The startup of the boost converter block operates in two steps:
- Input-to-output isolation switch (IsoFET)
As soon as the internal enable signal of the boost converter is activated, the isolation switch is slowly turned on, ramping up smoothly the current flowing from VIN into the output capacitors. The startup current is limited to 200 mA typically until VSWO > 3.5 V (short-circuit condition), and increases linearly with the output voltage. Once VSWO gets close to VSWI, the isolation switch is fully turned on and the boost converter starts switching. The soft-start function is also enabled. - Soft-start (SS)
To minimize the inrush current during start-up an external capacitor connected to the soft-start pin SS is used to slowly ramp up the internal current limit of the boost converter. It is charged with a constant current of typically 10 μA. The inductor peak current limit is proportional to the SS voltage and the maximum load current is available after the soft-start is completed (VSS = 0.8 V) or VDD has reached its Power Good value (90% of its nominal voltage). The larger the SS capacitor, the slower the ramp of the current limit and the longer the soft-start time. A 100-nF capacitor is usually sufficient for most applications. When VIN decreases below the undervoltage lockout threshold, the soft-start capacitor is discharged to ground.