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BQ24170: What is the expected behaviour when system load is directly connected to battery and battery is disconnected?

christoph baumann
Prodigy 30 points
Part Number: BQ24170

Hello,
we have a design that basically matches figure 23 of the datasheet (except we are using 2 cell batteries)


The system load is directly connected to the battery, before the charge current sense resistor.
The batteries we are going to use have integrated protection ICs that disconnect the battery under various fault conditions by switching off a pair of N-channel mosfets on the low side.

How does the BQ24170 behave when a good AC adapter is present and the battery is disconnected and a system load is present?



  • 0
    TI__Guru*** 147925 points

    Hi Christoph,

    The dc/dc converter will regulate that node to the battery regulation voltage if termination is disabled with TTC pin= GND as shown above.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 30 points

    Thanks for the response.
    I have one follow up question:
    In this configuration, when the programmed AC adapter input current (ACSET) is exceeded, the charger would regulate the charging output (SW) such that the battery charging current is minimized. Since the system is connected before the sense resistor (Rsr), System current is not regulated directly. 
    If i understand the datasheet correctly (section 9.3.13 Charge Overcurrent Protection), the charger still limits the total charging output current (out of SW) to 160% of the programmed charging current or the peak mosfet current, whichever is lower. That means, the charger should not be damaged in case the system draws too much current from the charger itself (for example in some fault or transient condition). If the charger cannot provide the current, the battery would supplement.

    Is that correct?

    Just for reference, i have attached our schematic:

  • +1 in reply to christoph baumann
    TI__Guru*** 147925 points

    Hi Christoph,

    If the charger hits the input current limit, the buck converter duty cycle is clamped to prevent the converter from pulling more input current.  Output current flows to the lowest resistive load which is ISYS, with charge current naturally reducing. If there is a constant load on SYS, the output voltage starts to drop as predicted by efficiency = VSYS*ISYS/(VBUS*InputCurrentLimit).  Luckily, since the battery is parallel with the load, it will supplement the ISYS load.

    Regards,

    Jeff 

  • 0 in reply to Jeff F
    Prodigy 30 points

    Thank for the response.