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5 - 9 V Input (USB-C), 4 cell NIMH to charge (i.e. 3.6 V - 5.2 V), Vsys1 = 3V3, Vsys2 = 6V

Niklas Roth
Prodigy 130 points
Other Parts Discussed in Thread: TPS63100, BQ25798, BQ25713, BQSTUDIO, BQ2000

1. Consideration - USB-C PD: Implementing USB-C I will mainly request for a 9 VDC supply. If negotiation wasn't successful only 5 V are provided.

2. Consideration - Battery: 4 NIMH cells => charging voltage: 5.2 V, delivered voltage range: 5.2 V down to 3.6 V (i.e. empty).

3. Consideration - Required system voltage: System supply voltages requirements are 6 V (P < 100 mW) and 3.3 V (P < 5 W).

What I found so far: Plenty of (hybrid) buck-boost battery management controllers (BQ2000T, ..., BQ25910) that doesn't fit the combination of boosting a 5 V USB supply for the required battery charging but also boosting later on to the required 6 V from battery (i.e. 3.6 to 5.2 V) if the USB source is absent.

I wonder if TI doens't have a battery management controller IC, that provides buck-boost from the input source as well buck-boost from the battery to provide a higher system voltage than the battery delivers?

A solution is of course a further booster circuit. But hope to find a chip that already organizes this application for me.

  • +1
    TI__Guru**** 151535 points

    Hi Niklas,

    TI does not have a single IC solution for your application. All of our chargers use either a buck, boost or buck-boost converter that provides a minimum system voltage to SYS output.  The SYS output connects to the battery through a BATFET.  When the battery charges above the minimum system voltage, the SYS voltage follows the battery voltage up and regulates slightly higher than the battery regulation voltage.  When the input source is absent, SYS = battery voltage.

    I recommend a BQ25798 with SYS output followed by a buck-boost converter like TPS63100 or similar from the TPS63xxx family.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 130 points

    Thx for your reply!

  • 0 in reply to Jeff F
    Prodigy 130 points

    I just saw that this isn't suitable for NIMH batteries. What I found so far are BQ25713, ...23, ...30, ...31. Unfortunatelly they don't provide integrated FETs a the BQ25798 does. You may know a buck boost charger supporting NIMH chemistry that has the FETs already integrated and can handle up to 800 mA?

  • 0 in reply to Niklas Roth
    TI__Guru**** 151535 points

    Hi Niklas,

    You can use the BQ25798 for NiMH through host software.  You have to do the same for BQ2571/2x.  Neither device provides the recommended dV/dt or dT/dt termination and only BQ25798 provides LiIon constant voltage charge termination.  How were you planning on terminating charge?  If charging at low enough current (<0.5C), it is okay not to use dV/dt or dT/dt termination and use either constant voltage or charge time to terminate.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 130 points

    Hi Jeff,

    Thank you very much for your valuable expertise. Good you mentioned "dV/dt" or "dT/dt" termination, as they fall out off my scope trying to find a proper buck-boost converter for NiMH batteries. How ever, is there a buck-boost converter at all that provides these kind of termination, which is a dedicated technique for NiMH (and NiCd?) batteries, as I understand it.

    Using a NiMH (AA) 4s1p configuration providing (i.e. 4 * 1.2 V * approx. 2100 mAh = 10 Wh) and required a max charging time of 5 hours leads to a charging current of 420 mA which is 0.2C (< 0.5C). So you mean that I could use any of the above mentioned (BQ25713, ...23, ...30, ...31, or even ...98) monitoring the battery voltage?

    With "host software", do you mean a firmware of an attached microcontroller that shall monitor it, or you mean using BQSTUDIO to download a specific battery management configuration so that the BQ... itself takes care of charging termination?

  • 0 in reply to Niklas Roth
    TI__Guru**** 151535 points

    Hi Niklas,

    There is no buck-boost converter that terminates with dV/dt or dT/dt without the help of host (u-controller) software (firmware).  The only charger with that capability is the BQ2000 buck controller.  It is u-processor based.  All of our other chargers have asynch state machines that make decisions based on I2C registers.  The host (u-controller) software (firmware) makes changes to those registers.  Both BQ2579x and BQ2571/2/3x have ADC which could be used by software (firmware) to measure the dV/dt or dT/dt and perform termination.  However, at your charge rate < 0.5C, I don't think it is necessary for dV/dt or dT/dt termination.  BQ2571/2/3x doesn't do termination and relies on a gas gauge IC, through software (firmware), to tell it when to terminate.  BQ2579x terminates based on V(BAT) = BATREG and ICHG < ITERM.  

    Regards,
    Jeff

  • 0 in reply to Jeff F
    Prodigy 130 points

    Ok, I gues I got it so far. Thank you very much for your inputs.