Hello all!

In this log I'm going to go over my current progress on the regulator design for the 3.3V and 5V supplies. This design is a bit more complex than I expected for reasons that I'll get into here.

Goals

• Supply both 3.3V and 5V on StampPCB to allow powering of different types of microcontrollers
• Regulated pins should always deliver around 3.3V and 5V even when other voltages are requested through USB PD
• Amperage needs to be enough to power a microcontroller and some supporting components. Current thinking is at least 500mA

Design Options

All LDO's (Turns out this is kinda bad)

Originally I had been thinking that I would just use two LDO's set to 3.3V and 5V and call it a day. LDO's are very simple to use and setup, however, I ran into some issues when looking at this design choice.

1. LDO's are not very efficient when there is a large difference between Voltage In and Voltage Out
   1. I did not want to waste a lot of power constantly generating 3.3V and 5V, especially at 28V where the efficiency could be as low as 12%
2. LDO's usually don't work when Voltage In = Voltage Out
   1. LDO's require more Voltage In than Voltage Out (Vin > Vout). This is called the Minimum Dropout Voltage. This can range from a couple volts to around 100mV on a nice (ie. expensive) LDO. This causes issues for the 5V regulation. I would need around 5.1V input to get 5V output. Since USB PD will have a minimum of 5V, I would expect to get less than 5V on the output pin for most LDO's. This might be ok for some microcontrollers and devices but I would like for this design to be as universal as possible so not having 5V exactly is not acceptable.
   2. Since we already have 5V on the USB, why not just switch the supply to go directly to the pin instead of through the LDO? There are 3 ways that I can think to do this: Schottky Diodes, MOSFETS, or a Automatic Load Switch. The Schottky Diode and MOSFET methods also have a voltage loss so I don't think these really solve the problem. The Automatic Load Switch can have low to no voltage drop but the logic is more complicated and most IC's that I viewed seemed rather expensive. Combine this with the already poor efficiency of the LDO and I've decided that I'd rather implement a more complicated voltage regulator

Buck Converters

Buck Converters are much more efficient than LDO's when the voltage difference is larger between Voltage Input and Voltage Output. That's good for our efficiency! However, using a Buck Converter has issues as well.

1. Buck Converters are more complicated to design for. They require an inductor to function and usually an output capacitor. Some even need a diode as well. This means we have more components to add and more math for me to do.
2. Buck Converters still can't regulate voltage when Voltage In = Voltage Out! This means we still have a voltage drop just like with an LDO!
   1. Some Buck Converters have the ability to run in what is called "100% Duty Cycle Mode" where they act like an LDO when Voltage Input = Voltage Output. This still has a voltage drop, especially at higher amperage. The IC's that have this are usually much more expensive and generally don't support the 5-28V we're looking for
   2. This means we still haven't solved our problem with the 5V Voltage In = 5V Voltage Out
   3. I want a perfect 5V. This means that Buck Converters can't be our only solution

Buck and Boost Converter 

While a Buck Converter can't regulate our Voltage In = Voltage Out issue for 5V, we could step up the voltage to 5V with a Boost Converter. There are two options for this with some trade-offs:

1. Combined Buck-Boost Converter for 5V
   1. A combined Buck-Boost converter can boost the voltage as needed. This means that when we input 5V we can output 5V!
   2. Buck-Boost Converters are more expensive and require more components (more math!), especially for our wide voltage range
   3. Most Buck-Boost Converters are small surface components that must use a hot plate. I would prefer the option to hand solder if possible
2. Use a Buck for 3.3V and a Boost Converter on the 3.3V for 5V
   1. Allows for voltage drops below 5V for Voltage In! Good for a noisy USB signal
   2. Possibly cheaper and there are more options for IC's in this range
   3. Less efficient since we are combining the efficiency of both the buck and boost converter
   4. Higher current draw from the buck converter. It must now supply current for both the 3.3V and 5V!
      1. There is a stronger current dependency on the 3.3V and 5V where drawing too much current on one voltage may cause issues in the other

Conclusion

So far this is my progress on the 3.3V and 5V regulation design. I plan to go with a Buck and Boost Converter but I'm still weighing out options 1 and 2. This is mostly a trade off between cost, IC availability, and design complexity. At this time I'm leaning towards option 2 since I think it will be easier to find IC's and it will be cheaper, even if it's not the most efficient design. I will be looking into more research for Buck-Boost Converter Combo's to see if there are options I have missed.