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I've designed the power supply section for powering an ESP32 board and for switching between USB Power and 5V external power, would this circuit be sustainable and work properly?

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  • \$\begingroup\$ When both VBUS and 5V_EXT are present, consider the tolerances on the supplies are well i.e., consider VBUS is at the highest level and 5V_EXT is at its lowest level. Can there be a current flow from VBUS to 5V_EXT. This is a critical point you need to look into. \$\endgroup\$
    – sai
    Oct 24, 2023 at 16:49
  • \$\begingroup\$ To prevent a high current path between VBUS and 5V_EXT I'm going to add another diode from +5V_EXT to C9. That should solve the issue as suggested right? \$\endgroup\$ Oct 24, 2023 at 18:57

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That should be OK, but it would be safer to add another diode to keep USB power from flowing your internal supply. If you're concerned about efficiency, you can replace the diodes with "ideal diodes" such as the MAX40203.

Of course, this is still simply voting according to which supply is higher. There's also options such as TPS2105-EP if you want more fine-grained control; with a fairly light external load on it so it doesn't float, you could probably use the input you want to prioritize as the select line (e.g., use USB power if it's there, but use internal power only if USB power is unavailable).

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  • \$\begingroup\$ To prevent a high current path between VBUS and 5V_EXT I'm going to add another diode from +5V_EXT to C9. That should solve the issue as suggested right? \$\endgroup\$ Oct 24, 2023 at 18:57
  • \$\begingroup\$ That should work, as long as you're OK with highest voltage wins. In retrospect, you're using an LDO, so dissipation isn't an issue...it's just a question of whether the LDO or the diode dissipates the excess power. \$\endgroup\$ Oct 24, 2023 at 19:02
  • \$\begingroup\$ Thank you so much kind sir. \$\endgroup\$ Oct 24, 2023 at 19:05
  • \$\begingroup\$ Glad to help. :) \$\endgroup\$ Oct 24, 2023 at 19:10
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When you are using a diode in series with VBUS and 5V_EXT, there is no risk of reverse current but, you need to check if the drop across the diode is acceptable because it will reduce the Vin of the regulator. For eg., for this 3.3V regulator, as per the datasheet, the recommended minimum Vin is 3.3V+1.3V = 4.6V if the load current is 800mA (dropout voltage specification). Take into account the diode drop and the tolerances of the supplies and see if this can become an issue.

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  • \$\begingroup\$ If you are convinced that you don't have the reverse current issue, you could remove D5. Otherwise, you could try the other options suggested by Cristobol \$\endgroup\$
    – sai
    Oct 26, 2023 at 2:21

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