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Original question:

BQ25882: BQ25882 D+/ D- short to GND

BQ25882: D+/D- & USB interface

Rez Ameli
Intellectual 2965 points
Part Number: BQ25882
Other Parts Discussed in Thread: BQ25798, BQ25792, BQ24179, BQ25723,

Hello Power management team,

I have a customer who is interested in the devices tagged below.

  • Vin = 5v (Vbus from USB3)
  • 2 cell series battery (2S2P)
  • 2A-4A charge 
  • I2C interface to get battery condition

the main question is:

  1. Do they need to hook up the USB DP, DN lines to make this work? Since they will be using the I2C interface to the host processor for battery status, what does the USB interface provide? This product would have a USB3 interface available for charging the battery and downloading collected data from but no need for the downstream device to see this battery charger.
  2. I was reading on our e2e forums that if D+/D- are left floating, it will default to unknown charger at 500mA. I would like to double check this before I get back to the customer. 

  3. My concern is I don’t want a charger that is trying to negotiate the Vbus current based on the old OTG standard when I am using USB-PD to negotiate Vbus current through the SS lines on USB3. Maybe there is one of these charger that works specifically with USB3 but I didn’t see that in my search.

This will be used in portable ultrasonic products. 

Kind regards,

Rez

  • +1
    TI__Mastermind 41675 points

    Hi Rez,

    Unfortunately I do not think that BQ2588x or BQ25790, BQ25792, BQ24179, and BQ25798 will work for the customer needs. With a minimum 2 A charge current with 8.4 V battery (16.8 W) and 5 V input (assuming no losses), this puts the input current at 3.36 A. All of these devices are rated for max input current of 3.3 A. With 4 A charge current this is 33.6 W on the output and requires 6.72 A on the input with 5 V (assuming no losses).

    Is your input voltage fixed, or can you handle more input voltage, say like 12 V? Stepping down the voltage allows for lower current on the input and is more realizable. Alternatively there are other options, such as a charge controller family. One such example is BQ25723. It can support up to 16.2 A charge current with up to a 10 A input current limit and 3.5 V to 26 V input operating range. 

    Regards,

    Mike Emanuel

  • 0 in reply to Michael Emanuel35
    TI__Intellectual 2965 points

    Thanks, Michael for your time and explanation. Let me clarify their input voltage and I will get back to you. 

  • 0 in reply to Rez Ameli
    TI__Mastermind 41675 points

    Hi Rez,

    Looking forward to your response!

    Regards,

    Mike Emanuel

  • 0 in reply to Michael Emanuel35
    TI__Intellectual 2965 points

    Thanks, Mike. 

    The customer is considering to limit the charge current to around .7C which will keep the charge current under the 2A limit while keeping the input current limit to under 3300mA. I could use another part but then I have to put in a boost converter and I would like to eliminate as many parts as possible. Does the BQ25882 seem like a good choice given these constraints or is there still something I am missing?

  • +1 in reply to Rez Ameli
    TI__Guru**** 169546 points

    HI Rez,

    From the efficiency curve below, if your customer uses a low DCR inductor for highest efficiency, we can estimate that efficiency at 8.4V and 2A is worst case 88%.  So 5V*3A*0.88/8.4V = 1.57A is an estimate of the max current.  It that works then BQ2588x is a good choice.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    TI__Intellectual 2965 points

    Thank you, Jeff. I appreciate it. 

    Seems like this will works for their application.

    Last question I have is point 1 of the main thread regarding USB OTG pins. If using the standard USB3 SS signal lines to negotiate power delivery, do they need to hook up the DP and DM pins to their USB3 interface? Right now they have them connected via zero ohm resistors but it would be good to know beforehand whether they are actually needed.

    Thank you all for your support. 

  • +1 in reply to Rez Ameli
    TI__Guru**** 169546 points

    Hi Rez,

    In charge mode, shorting the D+/D- pins results in the charger settings its input current to the lower of either the highest setting (3.0A) or the value set per the ILIM pin resistor.  The D+/D- pins can be connected to a legacy USB input source (not Type C) and the charger will auto set the input current limit register, unless limited by the ILIM pin resistor.  The charger does not use the D+/D- pins during OTG mode.

    Regards,
    Jeff  

  • 0 in reply to Jeff F
    TI__Intellectual 2965 points

    Thank you, Jeff. I appreciate your time and support.