Think about the last time you were in a race. Maybe it wasn’t a marathon or even a 5K, but we’ve all raced our best friends to the swing set during recess. Or maybe we’ve tried (successfully or not) to catch a bus or train. Now that you’ve reminisced about the race, how did you feel afterwards? If you’re like most of us, who didn’t stretch or warm up before trying to catch the bus, it’s probably not too well. In the same way, the power supplies in your system better start slowly (softly, as it is sometimes called) in order to not pull a muscle (or break another part of your system).
Soft start refers to how most modern power supplies ramp their output voltage from 0V to the regulation level. When done properly, soft start reduces inrush current drawn from the input bus and prevents the bus from sagging or even browning out. This is most critical for current limited input supplies, such as most batteries and USB ports. If excessive current is drawn from these sources, their voltage drops and the entire system might reset. Talk about a pulled muscle!
However, a soft start that takes too long might cause other negative consequences. For example, when booting your computer or exiting from standby mode, you desire a short wait time before you can use the device again. Taking 30 or more seconds to start all the power supplies in your laptop will not win the race. There must be a tradeoff between low inrush current and short soft start time.
Most power supply integrated circuits make this tradeoff in the direction of faster startup times with a shorter soft start period. This is done because it is easier for the engineer to lengthen the soft start time than it is for them to shorten it. This application note, "Extending the Soft Start Time in the TPS6107x Family of Boost Converters," details a simple circuit for extending the soft start time in a boost converter, while this application note, "Extending the Soft Start Time in the TPS63010 Buck-Boost Converter," has a circuit for a buck-boost converter. Clearly, in some applications it is more useful to stretch and warm up before running the race.
TPS61070EVM with added Soft Start circuit
Additional Resources:
- TI's family of low-power DC/DC boost converters and buck-boost converters
- Check out other PowerHouse blog posts
