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BQ25756: Design considerations for long cables

Darren DeBattista1
Intellectual 465 points
Part Number: BQ25756

Tool/software:

Hi, 

I am using the BQ25756 for a new battery charger design where the battery would be connected to the charger using considerably long cables (~2m). Considering cable drops, this could obviously cause inaccuracies in the battery voltage measurement and a Kelvin connection would be ideal. My first idea would be to connect the feedback potential divider "positive" part at the connector of the battery (thus connecting it to the charger power output at the battery connector, after the 2m cable). As for the negative Kelvin wire, I would be tempted to connect this to the analog ground of the charger.

To my surprise, the BQ25756 has no analog ground pin. Moreover, the negative connection of the feedback potential divider is connected to a dedicated pin (FBG). According to the datasheet (pg 5, Table 6-1), this pin is internally connected to PGND when the battery is charging.

Is it advisable to connect the negative return of the Kelvin connection to the FBG pin to have a more accurate measurement of the battery voltage? If not, are there any suggestions please?

Thanks

  • 0
    TI__Mastermind 20980 points

    Hello Darren,

    I would recommend connecting the negative return of the kelvin connection to the IC's GND pin. Also, I would recommend using a twisted pair cable to reduce EMI on the 2 meter long cable if EMI might be present.

    What gauge of wire are you using?

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Intellectual 465 points

    Hi Ethan, 

    Thank you for your reply. The first image below shows what I was describing before. If I understood correctly, are you suggesting to connect the negative terminal of the Kelvin directly to PGND instead of FBG as shown in the second image (recall that the BQ25675 has no GND pin but only PGND)?

    As for wire gauge, it is still to be decided according to the max current that will be set.

     

  • +1 in reply to Darren DeBattista1
    TI__Mastermind 20980 points

    Hello Darren,

    Yes, I was suggesting to connect the wire to PGND. I've thought of another suggestion though. You could leave FBG floating and connect RFBG2 to PGND. Then, you could connect Vin- to PGND.

    Connecting the FB voltage divider from VBAT to PGND will increase the quiescent current though. Will either of these suggestions work for your application?

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Intellectual 465 points

    Hi Ethan,

    From your second suggestion, I understood that basically, I just don't use the FBG (which is only there to remove leakage while not charging) and create my own AGND consisting of Vm- of the Kelvin connection and RFB2, than connect with a small track to PGND near the IC PGND pin as its usually done for other devices. 

    I think this would work. 

    Thanks