The boost converter of the TPS61098x is designed always-on, so there is no enable or disable control of it. The boost converter starts operation once input voltage is applied. If the input voltage is not high enough, a low voltage startup oscillator operates the switches first. During this phase, the switching frequency is controlled by the oscillator, and the maximum switch current is limited. Once the converter has built up the output voltage V_(MAIN) to approximately 1.8 V, the device switches to the normal hysteretic current mode operation and the VMAIN rail starts to supply the internal control circuit. If the input voltage is too low or the load during startup is too heavy, which makes the converter unable to build up 1.8 V at V_(MAIN) rail, the boost converter can't start up successfully. It will keep in this status until the input voltage is increased or removed.

The TPS61098x is able to startup with 0.7 V input voltage with ≥ 3 kΩ load. The startup time depends on input voltage and load conditions. After the V_(MAIN) reaches 1.8 V to start the normal hysteretic current mode operation, an internal ramp-up reference controls soft-start time of the boost converter until V_(MAIN) reaches its set value.

The TPS61098x does not support undervoltage lockout function. When the input voltage drops to a low voltage and can't provide the required energy to the boost converter, the V_(MAIN) drops. When and to what extent V_(MAIN) drops are dependent on the input and load conditions. When the boost converter is unable to maintain 1.8 V at VMAIN rail to supply the internal circuit, the TPS61098x powers down and enters into startup process again.