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BQ25820: Need some help with the battery charger design.

Part Number: BQ25820

Tool/software:

Hi,

I have designed a circuit using BQ25820 but with some variations from the datasheet and EVM schematic. I hope I can get some clarity on whether this can be done.

Some basic design parameters:

  1. Vin: 6V to 50V
  2. Vout: battery only
  3. Battery: 3.6V LiFePo4 battery
  4. Charge rate: 1.5A max
  5. Discharge rate: 8A max
  6. Switching frequency: 250KHz

Questions

  1. ACUV and ACOV
    1. I have used two separate resistive networks to control the AC input range, as I wanted to set them from 6V to 50V. The datasheet and EVM had them connected in a triple resistor network, so I'm unsure if there's some consideration that I might have missed here.
  2. ACFET and BATFET
    1. I kept ACFET to control input to the buck converter, but I bypassed BATFET, so battery is always connected to my system load. Also to note that the ACFET is not connected to my system load.
    2. My question is whether this IC could face any issues when the AC input is active, and the system load draws its maximum current of 8A. I anticipate that the buck converter will also experience increased load since the battery voltage may dip under these conditions, causing the buck to remain active. Although the maximum current is expected to last only a minute or two, could this pose any potential problems?
  3. Inductor rating
    1. My max Iripple_buck seems to be around 5A at Vac 50V and D = 0.5. Am I correct to size the inductor at Isat ~=12.5A, or would that be excessive?
  4. Input capacitors
    1. What is a good way to size the value here? The EVM had the input capacitors at 56uF; could I size it down? 
  5. Battery capacitor
    1. What is the typical acceptable ripple voltage range if I am using the BATFET?
  6. Design checklist
    1. I just wanted to check if you have any kind of Excel spreadsheet for calculating the design parameters.

I hope my questions aren't too much to ask in a forum. Thank you.

  • Hello Kenneth,

    Thanks for working on this.

    1.  This will work for the charger.
      1. This should work
      2. This should not be a problem, but your battery voltage may dip here.
    2. This will be fine
    3. The minimum capacitance is 80µF on the input and 80µF on the output. You might be able to use only 40µF on the input and output, but you'll have a lot more voltage ripple on the output.
    4. Can you rephrase this question? I'm not quite sure what you mean here.
    5. Yes, we do have a design checklist and a design calculator. We also have a general FAQ for this family of parts.

    One thing to keep in mind though, is that the BQ25820's trickle charge region is 0V to 3V. That means the IC will only enter fast charging from 3V to 3.6V unless you set EN_PRECHG=0. To work around the trickle charge region, I'd recommend using up two LiFePO4 cells in series or 1 Li-Ion cell to extend the voltage range of the circuit.

    By the way, will your system have I2C?

    Let me know if these answers makes sense and let me know if you have any other questions.

    Best Regards,
    Ethan Galloway

  • Hi Ethan,

    Thank you for your response. 

    1. Noted.
    2. Noted on your response for the two sub-points.
    3. Noted.
    4. I will go with 80uF. Just to clarify, does the input cap here refer to the cap after the AC FET? Also, how should I balance the cap on both ends of the current resistor? Do I place a 40uF (1/2 of 80uF) on the other end of the resistor?
    5. What I'd really like to know is how to properly size the capacitor. Due to space constraints, I'm aiming for the smallest possible capacitor. Some ripple is acceptable, but I want to ensure it doesn't destabilize the BAT FET to the point where it can't switch properly. Is this something I should be concerned about?
    6. Thanks. I will check on them.
    7. Regarding the I2C connection, I do have it linked to my MCU. My plan is to force `EN_PRECHG = 0` to address the concern you mentioned. Thank you for pointing it out! Please let me know if I've misunderstood anything.
  • Hello,

    Due to the U.S. holiday our responses will be delayed, sorry for the inconvenience.

    Sincerely,

    Wyatt Keller

  • Hello Kenneth,

    Thanks for the being patient with this.

    4.

    Also, how should I balance the cap on both ends of the current resistor? Do I place a 40uF (1/2 of 80uF) on the other end of the resistor?

    Yes, putting 40µF on both sides of the current sense resistor will work. You can see this FAQ for more information.

    5.

    What I'd really like to know is how to properly size the capacitor. Due to space constraints, I'm aiming for the smallest possible capacitor. Some ripple is acceptable, but I want to ensure it doesn't destabilize the BAT FET to the point where it can't switch properly. Is this something I should be concerned about?

    The power path switchover for the BATFET is controlled by ACUV and ACOV. I don't think you should be concerned about the BATFET not being able to switchover.

    Best Regards,
    Ethan Galloway

  • Hi Ethan,

    Thanks for replying. The FAQ clarified my doubts about the capacitors, and I appreciate your timely and detailed feedback.

    Kenneth