1. **PCB Size**

2. The microcontroller used is the Silicon Labs EFM8 Sleepy Bee. The relatively large connector on the left provides a direct link to the SiLabs USB debug adapter. This connector occupies a significant amount of PCB space, making the overall design appear larger than it actually is.

3. The figure below illustrates the PCB dimensions in inches. The shorter horizontal dimension represents an estimate of how compact the board could be if the debug connector were removed and other components were rearranged.

Here are some other ways to reduce the size of the board:

1. I chose the larger passive component sizes (0805 and 1206) because they are easier to assemble. If you plan to assemble boards, consider using 0603 or even 0402 sizes (you may find acceptable 2.2μF capacitors in an 0402 package, but for 0.1μF capacitors and resistors, 0402 is suitable).

2. I opted for a larger microcontroller package; this is a 9mm x 9mm QFP32. A 32-pin QFP is significantly smaller (5 mm x 5 mm), and there’s an even smaller 24-pin QFP (4 mm x 4 mm). In my opinion, most applications using this power supply do not require more than a few I/O pins, so a 24-pin package is likely the best choice. I used the 32-pin version because the microcontroller does not offer a leadless package.

3. I use a 32.768kHz crystal oscillator for real-time clock applications; it is about the size of an 0805 component. The microcontroller includes an internal low-power oscillator with ±10% accuracy, so if precise timing isn’t needed, the crystal can be omitted.

4. Charge pump switch regulators currently have four 2.2μF output capacitors, though only one is necessary.

5. The LED and its resistor are used solely for debugging and can be omitted from the final design.

6. You might consider removing all circuits (switches, LDO, and two capacitors) associated with debugging power supplies. However, I do not recommend this because solar power is not an ideal source for firmware development and testing.

Double choice

The final suggestion for reducing size involves placing components on both the top and bottom of the board. As I wrote this article, I considered whether the entire circuit could fit within the solar cell’s footprint, allowing for a design with the solar cell on top and all other components underneath. I removed some unnecessary components from the schematic to test this idea, and here’s what I found (in inches):

It’s a rough approximation, but as shown, we are close to fitting all the circuits into the PCB space occupied by the solar cells. To achieve this layout, I eliminated three of the four output capacitors, the crystal, the LED, and its resistor. I also switched the microcontroller package to QFN24. The passive components remain 1206 and 0805, but these larger packages help manage the connection to the debug adapter. While routing space is limited, using a four-layer board (with ample room below the solar cell) should address this issue.

Conclusion

We’ve examined the PCB layout of my recent solar microcontroller board and explored a more spatially optimized design that closely matches the size of a solar cell. If you have experience with space-constrained designs for low-power embedded devices, please share your thoughts in the comments.