# How does soft start work in TI buck regulators?

I am planning to use "TPS54341" in my design. I am curious about how the soft start circuit internally works in TI regulators.

The datasheet explains to how to use it, however, does not go in much details on what's the theory of operation. http://www.ti.com/document-viewer/TPS54341/datasheet/detailed-description#slvsc612433

There is a block diagram in the section 7.2

Figure 28 of your datasheet shows how startup is delayed until the SS/TR pin rises above 800mV. It has an internal current source and recommends the delay capacitor have a value less than 470nF and greater than 470pF. In many ways it is just another comparator to judge if the IC should be ON or OFF-or delayed.

Soft-start can help prevent overshoot of the output voltage compared to IC's that are ON at full voltage very fast-too fast to control any overshoot which could harm some types of logic IC's.

This is a quote from the datasheet about softstart:

7.3.7 Soft-Start/Tracking Pin (SS/TR) The TPS54341 device effectively uses the lower voltage of the internal voltage reference or the SS/TR pin voltage as the reference voltage of the power-supply and regulates the output accordingly. A capacitor on the SS/TR pin to ground implements a soft-start time. The TPS54341 device has an internal pullup-current source of 1.7 μA that charges the external soft-start capacitor. The calculations for the soft-start time (10% to 90%) are shown in Equation 5. The voltage reference (VREF) is 0.8 V and the soft-start current (ISS) is 1.7 μA. The soft-start capacitor should remain lower than 0.47 μF and greater than 0.47 nF.

It often works by limiting the maximum pulse width of the output MOSFET's which would normally start at maximum pulse width with a heavy surge of current and a possible brief overshoot of the output voltage. Soft start puts a throttle on that so it starts at a narrow pulse width but quickly allows the pulse to widen to its normal width for a given load, so the loop stabilizes quickly. That is why the soft-start capacitor has a narrow range of values. Soft-start is a brief choking of the MOSFET pulse width-no more than that.

• Right, that's delayed operation. How does it actually slow down the output voltage ramp? – Nilesh Dattani Nov 15 '18 at 3:46
• This 'soft-start' feature is included in almost all high-end power controllers, regardless of who makes it. Those IC's without it start up as soon as input voltage is above a safe level, with no delay in output voltage rise-time. – Sparky256 Nov 15 '18 at 3:54
• It often works by limiting the maximum pulse width of the output MOSFET's which would normally start at maximum pulse width. Soft start puts a throttle on that so it starts at a narrow pulse width but quickly allows the pulse to widen to its normal width for a given load. I put this in the answer. – Sparky256 Nov 15 '18 at 4:28