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bq24171 low charge current

Other Parts Discussed in Thread: BQ24616, BQ24616EVM, BQ24171, BQ24600, BQ24600EVM

I have designed a PCB based on the TI design notes, using the suggested component values. I want to charge 3 LiPo cells at 4A maximum. No matter what values I change the output is 2A. The converter is running at 1.6 MHz. but the duty cycle is about 20%. Any suggestions will be appreciated.

Thanks,

Bruce

  • Bruce,

    Can you include a copy of your schematic? What is your input voltage, input current, battery voltage and system load? Are you using a real battery? Please take the measurements close to the IC if possible.

    Thanks,
    Rachel
  • Hi Rachel, thanks for your response.

    The input voltage is 15VDC, input current is 10A (available), the battery is an 11.1V 5000 mAH LiPo we want to charge @ 4A, and I have no system load because the design is for a battery charger only. I will attach the schematic for our proto board. It has all of the components shown in the TI spec sheet (including the system load and battery switching which we will not be using in the final product). I just wanted to have everything on the proto board for eval purposes.

    Some of the component values have been corrected such as C4 is 10uFand C9 is 1uF.

    We are considering changing the device to a bq24616 for future capability and design margin on this product. I've ordered the bq24616EVM and should have it in a few days.

    If you can't view the schematic let me know and we'll see what else I can do to get it to you.

    Thanks, Bruce Pearman

  • Bruce,

    Thanks, I was able to view it. Unfortunately nothing is jumping out to me for why you're seeing the behavior.  Only thing I see is that you'll need to change R17 and R18 to adjust the battery voltage to 11.1V.

    Can you measure the voltage on the AVCC pin? Can you also try adjust ISET from less than 2A and slowly sweep it up past 2A - 3Aand see if you're able to control charge current? 

    Did you try this on a modified bq24171 EVM? Just to make sure it's not a layout issue. 

    The bq24616evm would be good for you to check out as well since it will allow you to have higher charge currents.

     

    Thanks,

    Rachel 

  • Rachel, thanks for your quick response!

    I have made numerous adjustments to the divider ratios (for example R17 & R18). I am unable to change the current and voltage on VBATT with any adjustment. It seems like there just isn't enough energy going to the battery to push the current up to 4A. When I change the voltage divider for iSET the PWM waveform does not change in duty cycle. I thought it was at 20% but I had the scope channel inverted--it's actually more than 80% on. 

    And as a side bar, all of the safety functions (over voltage, under voltage, shorted output, temperature, etc.) work perfectly.

    I didn't have the luxury of an EVM for the bq24171. It was while looking at that we decided to change to the bq24616. The cost difference isn't enough to discourage that. I ordered the EVM for the bq24616 today. I should have it by Friday.

    Do you think this could be a layout problem? I don't really have any way to judge how well the Switch Mode circuit is working. I think my grounds are pretty good and I don't see much noise on anything.

    If you think of anything else I can try please advise. I'm hoping that getting the EVM, trying it on the test battery and using the layout as a guide all of this will become moot!

    Thanks a lot; I'm a big TI fan and use TI components in most of my designs.

    Bruce

  • Bruce,

    Glad to hear you like our parts!

    Regarding the Battery Regulation Voltage (set by R17&R18), you'll only see that voltage on VBAT when the charger enters CV mode. Looking at the datasheet, the equation is:

    So, for your schematic you should just set R18 = 100k and R17 = 430K.

    Instead of changing resistors out on ISET, for debug purposes it's sometimes easier to just attach an external power supply to adjust the voltage.

    The best way to debug from here is going to be to probe voltages as close to the IC as possible. At high currents, you can see significant voltage drop over long cable leads and sometime that means that you're not getting a good sense of what inputs/output are actually happening at the IC.  It would really reduce the variables if you tried on the EVM so we know there isn't a problem with the set-up then we could better troubleshoot your custom board.  

    The BQ24171 should be able to do the 4A charging without any problems, but since it sounds like you're already considering moving to the bq24616 go ahead and check on the EVM when it gets there and then if we need to come back and still work on the bq24171 I'll be happy to keep supporting you. 

     

    Thanks!

    Rachel

  • Hi Rachel,

    It's the pest from Colorado again!

    I received the bq24616EVM this morning and I'm working through the changes I need to make for our design and batteries. I've already changed the Vfb resistors to 430k and 100k. The parameters and circuit for this device are slightly different so there's a small learning curve too. If you could tell me the correct resistor values for the other dividers it would help me considerably.

    I have an 3-cell, 11.1V LiPo battery that I want to charge at 4.5A. The input current should not exceed 6A and the input voltage will not go above 20V.

    Thanks, Bruce

  • Bruce, 

    The other adjustments should be:

    • VISET1 = 0.9V (4.5A Limit)
    • VISET2 adjusted to desired precharge current
    • VACSET = 1.2V (6A limit)

    You can try the calculator tool (excel spread sheet) to help you quickly adjust values to match your design requirements:  http://www.ti.com/lit/zip/sluc175 

  • Thanks Rachel. The calculator is a bonus.

    I got the bq24616EVM working and feel confident that I can layout a PCB to do what we need. I'm looking forward to ordering thousands of this part.

    Bruce

  • Rachel, I hadn't really looked at the spreadsheet calculator before my previous reply. I just wanted to add one more comment: WOW!

    Thanks again, Bruce
  • Hi Rachel, I have one more question for you. The typical design shown in the bq24616 datasheet has all of the capabilities of switching the battery between the charger and the system etc. I don't need all of that capability because my design is just a battery charger.

    The question is: can I eliminate most of the components associated with the input and battery switching? If so what do I need to do with the unused pins? Tie them to a rail or let them float? What pins are necessary for a minimum configuration? ACN & ACP for example.

    Thanks a lot for all of your help. Who can I send a glowing report to? 

    Bruce

  • Hi Bruce,

    Glad to be of help!

    You should be able to eliminate quite a few of the external components to simplify your design down to a charger only, but we also offer a few charger only options without the power path in this same family of parts, so you might look at the bq24600.

    I'm also going to pass this thread on to the apps engineer supporting this device family so he can give you feedback regarding switching to the bq24600 vs which components you can remove from the bq24616.

    Thanks!
    Rachel
  • Thanks again Rachel.

     

    I think I have most of it figuered out but I'd be happy to hear from the apps engineer too!

     

    Bruce

  • Bruce,

    I agree with Rachel that the bq24600 is probably your best bet. Since the bq24600 switches at 1.2MHz (vs. 600kHz for bq24616), you can also get away with small LC filter. And since the bq24600 is a smaller package, in the end you end up with a smaller solution size. I'm not sure if space is a concern for you, but I thought i would point this out.

    What are the requirements for the charger for your application?

    For the bq24616, all the changes that I would recommend would just make it look more like the bq24600 with unused pins on the bq24616.
  • Hi Smriti,

    Thanks for responding. The specs for this design are: 12.6VDC to 19VDC input, 3-cell @ 11.1VDC, 4.5A hi-charge and current, good thermal characteristics so it can be in a closed enclosure.

     I have ordered a bq24600EVM so we can do more evaluation.

    Bruce