4 Replies Latest reply on Dec 1, 2017 6:16 AM by rachaelp

    Power management design confusion

    mike24

      Hello all,

       

      I have a project in which I am making a data logger for different temperature sensors and comparing both at the same time, the thing is that I have 2 boards, a main board with female socket pins like an arduino and connected to them is an adapter board that looks something like this

       

      On the main board I have a USB male plug that powers the whole device with 5V but all my components use 3.3V so I have a LDO in the main board that lowers the 5V into 3.3V.

      The problem is that I want to make it as a mobile device that you can take anywhere you want and log the data from the sensors, therefore I am adding a LiPo battery in the adapter board that is on the top and also adding a power management device that handles the recharging of the battery and the supply of the whole device and disconnects the battery from the system when the USB is plugged in a PC, I cannot change the design of the main board because it is more time consuming and would take more than a month to have the PCB made.

       

      So after the short explanation of my device please see the image below to get a better understanding of my problem.

      Would there be a problem if I place the connections like this? Would there be a short circuit where the pin 13 and 14 meet?

      The BQ24075 has overvoltage protection, it supports up to 1.5A charge current, also has reverse current, short-circuit and thermal protection.

       

      May be I have already answered my question but I just want a second opinion on this forum, multiple brains helping each other are better than having one confused brain trying to solve multiple problems at the time.

        • Re: Power management design confusion
          michaelkellett

          It's not good, if the USB source loses power then your battery tries to power it.

           

          If the USB lead is plugged in live your main board sees a voltage spike (which it may not care about).

           

          You could add another diode in series with the USB but it won't reliably charge the battery if you do (but depending on the application you might get away with it).

           

          MK

          3 of 3 people found this helpful
          • Re: Power management design confusion
            jw0752

            Hi Miguel,

            Connect your Main Board to the output of the BQ 24075 Pin 13 on your diagram. Do not connect pin 13 and Pin 14 together. The BQ 24075 is designed to simultaneously power your device and charge the battery so you use it portable and when you are hooked to the USB it will run the unit and charge the battery. Here is the Data Sheet. Check out the 3rd paragraph in the introduction.

             

            http://www.ti.com/lit/ds/symlink/bq24075.pdf

             

            John

            3 of 3 people found this helpful
            • Re: Power management design confusion
              mcb1

              mike24

              I suspect if you used two diodes it would work.

              Feed the USB power to the IN of the BQ24075, and thru a diode to the Main board.

              Feed the OUT of the BQ24075 thru a second diode to the Main board. (ie both anodes join)

               

              This will stop the battery trying to back feed the USB and may solve your power issue.

              The better silution is how Arduino and others use a transistor to switch power sources, but as you indicated the boards are done already.

               

               

              Mark

              1 of 1 people found this helpful