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I'm designing a project at the moment that will be powered by USB-C. There's more information about that in this previous question of mine.

The project can be easily split into two parts:

  1. Control/logic side which requires 3.3V/500mA
  2. LED side which requires 5V/6A

What makes this slightly more complicated is how the power is supplied from the USB-C. On connection, the power adapter will provide 5V (current can depend on a few factors) on the VBUS pins. A USB-C controller can then negotiate with the power adapter to provide up to 20V/5A on the VBUS pins. For my particular project, I'll be using a power adapter than can provide 20V/3A.

At the moment my thoughts are:

  • Use the power from the VBUS pins to power a wide-range LDO that takes 5V upto 20V and has a fixed output of 3.3V. This can control the logic side of things.
  • Provide VBUS to a buck converter, which is only enabled once the VBUS voltage reaches 20V, to provide 5V to the LEDs.

My primary concerns are:

  1. The efficiency and heat dissipation of an LDO that has a 20V input and outputs 3.3V (even only with 500mA maximum current).
  2. The stability of the output from the buck converter when supplying the LEDs. These LEDs are similar to the WS2812B
  3. The heat from the buck converter running with the relatively high current.

Is there a better way of doing this, or is the buck converter for the LEDs and the LDO for the logic the best way around this? In particular are there any issues that might occur that I haven't mentioned above?

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  • \$\begingroup\$ why not borrow 500ma from the 5v line to get 3.3? a lot les heat... \$\endgroup\$ – dandavis Jan 13 '18 at 20:38
  • \$\begingroup\$ @dandavis That’s another viable solution if the two supplies are always enabled at the same time. If the 3.3V logic has to switch the 5V on and off it might need an extra high current load switch. \$\endgroup\$ – John D Jan 13 '18 at 21:07
  • \$\begingroup\$ The USB supply will only supply 5V until the MCU (which requires the 3.3V) negotiates 20V from the USB power supply. So once the MCU has negotiated 20V and confirmed that is present, it'll enable the 20V -> 5V buck converter. \$\endgroup\$ – CircularRecursion Jan 13 '18 at 22:25
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Your concern #1 is valid. You have 16.7V at 500mA meaning over 8W of dissipation in your LDO. Not a realistic option.

Use a buck converter for both applications. There's no concern about output stability in a properly designed converter. You can get efficiency of high 80's low 90's meaning you have to deal with a few watts of dissipation with a 30W load.

Proper thermal design should handle that without an issue.

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