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Dual Input w/ PowerPath

Other Parts Discussed in Thread: BQ25703A, BQ24773, BQ24232H

Researching for a new product in the concept phase. Here's the bullet points:

  1. Battery charging of different Lithium based batteries.
  2. Power for the system can come from 
    1. DC input up to 22V
    2. Solar Panel (specs TBD)
    3. In the absence of either charging source the system will run from battery
  3. System runs at ~3VDC and max 300mA draw

I think the high DC voltage input limits my choices severely so that could get a switcher to bring down the voltage but I'm hoping to get lucky and find something that can handle the higher voltage. The solar panel will be somewhat small (maybe no more than 2" x 3").

Suggestions and thoughts on this are greatly appreciated.

Thanks,

George

  • 1. What is the solar panel power level and the battery capacity? Did you already find smaller than 2"x3" panel with 22V output? What is the maximum power point voltage for that panel?

    2. "System runs at ~3VDC and max 300mA draw" So, is the battery single cell Li-ion battery?

    3. Dou you have one battery or two batteries in your system? That decides that the solution uses one charger or two chargers.

    4. When two input sources are plugged in, do you want to use both power sources to charger the battery? Or, one input source has a higher priority? That decides the solution uses one charger or two chargers.

    5. In your system, do you have a MCU or other embedded controller which can decide charging profile and support I2C communication?

  • 1) The solar panel has not been selected yet but the size is somewhat fixed based on the product concept. I imagine this small of a panel will have low voltage, less than 6V

    2) Yes we are planning on using a single cell, Li-Ion or perhaps LiFePO4

    3) Only planning on 1 battery

    4) The 22VDC max input has much higher power so it should be preferred. For the most part these 2 sources will not be available at the same time but there could be situations when both are available.

    5) The system will have a MCU so it can interface with the charger. Ideally the charger should still be able to function if for some reason the battery is removed or dies and the system has to rely on the input power sources to operate.

  • You can select two channel ORing diode circuit in the front which is connected to 22VDC in one input port and solar panel in another input.
    Two channel Oring diode outputs are connected together as a charger input source. If the solar panel MPP voltage is higher than the Li-ion full charge voltage, you can use a buck charger bq24773. If the solar panel MPP voltage could be lower than the LiFePO4 full charge voltage, you can use a buck boost charger bq25703A.

  • Those are interesting parts that I overlooked. Did you select those because of the 22V input? What happens if I lower that input a little, does that offer me more solutions?

    Can you think of any reference designs that are similar to what I'm trying to achieve?
  • Yes. Pick bq24773 and bq25703A because of the 22V input. If the input voltage is lower, you have more options. You can find more available parts from battery charge selection guide in TI.com. Some parts may already support dual inputs, you don’t need a reference design. You can just check the EVM.

  • If I look at the bq24232H I'm confused by the Vin spec. It has a line that says Vin can have a max of 26V but then the line below says operating voltage range of only 10.2V max. Are they just trying to say that you can have spikes of up to 26V but the part won't operate above 10.2V?
  • That is correct. You can have spikes of up to 26V but the part won't operate above 10.2V