I'm a hobby home cockpit builder, and making my panels myself. Panels have backlighting, for which I use LED's. To control the LED's, I'm using STP08CP05TTR having 8 constant current outputs. I use the OE pin for brightness using PWM. Because the current is constant, I'm sure that all LED's will have the same brightness. But to allow at least 5 LED's on one output, and having only a 12V power supply, I'm upconverting from 12V to 18V. Below is an extract from my schematic. I tune my 18V output voltage with potentiometer R3.
The problem is that when I run my software, and I enable my backlight, that the 18V output power seems to drop sometimes to 16V, or even lower to about 12V (= input voltage?). If I disconnect the input power, and connect it again, then it goes immediately up to 16V á 18V. It's not really stable. The strange thing is that this happens when I start my software, which means that the LED's are temporary switched off and then on while the power supply is working continuously. Could this be because of a peak load? When I unplug/replug the power, then the dynamics are obviously different, because then the software starts almost at the same time as the LED's are being enabled, and then the power seems always high enough to drive the LED's.
Any ideas on what I'm doing wrong or where to look at?
PS. I'm not an electronic expert, rather a software guy. But so far my devices that I made work pretty well. This power issue is the ONLY issue I'm having so far.
Additional info:
As asked in some comments below, here are the specifications of some components - I provide the links to the LCSC Electronics website in which you find the datasheet links:
- L1 - SWPA6045S220MT - 220uH
- U3 - MT3608L
- U5 - STP08CP05TTR
- D1 - SS34
- LED - Hubei KENTO Elec KT-0603W
The 1K resistor (Rext) on the LED driver (U5) controls the output current of the outputs to slightly lower than 20 mA - see page 14 of the datasheet. As we have 6 chains of LEDs connected to the 18V bus, it means that the maximum current drawn will be 120 mA.
Update 18-May-2024
It looks like the issue is caused by erasing my microcontroller!
I used one of the outputs to trigger my oscilloscope (RICOL DS1202) in "Single" mode. The goal was to see if there are any disturbances on my 12V and 18V bus. By doing that, I discovered that my 18V bus dropped far before my code starts initializing. I even commented out all my code, and still I see that my 18V bus dropped. Coincidently, I saw that the drop correlated somehow with the message "Erasing..." on my IDE output screen. With this knowledge, I powercycled my board (which always brings my 18V bus back to 18V) and I just issued the command "Erase Device Memory Main Project", and I immediately saw my 18V bus dropping to 12V.
I'm using a PIC18F47J53-I/PT from Microchip. My IDE is MPLAB X IDE v5.45 which is connected via PICKit 4 through connector H4. See below schematic.
So, the mystery now is - how is it possible that this erase cycle is influencing the 18V bus. My PICKit 4 is powered by my board ("Power target circuit from PICKit 4 = disabled), and I have also experimented with the 2 options of "Programming mode entry", which can be "low voltage" and "high voltage", but these options seem also not to make any difference.
This also kind of explains why power cycling my board, which restores the power and restarts my board, seems always to work flawlessly. In this case there is no erase cycle, which seems the reason for my issue.
Update 20-May-2024
I tried to measure the FB voltage on pin 3 of the MT3803L. Seems impossible, because if I only touch it with my scope or my multimeter, I influence the output voltage. It always is "around" 0.6V (as mentioned in the datasheet), but pretty "spiky". In some cases, when I touch it or release it, the output goes from 18V to 15V (or other voltage).
Conclusion
Looks like I don't get the step up convertor under control :-( Luckily, my Power Supply has a 24V output on spare, which I can connect to a 7818. Probably easier and a lot more reliable.
