Need help with Boost Regulator & Optocoupler Circuit - Page 1
Forum section: electronics / beginners

author: Marcus_DLG
author_rank: Newbie
timestamp: January 13, 2021, 11:39:00 pm
content: Hi,I recently found some HPDL-1414 led modules and I thought it would be cool to make a 70's style LED watch with one of them. My goal is to make it as small as possible and maximize battery life from a 3V CR2032. Right now I'm using an STM32L011 microcontroller to run the RTC and drive the display. So far everything works but the microcontroller is running at 3V and the LED display is designed to run at 5V. It does work, but it is very dim and not ideal. My next thought was to use a boost regulator to increase the battery voltage to 5V just for the display that could be turned on and off. I went with the TI TPS61222DCKR because of its small size. I bought it and built the example circuit from the datasheet and it worked great, the only problem is that even when the enable pin is low it still passes the battery voltage through to the output. This is a problem because even when the display is not showing any characters, it still draws about 200uA from the battery. My next idea was to connect the output from the boost regulator to a PNP transistor so that I can switch the power to the display on and off. I tried various configurations but couldn't get any of them to work. I figured I would have better luck if I triggered the PNP with an optocoupler instead. I breadboarded that circuit with 5V on the PNP collector, and 3V on the optocoupler anode all from my bench power supply, and sure enough that worked great (got 5V on PNP Emitter). However, when I connected the boost regulator and the transistor circuit together it never functioned correctly. The most I would ever get on the PNP Emitter was about 2.5V. I don't have a lot of experience with PNP transistors or optocouplers so I'm hoping I've just made a simple mistake that someone will point out. I attached the schematic, let me know if you need any clarification on it. Thanks in advance!

author: m k
author_rank: Super Contributor
timestamp: January 17, 2021, 11:57:39 am
content: Consider reshaping your message.Add paragraphs and more conservative picture, even hand drawn if needed.(20/77)

author: Marcus_DLG
author_rank: Newbie
timestamp: January 21, 2021, 12:49:07 am
content: I figured out what the problem was. It had nothing to do with the transistor circuit, it was all down to the boost regulator. Since I was enabling the boost regulator at the same time I started drawing current from it, it did not have enough time to build up the voltage. Theoretically the solution would be to enable the regulator a few seconds before I turn on the display so that it can charge up the output capacitor to 5V and supply more current.Unfortunately there are 2 problems with this in my particular application. After testing with multiple output capacitors, even with a massive 4700uF cap, the 5V collapsed after about 3/4 of a second. The second problem is that having to wait a few seconds after hitting the button to see the time on the display would be a horrible experience.Based on everything I've learned, I'm going to have to change my plans. My current idea is to use 2 stacked 2016 coin cells to get 6V from the battery's. Instead of stepping up the voltage I will have to step down the voltage to 3.3V for the microcontroller and potentially 5V for the display (although I might just try running it at 6V if I can get away with it). My only concern with this method is that because the regulator will always be on for the microcontroller, the battery life will be considerably worse than I was hoping, especially since I would now be using 2016's instead of a 2032.If anyone has a better idea on how I should approach this project I would be happy to hear it, otherwise the question I originally posted has been solved.

author: Infrared_Fred
author_rank: Regular Contributor
timestamp: March 02, 2021, 09:35:14 pm
content: To me, it sounds like you may be overdrawing the battery. If the ESR of the battery is high enough, assuming a ~75mA load x 5V = ~0.4W, the load current on the battery is going to be about 130-160mA which sounds too high for such a small size cell. Also, I have heard of being able to run an LED of the battery without a series resistance since the internal resistance is high enough so running a large current of it probably doesn't work. See thisCR2032datasheet for further information.Maybe a battery like this onePRT-13851would work with an appropriate charger board.