TPS55340: how does soft-start work?

Hello Nan,

Thanks for reaching out to us via e2e.

The diagrams in the datasheet are only meant to illustrate certain features, but they do not show the complete logic/system inside the part.
In this case, the diagram you added is meant to show the influence that the SS pin voltage has on the output signal of the error amplifier.
Neither the current source nor the discharge resistor are shown there.

Important is the text:
When the IC is enabled, an internal bias current source (6 µA, typical) charges a capacitor (CSS) on the SS pin.
The size of the connected capacitor defines how lon it will take to get it charged up to 1,8V with this constant current.

The voltage at the capacitor clamps the output of the internal error amplifier that determines the peak current and duty cycle of PWM controller.
This is what the diagram shows.

Limiting the peak switch current during start-up with a slow ramp on the SS pin will reduce in-rush current and output voltage overshoot.
Once the capacitor reaches 1.8 V, the soft-start cycle is completed and the soft-start voltage no longer clamps the error amplifier output.
The effect is shown in figure 8-15 in the datasheet.

When the EN is pulled low for at least 1 ms, the IC enters the shutdown mode and the SS capacitor is discharged through a 5-kΩ resistor to prepare for the next soft-start sequence.
The time depends on the capacitor size.
You can calculate the 5 tau time to charge/discharge a capacitor via a resistor yourself.

Best regards,
Harry