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Looking at the datasheet for the ESP32-WROOM-32E module, it appears that it takes 3.3V input at pin 2. Do I need a smoothing and/or bypass capacitor on this line?

Figure 6 shows a 22uF and 0.1uF capacitor on the input, but it does not mention it anywhere else in the datasheet.

enter image description here

Link to datasheet:

https://www.espressif.com/sites/default/files/documentation/esp32-wroom-32e_esp32-wroom-32ue_datasheet_en.pdf

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3 Answers 3

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All CMOS creates switching pulse currents and need a cap and big chips especially. Keep them as close as possible.

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  • \$\begingroup\$ to be fair, this is a castellated edge PCB, which in itself might carry capacitors. Wouldn't be to crazy to expect it to be self-sufficient in decoupling caps for "reasonable" supply/ground connection. \$\endgroup\$ Jan 1, 2022 at 1:17
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    \$\begingroup\$ However, caps are cheap, and unpopulated footprints are free, so this question simply doesn't arise: if there's a chance you might want these capacitors, you'd want to put them there; can remove them in a later revision if you unsolder and can prove the system still works well. So, classic Tony: +1 (and a happy new year!) \$\endgroup\$ Jan 1, 2022 at 1:18
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There's no height for a large capacitor below the ESP32 shield so you have to add it externally.

Its value depends on what LDO you'll use for 3V3. You should read the LDO datasheet and pick a capacitor value that makes your LDO stable.

Most modern LDOs work with low ESR ceramic caps. If you pick a fast transient response LDO like LDL1117, a 10µF 16V X7R cap is sufficient. It should be close to the 3V3 and GND pins on the ESP32.

Some older LDOs require capacitor with a large ESR, and having large ESR means the cap won't help much with ESP32's pulsed current. In this case to achieve these two contradicting objectives (LDO likes ESR, and high ESR screws up transient response) the solution is to use a larger cap value, more than 100µF electrolytic or tantalum.

Don't use AMS1117.

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  • \$\begingroup\$ why not AMS1117? (just curiosity) \$\endgroup\$ Aug 9, 2022 at 15:46
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    \$\begingroup\$ It has a very slow transient response, so your ESP32 will crash sometimes when its current draw steps from very low (sleep) to high (wakeup). \$\endgroup\$
    – bobflux
    Aug 9, 2022 at 18:05
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The ESP32 draws significant current at startup and when transmitting. You need to have the 22uF capacitor close to the module otherwise it will be highly unreliable. The choice of 22uF capacitor is critical - it needs to be a ceramic and therefore you need to consider the voltage derating on the capacitance. The capacitor will probably be a size 1210 and 25V. Every manufacturer is different and you need to carefully read the datasheet. If you choose a capacitor of size 0805 and voltage of 6.3V, you'll be unlikely to achieve anywhere near 22uF of capacitance. If you're height constrained, use a number of smaller capacitors.

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  • \$\begingroup\$ Why does the bulk cap have to be ceramic? I have very reliable custom ESP32 boards with low ESR electrolytic bulk decoupling caps 2-3 cm from the ESP32 module. (There are smaller 0.1uF X7R ceramics close to the power pin.) My research led me to believe the placement of the bulk cap is much less critical. There are practical demonstrations of this online. \$\endgroup\$
    – TypeIA
    Jan 1, 2022 at 7:58
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    \$\begingroup\$ @TypeIA - according to Espressif you want to place the capacitor as close as possible. espressif.com/sites/default/files/documentation/… Regarding the 10uF vs 22uF debate, on their dev boards, Espressif use either a 22uF 10V ceramic or a 10uF 25V ceramic. Due to the DC bias derating, the 22uF 10V will hopefully yield around 10uF of capacitance and the 10uF 25V will suffer significantly less derating, so you'll get more of the 10uF capacitance you paid for. \$\endgroup\$
    – Kartman
    Jan 1, 2022 at 11:11
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    \$\begingroup\$ As for ceramic vs low ESR electro, the current transitions are fast, so you really want a ceramic. I've not seen a design by Espressif or other commercial design that uses anything other than a ceramic. Maybe one day I'll get motivated to do some tests with my new 600MHz scope to prove/disprove. \$\endgroup\$
    – Kartman
    Jan 1, 2022 at 11:14

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