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BQ25895: Generic Questions

Guillaume Carles
Prodigy 70 points
Part Number: BQ25895
Other Parts Discussed in Thread: , TS3USB221A, TLC59116, LM5175

Dear all,

I need some information that are not clear enough for me in the BQ25895 datasheet.

First let me explain what i want to achieve: I need a host-controlled battery charger, included in the system, which is able to charge from an USB source (SDP/CDP/DCP) and later HVDCP adapters, while outputting up to 3A to 3.2V blue LEDs (OTG never used). The BQ25895 seems to fit that role pretty well.

Some generic questions:

  1. I read that the only difference between BQ25895 and BQ25895M is the charge voltage limit (4.208/4.352V), but it seems that others registers differ too. Any other important change I'm missing?
  2. The MCU needs access to the USB port for communication. I consider using TS3USB221A USB switch for that purpose. A 10k pull-up resistor on DSEL is the only part needed?

And more specific information:

  1. Can I left STAT floating if unused?
  2. As I am not using OTG feature, nothing will be connected to PMID. I get the feeling that letting PMID in a floating state is not a good idea, as it seems the cap is used somehow in the buck/boost process for the SYS output. No information is given on how to choose the PMID cap value in that case, or even if that cap is needed at all.
  3. Is Vsys regulated at Vsysmin (3.5V) in battery-only mode (Vbus=0) while Vbat < 3.5V?
    If so, it could save me a boost converter, as Vsysmin can be set to 3.7V by register.
    I asked because I wonder how the BQ25895 is able to achieve, at the same time, 5V on PMID in OTG mode and 3.5V on SYS, while Vbat<3.5V, in a single Boost topology.

Best regards,
Guillaume

  • 0
    TI__Guru**** 166996 points

    Hi Guillaume,

    The key difference between bq25895 and bq25895M is the default charge voltage as you have mentioned.  The only other change not related to boost mode is the timing of relative VINDPM threshold computation at plug in.  Using a TSUSB221A is acceptable.  After ~2 seconds following adapter plug in, the bq25895M does not automatically access the D+/D- unless forced by an I2C write from the MCU.

    STAT can be left floating.  If boost mode is not being used, then PMID needs a minimum of 8.2uF (typically a 10uF ceramic capacitor is used).  When an adapter or other power source is applied at VBUS,  V(BAT) < VSYSMIN, the charger maintains V(SYS) = VSYSMIN.  In battery only mode, V(SYS) = V(BAT)-ISYS*Rdson. 

    Please let me know if you have more questions.  If not, please confirm that this post has been answered.

  • 0 in reply to Jeff F
    Prodigy 70 points

    Thanks Jeff for your quick answer !

    I'll stick to BQ25895 and use TS3USB2211 as a switch.

    Some final questions:

    1. Using the TS3USB221A switch, do I need to take specific care for USB detection (BC1.2) to work properly?
    2. You told: "the bq25895M does not automatically access the D+/D- unless forced by an I2C write from the MCU". Is that the case for the BQ25895 too?

    Bonus:

    Following you answer, I will need to use a Boost converter.
    What I need: 2.5-3A output current with integrated switch, 4V output from 3.5-4.2V (from BQ25895), not in a BGA package.

    I found interesting parts, but none fulfill all requirements:

    1. TPS61230: great zero duty-cycle mode but not enough current capability
    2. TPS61230A: Better output current, but zero duty-cycle is absent
    3. TPS61236P: It's current output is perfect but it requires Vin<Vout-0.6V, meaning an output of 4.8V min
    4. Other parts were in a BGA package

    For those who wonder why 4V out:
    I'm using some TLC59116 LED drivers to, well, drive LEDs. For the constant-current path on each LED to function normally, it needs Vfw+Vol=Vled, with typical blue LED Vfw=3.4V and, @20mA, Vol is between 0.4 and 0.6V, so, in the worst case, I need Vled=4V to have full Vf accross the LED.

    Going higher than 4V is just wasting power, dissipated as heat by the chip. For example, using a 5Vout boost converter, we have (5-4)*2.5=2.5W pure loss, not ideal on battery-powered device.

    Best regards,
    Guillaume

  • 0 in reply to Guillaume Carles
    TI__Guru**** 166996 points

    Guillaume,

    For BC1.2 detection to function, the switch needs to ensure that the charger's D+/D- pins lines are connected to the USB bus.  The DSEL drives low at power up to indicate that BC1.2 detection is in progress and could be used to drive the USB switch.  Once complete, the input current limit and power good status registers auto-update.  If SDP or a nonstandard adapter are detected, the DSEL pin goes high.  If DCP or CDP are detected, the DSEL pins remains low.  Please note that we have found some CDP ports are not compatible with the detection and will report as SDP.  In these cases, the charger relies on the host to perform full USB enumeration to get the information about the USB port and update the charger input current limit accordingly.  bq25895 and bq25895M perform the same in this instance.

    Regarding the boost circuit, if you need a regulated 4V output when battery=4.2V, then you will need a buck-boost circuit.  I used TI's webench tool and found the LM5175 as one option.  There are likely other options. I suggest reposting to the dc/dc converters forum.

  • 0 in reply to Jeff F
    Prodigy 70 points

    Thanks again Jeff

    Good to know for that CDP->SDP erroneous detection, it could have taken me some time to find out the problem!
    I think I have all the info I need now.

    And for the boost circuit, I'll ask directly to the dc/dc forum ;)
    The LM5175 seems overkill for my need, and too space consuming.


    Best regards,

    Guillaume