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I am learning PCB design by myself via the internet, I am using KiCad 8, I am trying to make a dev kit using:

  1. USB (Type C)
  2. An external power supply from 7VDC -> 40VDC
  3. CP2104
  4. ESP32-WROOM-32E

KiCad 8

In the current technical drawing I have not included ESP32 because I still have questions about the power, I have looked at the datasheet of the devices and other people's online tutorials then drew the schematic diagram. My goal is to use the VBUS of Type C to signal to CP2104 that I have plugged in USB, then use the 5V power generated from LM2576HVS to transmit to the REGIN pin to power the CP2104 instead of using the VBUS power like that, is it stable? According to the principle I read that after supplying +5V to REGIN, VDD of CP2104 will supply +3.3V, but AMS1117 is also supplying +3.3V from 5V source of LM2576HVS, so can I combine these two +3.3V sources into the power source of the whole system? Or do I need to isolate them?

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  • \$\begingroup\$ According to the datasheet of CP2104 (table 5), the typical output voltage VDD of the built-in voltage regulator is 3.45V. So in your current design I would not recommend connecting VDD with your 3.3V voltage rail since there will be some voltage difference. However, according to section 9 of the datasheet, you can power the CP2014's regulator with 3.3V and then connect the VDD pin to the 3.3V rail. \$\endgroup\$
    – l.voe
    Commented Sep 9 at 7:32
  • \$\begingroup\$ Thanks for your suggestion. So I will completely isolate CP2104 from the system by using +5V power from VBUS instead of +5V power from LM2576HVS, 3.xV power from VDD will also be isolated from the system, will that be stable? \$\endgroup\$
    – Cosi Nguyen
    Commented Sep 9 at 11:38

1 Answer 1

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That schematics will not work.

Please refer to CP2104 data sheet for a reference design which tells how to make a self-powered CP2104 design correctly.

You need to supply it with the 3.3V from the regulator, and provide 5V via resistor divider to VBUS to detect when something is connected to USB port or not.

So connect the regulator 3.3V output to REGIN, VDD and VIO of CP2104.

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  • \$\begingroup\$ Based on the suggestion of CP2104 which can use 5V power from USB to REGIN, can I make it a standalone part of the board by using 5V power from USB, isolating the 3.xV power generated from VDD from the board, will it still work best? \$\endgroup\$
    – Cosi Nguyen
    Commented Sep 9 at 11:42
  • \$\begingroup\$ @CosiNguyen No, that will be the worst possible scenario, having data lines between powered ESP32 and unpowered CP2104, and the CP2104 internal regulatot has higher output voltage. Just look at the datasheet example and copy it as it is for best results. \$\endgroup\$
    – Justme
    Commented Sep 9 at 12:25
  • \$\begingroup\$ My intention is that I will use VCC from the flexible input source for the purpose of flexible power supply of the Relay later. So only when I need to transmit data or need to debug, I will use CP2104, otherwise it can be turned off. In my new schematics design, both CP2104 and USBLC6-2P6, will use VBUS power. Does that still work? Sorry if my understanding is limited IMG: imgur.com/a/lMmCveM \$\endgroup\$
    – Cosi Nguyen
    Commented Sep 9 at 14:58
  • \$\begingroup\$ @CosiNguyen But that is exactly not why my answer and the data sheet tells you to do. That is exactly a way that leads to trouble. You cannot have unpowered chips connected to powered chips. If your ESP32 sets the TXD pin as high output, it will (usually) lead to problems like CP2104 trying to power through the IO pin. \$\endgroup\$
    – Justme
    Commented Sep 9 at 16:52
  • \$\begingroup\$ Understood, I will split into 2 independent circuits, that is the best way \$\endgroup\$
    – Cosi Nguyen
    Commented Sep 10 at 1:41

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