Design of a solar charge pump power supply circuit

In this paper, we discuss a new method which centers on the non - inductive DC/DC Converter and solar cell 3.3 V Power circuit .
I recently designed a circuit board , Let me have the opportunity to explore the application of solar energy in embedded devices . My thoughts are as follows : I want a reference design , Will act as a very compact , Using ambient light in a very direct way 3.3 V Microcontroller based embedded system . I've tested enough boards , To confirm that it works , Although it has major limitations . In this paper , I will focus on the schematic design of the board .

Keep it small
The overall goal of my project is to develop a simple method , Integrate solar energy into small low-power devices , Such as wearable devices and IOT sensors . up to now , The performance I observed didn't impress me very much , But the scale of the whole solution is quite good . The main components are 7mm × 22mm Single crystal solar cells (p/n KXOB22-01X8F) And micro charge pump DC/DC converter (p/n LTC3204EDC-3.3).

Images taken from this data table

Picture by Digi-Key Provide

In addition to solar cells and charge pumps , All you need is a capacitor : An input capacitor 、 The capacitance of a charge pump circuit and an output capacitance . I'm glad to know , You can convert sunlight into a stable 3.3 V The power supply has only one solar cell , A miniature six pin integrated circuit , And three capacitors . What's better is , All three capacitors can be of the same value , This makes it easier to order parts and assemble circuit boards .
My execution
The following sketch excerpt shows the solar energy I designed 3.3 V Power Supply . I include four output capacitors , This enabled me to add and remove effects as a way to experiment , Higher or lower output capacitance . As mentioned above , although , There is only one output capacitor ( The minimum 2μF) It's necessary .

As you can see , It is difficult to compete with the simplicity of this approach .BOM There are three line items , And the interconnection is very simple . All capacitors can be surface mount ceramic type ; I recommend X7R. The most difficult part is to install LTC3204, It's not just tiny , And it is a leadless software package with a thermal pad . With the help of my hot air gun and some low temperature bismuth based solder paste , I can finish it easily .
Limit
Before I say anything positive about this design , Let's review the negative aspects :

• It has no supercapacitors or batteries , let me put it another way , It has no way of storing energy for a long time . This is a “ Work in the sun ” Device of .
• The charge pump IC can achieve proper adjustment only when the solar cell is exposed to the sun . This is an unexpected limitation , I will explore this further in the next project article . The bottom line , As far as I know , Only sufficient solar energy can generate enough solar cell current , To adequately supply the required input current LTC3204.
• Even in full sunlight , The available output current is also quite low . The maximum capacity of the solar cell is 4.4 Ma , This is not enough for the operation of high-speed microcontrollers , And I guess RF communication is not very feasible .
  homicide
  My circuit board includes not only the solar charging pump power supply , It also includes a microcontroller circuit :

Click to enlarge.
Click to enlarge

• The microcontroller is from the silicon lab EFM8 Sleepy bee . stay 32.768 kHz At the operating frequency of , It just needs 90μA Power supply current .
• A switch allows me to switch from a solar cell circuit or from a silicon lab USB The debug adapter supplies power to the microcontroller . The debug adapter provides 5 v , So you need a LDO ( Sleepy bee VDD The scope is 1.8-3.6 v ).
• 32.768 kHz The crystal enables me to realize low current consumption and high-precision real-time clock applications . No capacitors are required ; Connect the crystal directly to XTAL3 and XTAL4 Pin , Then configure the load capacitance through the firmware .

Charge pump
LTC3204 There are four styles : ( Fix ) The output voltage can be 3.3 V or 5 V, The unit can operate in burst mode or constant frequency mode ( No, “ Mode selection ” Pin ; You must specify the part number corresponding to the desired mode ). I chose burst mode . I'm not sure if there's any obvious benefit in solar energy , But in burst mode “ No load input current ” Much lower , Considering the output current limit of my solar panel , This seems to be a useful feature .

Taken from the LTC3204 Data sheet . with “ B” The suffix part number is a non burst mode version
LTC3204 Stable 3.3 V Output , The input voltage is from 1.8 V To 4.5 V; This makes it associated with KXOB22-01X8F Solar cells are a good match :

The voltage of a solar cell can be as high as 4.7 V, Strictly speaking, it goes beyond LTC3204 Input range of . However , The absolute maximum input voltage is 6 v , So the voltage of the solar cell will never exceed or even approach the absolute maximum , I doubt that 4.7 V input will cause any problems .

Conclusion
We have seen the schematic design of a solar powered microcontroller board , It uses a simple , The compact charge pump IC produces an adjustable 3.3 V Power track . This is a small and fairly inexpensive solution , Can be integrated into a variety of low-power devices . It has great limitations , although , I currently have a plan to modify the design , More is needed PCB The real estate , But in exchange , Provides higher performance and easier assembly .