Anton
The smallest ESP32 chip available is 5x5 mm only and its C3FH4 modification offers a small amount of flash memory internally! Thus I need just a crystal oscillator and a couple of discrete passives. Two more large components left - power supply and antenna. Previously I successfully used the SMD-type ceramic chip antenna which comes in various sizes from Arbacon or Johanson Dielectrics for instance. Thanks to a wide range of antenna models it is possible to pick up some which require less attenuation components or even could run without them.
The smallest possible I found is only 5x2 mm
So how the board could be powered and what kind of the form-factor I could use to beat all the competitors? I believe I could fit all the required components on the board plugged in a regular USB header! Thus the board could be accurately hidden inside a PC or laptop. No sticks around!
Back to components selection, the last thing is to supply the ESP32 chip with the 3.3V at 1 Amp (peak loads). Getting power from the USB means I need to step the 5V input down to 3.3V. I also need to keep in mind space constraints while 10x12 mm is desirable as a final board dimensions. The TLV75733 I successfully used before comes in SOT-23-5 package and is good enough, seems stable but heats too much as to me.
Thus I decided to replace it with the TPS74801 from TI again. Unfortunately it requires a pair of additional resistors to set up the output voltage level in contrast to the previously mentioned TLV75733 which is fixed. However TPS74801 is able to provide up to 1,5 Amps which is more than enough for my load even considering a poorly attenuated antenna.
This voltage regulator comes in VSON-10 package which seems bigger, but there are no external pins like SOT-23-5 has. So on the whole it takes just 1 extra millimeter at width. And the output should be stable with the 2,2 uF capacitor only. I strictly adhered to the ESP32 guidance and used 10 uF capacitor together with the 1 uF and 100 nF caps on its critical pins. That should be more than enough even keeping in mind the capacitance vs. DC bias effect.
I started my design using 0402 SMD components. However it was almost impossible to place and trace them properly. So I was forced to switch on 0201 mostly, except the main capacitors (0402 and 0603) and power inductor (0402). And I still need to minimize the number of required components as much as possible to fit the board. There will be no power management circuit therefore - once the board is plugged in USB it could get the power constantly. I even abandoned the Reset button because (or thanks to) of that form factor.
Now dealing with the board height. The higher component is ceramic chip antenna which is 1,1 mm. The overall board height should not exceed 2 mm (including components) while mostly used PCB thickness is 1,6 mm. That means I need to pick the lowest possible option. I really like the price-to-quality offerings of the JLCPCB company and am happy to produce a few prototypes there. Thus I need to figure out the lower available option for the PCB thickness on their website.
It looks like 0,4 mm fits well to my requirements: the MCU and voltage regulator chips as well as crystal oscillator are not higher than antenna, but that thickness turned out extremely expensive and I decide to select the next one suitable and more popular - the 0,6 mm. And of course to hide the device well it is better to select black color of the solder mask.
Finally I assume to cover all the components with the epoxy resin in a form to get one solid black or dark grey block with the pads on the reverse side. That block could be easily plugged in as planned initially.
This is a 2-layer board only which was also the challenge for me to stay in 2 layers only while it is possible. Needless to say how the production cost will change with the growing number of PCB layers. So the production costs are reduced as much as possible.
I am...
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