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