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BQ24170: Charging and input current not matching settings

Part Number: BQ24170
Other Parts Discussed in Thread: LMZ31503

Hello,

I'm using the BQ24170 for charging a two cell Li-Ion battery in our application. We are using a circuit similar to the one shown on page 4 of the datasheet. Power from the charging circuitry is fed into the TI LMZ31503 and regulated to 5V for our application. In general the chip and circuitry has been working well for charging our batteries, but I've been looking more closely at setting the current for the input and charging, and things aren't working as I expected.

I'd like to set the max input current to 1.5A and since we are using 2.2Ah batteries, I was planning on setting the charging current to 1.5A, too. In our application, when the device is on at full power with the battery disconnected it draws 800mA from a 12V supply.

In our circuitry, I'm using 10K and 100K resistors for the voltage dividers required on the inputs to ISET and ACSET (R4&5 and R2&3 on the datasheet schematic). We are using 0.01 ohm resistors for Rsr and Rac, so according to equations 2 and 4 in the datasheet, the currents for both the input and charging should be set to 1.5A. When I actually look at the current draw when using a 12V input, I get 1.1A just charging and 1.76A charging and running at full power. If I lower the input voltage to 9.5V, I get 1.36A just charging and 1.63A charging and running at full power. 

I tried a similar set up using 1K and 10K resistors instead and got similar results. I've also had similar results at different current settings, the charging current is less than expected and the input current is more.

Have you seen anything similar to this? Am I doing something wrong or misunderstanding how the power management works? Should I expect the current draws to change if the input voltage changes?

The behavior seems fairly consistent across all my boards, so I'm fine just adjusting the resistors until I find something that keeps the currents below the max current we want. But, I do want to make sure doing that isn't going to come back to bite me in the butt once we go into production.

One last thing to note is we designed the PCB layout by referencing the eval board and layout guidelines in the datasheet; however, our board is only 2 layers.

Thanks,
M

  • Hi M,

    I am sorry for such a delay in response for this.

    I would like to point you to this other E2E post that I believe will help you understand the dynamic power management better.

    e2e.ti.com/.../1116989 dpm#1116989

    The ACSET current is the input current threshold for when the IC will begin to reduce charge current to keep the input current below the set threshold. However, if the charge current is reduced to 0A then the DPM loop can no longer hold the input current below the set threshold. At this point the system will directly draw as much current as is needs from the adapter on the input.

    The difference in current values as voltage changes is expected. 12V at 1.1A and 9.5V at 1.36A is about the same power. For the currents above the 1.5A threhsold, the IC can no longer regulate the input power, because the system is taking what it needs from the adapter.

    Let me know if this answers your questions and concerns.

    Regards,

    Joe
  • Hi Joe,

    Unfortunately, I get this error when I click on that link:

    "Access Denied (Note: a common reason this message is displayed is because the thread was deleted)
    You do not have permission to view/download this item."

    However, I think the rest of this post puts my concerns at ease. I had noticed that the power draws were similar, but I wasn't sure if things were working right because the datasheet lists equations for the input and charge current without regards to the input voltage. Anyway, it seems like it is working as expected and I just need to find a resistor set that will keep the input current below my adapter's capabilities.

    If you can post the info in that link some other way, I'd still be interested in reading it.

    Regards,
    M
  • M,

    I apologize, it looks like that post is restricted. The description I posted is very similar.

    Instead of that post, this article goes into more depth about DPM: Dynamic Power Management for faster, more efficient battery charging\

    If you feel your question was answered, could you please click the "Verify Answer" button?

    Direct link if hyperlink doesn't work: http://www.ti.com/lit/an/slyt546/slyt546.pdf

    Best Regards,

    Joe