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BQ25758: Buck mode for 50A

Nuno Martins
Prodigy 40 points
Part Number: BQ25758
Other Parts Discussed in Thread: BQ25756, BQ25750, BQ25756E, LM5122

I am planning to use the BQ25758 in Buck mode for a bidirectional battery charger from 50V to 8V with charge current 5A and discharge current 50A.
One limitation I found is the minimum inductance DCR is 2.6mR and current sense resistor 5mR. That means high losses in discharge mode.
How critical are these parameters? It's possible to do that with lower inductor DCR and CS resistor?

  • 0
    TI__Expert 3070 points

    I just want to let you know and confirm that BQ25758 is not a battery charger, it is a bi-directional buck-boost controller. Please let me know if that is what you meant or if you want a battery charger ?

  • 0 in reply to Utsav Gupta
    Prodigy 40 points

    Yes, that's right. I'd prefer a battery charger but there's not much options for this application. The BQ25756 works only in Buck-Boost mode, which means an additional switching stage that is not needed.

    Do you think it can also be used as battery charger?

  • 0 in reply to Nuno Martins
    TI__Expert 3070 points

    Hi Nuno, The BQ25758 cannot be used as a battery charger. It is a converter. The BQ25756 does not work only in Buck-Boost mode. That is not correct. It works in buck, boost and buck-boost mode. It can work in any of the 3 modes depending on your inout and output voltages. I would suggest BQ25756 if you have high voltage applications, BQ25756E if you only have low voltage applications and BQ25750 if you want power path transfer between VIN and VBAT to power a system load.

  • 0 in reply to Utsav Gupta
    Prodigy 40 points

    Hi Utsav, thank you.

    It's possible to use the BQ25750 / BQ25756 with just 2 MOSFET? From what I found I think there are required all the 4 to work properly.

    Anyway, there's the same limitation with the output current sense resistor which is specified a value of 5mR. It's possible to have lower resistance for higher current, for exampe 2mR for 50A?

  • 0 in reply to Nuno Martins
    TI__Expert 3070 points

    Hi Nuno,

    The default input sense resistor (RAC_SNS) is 2 mΩ, and the register allows for a range of up-to 50-A input current limit. If lower currents are desired, it is possible to use a higher resistor, such as 5 mΩ. In this case, the IAC_DPM register value should be multiplied by a factor of 2/5 to program the correct current. For example, if a 5-mΩ RAC_SNS is used, and the register is programmed to a value of 0x60, the true maximum current across the RAC_SNS will be: 12A * 2/5 = 4.8 A

  • 0 in reply to Utsav Gupta
    Prodigy 40 points

    Yes, that´s right, but the problem is with the RBAT_SNS that must be 5mR and we have a low charge current, but high discharge current.

    One option could be bypassing this resistor with a MOSFET when in discharde mode (probably with one of the enable features of BQ25750)

    Or, in alternative using the BQ25756 for battery charging and another DC/DC (like LM5122) for discharge.

    What do you think about that?

    It would be possible to use the BQ25750/6 with just 2 MOSFET?

  • 0 in reply to Nuno Martins
    TI__Expert 3070 points

    Hi Nuno,

    It is not possible to use the BQ25756 or BQ25750 with just 2 FETs. We have a device coming out later this year which is a buck only charger.

    As for the DCR, the minimum value is fixed for proper functioning of the system as we have a feature in our IC called adaptive inductor identification.

    I don't think the alternatives you suggested for RBAT_SNS would be possible. The feedback circuits were designed to work according to that 5mOhm resistor.

  • 0 in reply to Utsav Gupta
    Prodigy 40 points

    Ok, that way is better to go with BQ25756 for charger and additional Boost converter for discharge, right?

    There's any expected release date for this new device?

  • 0 in reply to Nuno Martins
    TI__Expert 3070 points

    Since it is not possible to change the BRAT_SNS, I think you can use a boost converter for discharge. I must ask, even at max 20A current, the power loss is only 2W, how much power loss can you handle maxmium ?

    The expected release date is end of June.

  • +1 in reply to Utsav Gupta
    Prodigy 40 points

    The charging current will be lower than 10A, so below 0.5W, which is acceptable for this application and also for a 2512 resistor.

    I'll start with the BQ25750 and probably change it later for a Buck-only converter when it's available.

    Thank you for the support.

  • 0 in reply to Nuno Martins
    TI__Expert 3070 points

    Thank you. Let us know if there are any more issues