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BQ25895: Is an I2C buss required to use this part?

Matt Swider
Prodigy 70 points
Part Number: BQ25895
Other Parts Discussed in Thread: TPS3839

I have inherited a battery charger design from someone and am evaluating it.  The design application does not have a practical access to an I2C buss for setting up the device.  I want the device to be capable of running as follows:

1) Providing power from a bank of rechargeable Li-ion batteries to a load.  

2) Accepting input from a USB source to charge the bank of rechargeable Li-ion batteries.  Would like for the battery charge current to be 2A, which appears to be the default.  It also appears that the input current limit from VBUS, which I believe would be the source of power to charge the batteries, is by the resistance between ILIM and ground, as the this is the default state for ENILIM.  

Based on what I have learned so far, it does not appear that use of the I2C buss is required for my simple application.  Is this correct?  

I am also wondering if the original designer over specified in selecting this part, that is I wonder if a simpler part, with fewer supporting components, could be used in this same application.  Actually when I look at the schematic it is pretty simple already, but I thought it did not hurt to ask.  

  • 0
    TI__Guru**** 166416 points

    Hi Matt,

    Regarding 1, yes.  The bq25895 automatically enters OTG mode and powers a load connected at PMID (after a short delay) when no VBUS is present.

    Regarding 2, yes.  The bq25895 automatically starts charging up to 2A charge current, assuming both the ILIM pin resistor and the D+/D- pins are configured to allow enough input current.  The D+/D- pins could set the input current to  500mA if an SDP port is detected.

    No I2C bus is required but does make it easily to modify the charger's internal register settings and monitor status.

    TI does not have a simpler charger that automatically switches between charging and OTG boost mode.

  • 0 in reply to Jeff F
    Prodigy 70 points

    Thank you for your prompt response.  I have a follow on question regarding the bq25895.  This application will not have access to an I2C buss.  Can the Ilim pin configure the chip to limit the input current, and hence the current delivered to the batteries being charged?  I think what I really want to do is limit the input current such that I don't load down a USB power source, and allow as much current as is available to charge the batteries.  Likley want to limit the input current being supplied to VBUS to 1 or 1.5A.  Is this a simple as selecting the correct resistor on the Ilim pin (pin 10)

  • 0 in reply to Matt Swider
    TI__Guru**** 166416 points

    Matt,

    yes, the ILIM pin limits input current which is directly proportional to output current SYS and BAT.  If you have no load SYS, then the relationship is simply efficiency=(VBAT*IBAT)/(VBUS*IBUS) where IBUS is clamped to ILIM resistor setting.  The charger also has the VINDPM feature and ICO features which prevent the charger from collapsing its input source.

  • 0 in reply to Jeff F
    Prodigy 70 points

    Thank you for your helpful response.  I have an additional follow on question regarding the OTG pin.  I gather that if I pull that low it will disable the boost regulator.  Am I correct in assuming this will cause the output at PMID to be disabled, as well as disabling switching on SW? 

  • 0 in reply to Matt Swider
    TI__Guru**** 166416 points

    If OTG pin is pulled low, the converter is disabled but there is a still a path from BAT to SYS through inductor then diode of Q2 to PMID.

  • 0 in reply to Jeff F
    Prodigy 70 points

    I don't follow your response this time.  When I look at the functional block diagram on page 14 of the data sheet, I do not see a path from BAT to SYS.  The symbol for the BATFET does not have a parallel diode (JFET I assume?).  It appears that BATFET, if fully turned off, isolates BAT from other nodes.  I assumed that the current into BAT at 5uA max with no VBUS input supported this isolation.  Please explain.

  • 0 in reply to Matt Swider
    TI__Guru**** 166416 points
    Think of the BATFET as two FETs with body diodes pointing toward each other. In HiZ mode when no VBUS is present and OTG is not enable, both FETs are turned on. If your host writes to the register to turn off the BATFET, then there is no path to SYS.
  • 0 in reply to Jeff F
    Prodigy 70 points

    I am hoping to use the BQ25895 in the following application.  I am designing a rechargeable Li-ion cell powered light.  I would like for the light to be able to accept a USB input to charge the cell.  I would like for the light to run off of the cell when there is no USB source.  And I would like to be able to provide power from the cell to a USB device.  It appears the BQ25895 can facilitate these tasks.  The most likely use will be to run off of the cell, not off of USB.  The one thing I don't care for is the Vbatt(DPL) can be as low as 2.15V.  The users would much prefer this threshold to be closer to 3V.  I am hoping to use a MCU to sense VBAT and provide an appropriate command to the BQ25895 to remove all loads from the cell.  I need the standby demand from the cell to be 50uA or lower.  It appears based on what I am reading that may not be possible without utilizing the I2C buss interface.  This application can not utilize that interface.  Can I use the BQ25895 in the way I intend, or if not can you recommend a different IC for this application?  

  • 0 in reply to Jeff F
    Prodigy 70 points

    Can I control the connection to the BAT pin via the CE pin?  I am thinking/hoping if I pull that high the BATFET will be disabled, and I can have a high impedance connection only to my battery.  

  • 0 in reply to Matt Swider
    TI__Guru**** 166416 points
    No. /CE only disables current flow from SYS to BAT. Current flow from BAT to SYS is still enabled when /CE is high.
  • 0 in reply to Jeff F
    Prodigy 70 points
    I am hoping to use the BQ25895 in the following application. I am designing a rechargeable Li-ion cell powered light. I would like for the light to be able to accept a USB input to charge the cell. I would like for the light to run off of the cell when there is no USB source. And I would like to be able to provide power from the cell to a USB device. It appears the BQ25895 can facilitate these tasks. The most likely use will be to run off of the cell, not off of USB. The one thing I don't care for is the Vbatt(DPL) can be as low as 2.15V. The users would much prefer this threshold to be closer to 3V. I am hoping to use a MCU to sense VBAT and provide an appropriate command to the BQ25895 to remove all loads from the cell. I need the standby demand from the cell to be 50uA or lower. It appears based on what I am reading that may not be possible without utilizing the I2C buss interface. This application can not utilize that interface. Can I use the BQ25895 in the way I intend, or if not can you recommend a different IC for this application?
  • 0 in reply to Matt Swider
    TI__Guru**** 166416 points
    In charge and HiZ mode, VBATT(DPL) is 2.15V minimum but the battery's internal protector generally opens in the 2.6V-2.9V range. In OTG mode, V(OTG_BAT) = 2.6V to 2.9V is when OTG stops. The bq25895 is our only charger that automatically enters OTG mode and provides power to its PMID pin instead of VBUS when VBUS is removed (assuming the OTG pin is high).

    All of our chargers require an I2C command to disconnect BAT from SYS (i.e. turn off the Q4 BATFET) if they have the feature.. However, you could add an external low RDSon PMOS between the BAT pin and battery that disconnects the battery from the BAT when told to do so by something like a TPS3839 supervisor circuit (after you invert its active low reset with another very low cost NFET). You will need to AND the supervisor output with VBUS signal so that the PMOS turns back on if VBUS is applied for charging.