ESP32 based device schematic review

Question

I am building an IoT device with ESP32 WROOM as a host MCU.
I have already made two test PCB with it, and show somewhat unstable behavior.
The issues are;

• It seems like USB-UART is unstable. Sometimes a computer recognizes the UART properly but sometimes does not. Also, flashing and monitoring with terminal(esp-idf monitor) works just fine, but monitor with putty throws trash characters.

• It also looks like have some power issue. When I build the firmware with 'brown-out detection activated, it continues to get restarted with a brown-out detection message. However, when I check the voltage of the 33V pin of the ESP32, the voltage is stable at a sufficient level (~3.4V).

• The L80-R is GPS module, and CMWX1ZZABZ-091 is LoRa module.

Can't find the reason by myself. Here is my schematic. Please, let me know if you have any opinions.

Answer 1

First: How did you check the voltage on the 3V3 ? using an oscilloscope ? A multimeter won't do the job here. It won't catch the very narrow voltage drop due to the huge current pulses of the ESP32. And if you are experiencing brownout reset, there is an very high probability that there is such a voltage drop, even extremely short.

You may also have a look at that video : https://www.youtube.com/watch?v=LUB8RWzzLWc&t=2s : It is related to measuring small currents, but at some point, it shows brownout issues with an ESP32 and measures its associated current peaks. Might be informative for your particular issue.

As said by @Kartman, the kind and placement of the decoupling capacitors can make a big difference here. What kind is it ? Ceramic I hope. Large electrolytic capacitors have poor ESR. They are good for storing energy "slowly", but they are useless for absorbing current peaks of digital chips. Check the datasheet of the various chips. 100nF might not be the way to go, especially with radio chips that may require much lower value to catch very fast transients.

To illustrate the remarks of @Kartman, real capacitors are not like ideal ones. The capacitance derating vs DC voltage can have a very strong impact. Here is a derating curve for a Y5V dielectric capacitors.

Imagine that you are using a Y5V 10uF 4V ceramics caps on your 3V3 power rails. 3.3V is 85% of the 4V rating. Thus looking at the curve, the derating is -90%. Your 10uF capacitor behave like a 1uF !! Thus the exact choice of the component matter. For instance, you could use other dielectric such as X7R which has a much much lower DC voltage derating, or choose a part rated for 25V or more.

With only the schematics given, the only thing that makes me unhappy is the location of Q5. It adds a small resistor between the VCC pin of the GPS and its decoupling capacitor. That is very bad, especially for a device like a GPS that is doing radio stuff.

Please post screenshots of the layout as well and I could extant my answer accordingly. The component placement and the power routing scheme is likely to the the source of your issue, but we can't look at it without a picture of the layout.