circuit protection mode, at which time the input current limit is roughly 20mA to mitigate the power loss. As the output voltage ramps higher than 0.5V, whereas before reaching 1.8V, the device operates at the boundary of Discontinuous Conduction Mode (DCM) and Continuous Conduction Mode (CCM). As a result, the inductor peak current is limited to around 350mA during this stage, and the average inductor current is limited to 80mA. After the output voltage rises to 1.8V and higher, the TPS61299x starts to detect the output voltage configuration of the VSEL pin, then latches the configuration. The version detection time depends on the resistance at VSEL pin, the higher resistance, the longer version detection time. E.g. for 5V normal version, the TPS61299x needs approximately 170μs for version detection. After version detection, TPS61299x continues switching and output rises 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 average input current limits after version detection stage are slightly different among TPS61299x family devices. In terms of the higher input current limit version device, TPS61299/6/7 and TPS61299/6/7, the actual average inductor current limit is smaller than defined input current limit, when output voltage is lower than 2.5V and higher than 1.8V, in order to reduce the inrush current during start up. For instance, the TPS61299 and TPS612997 limit the inductor average current lower than 500mA and TPS612996 limits the inductor average current lower than 250mA, when output voltage is ranging between 1.8V and 2.5V. After output voltage increases to 2.5V, the input current limit is back to normal defined value, namely TPS61299 back to 1.2A, TPS612996 back to 500mA, and TPS612997 back to 1.9A. For the low input current limit versions TPS612991/2/3/4/5, the input current limit is the same as the defined value once the output voltage is higher than 1.8V. Typically the device works at DCM during start up.

Figure 7-3 Soft-Start Timing