Bootstrapping board for programming the ESP32

[DihydrogenMonoxide]

I have a PCB design with an onboard uC (ESP32-WROOM-32D) and voltage regulator (AMS1117-3.3).
The PCB doesn't have USB comms or power, so I want to have a separate "bootstrapping" board which has those things and connects to the uC via a 4-pin header.
Should I add a diode to protect the voltage regulator on the board with the uC when power is coming from the chipless board?
If so, any suggestion on what kind of diode to use? I saw some designs using BAT760 Schottky diodes but not sure if that's appropriate.

 

[DihydrogenMonoxide]

Anyone have any idea?
How to protect an unpowered voltage regulator when the Vout line is already driven by another regulator?

 

[KlausST]

Hi,

Why not use the 5V input and use the internal regulator? This is the safe solution.

Many linear regulators are able to withstand voltage applied to the output as long as the input is left floating.

Klaus

 

[betwixt]

As the USB interface draws little current and will already have it's own protection at it's USB side connections, just draw power for it from the 5V and AMS1117. That is what most WROOM based modules do already. Take a look at the schematics for the various ESP8266 and ESP32 modules (NodeMCU etc.) and see how they do it, then move the programming interface to your bootstrap board. I think you only need TX, RX and one pin to put it in programming mode so the connections are very simple.

Brian.

 

[DihydrogenMonoxide]

Let me see if I understand.
You guys are saying I can use the AMS1117 to power both boards at 3.3V. The AMS1117 would be powered from the 5V USB line during programming, and from barrel jack at production time.
The datasheet of the CP2102N shows a configuration for ignoring the internal voltage regulator (page 6 of https://www.silabs.com/documents/public/data-sheets/cp2102n-datasheet.pdf).
Am I on the right track?

 

[DihydrogenMonoxide]

In the AMS1117 datasheet that you can have a crowbar circuit on the AMS1117. If I add that to my board, would that be safer than leaving the input floating?

Also, general question, how do voltage regulators like AMS1117 behave when input and output are shorted. Do they act as passive devices or can they wreak havoc on the rest of the circuit?

 

[betwixt]

I have attached the schematic for the ESP32 dev board. If you move the parts in the "Micro USB 5V&USB-UART" box to your bootstrap board and make connections of GND, TXD, RXD, EN and IO0 to your main board it will allow programming and serial coms through the USB socket. With it disconnected you can use the pins for other purposes. The CP2102N needs a 3.3V supply, ideally the same supply line as the ESP32 so the logic levels exactly match but that means an extra connection to the bootstrap board to carry 3.3V to it. If you want to, use a second AMS1117-3.3 to generate 3.3V from the USB 5V supply on the bootstrap board itself.

No harm will come from linking the input and output of the AMS1117 together but beware that the ESP32 WILL be damaged if more than about 3.6V is applied to it so I would not recommend you do it.

Brian.