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BQ77915: RCTRx Minimum Value

Chuchen Wang
Expert 3470 points
Part Number: BQ77915
Other Parts Discussed in Thread: BQ76200, BQ77905

Hi,

My customer has the following question:

We are trying to use 2 of the BQ77915 in a stack configuration, and wondering if we can lower the resistance value of the current limiting resistor between CTRD/CTRC and DSG/CHG. If so, how low can we go, and how much of an increase in terms current/power we will get across different modes of operation. We do want to maintain a low sleep mode (hibernate mode) current to not drain the battery when standing by. Our end customer have some concern of using the 10Mohm resistor at this point, so we are trying to see all options we have.

I didn't see the minimum requirements for RCTRx in the datasheet.  Please let me know.

Thanks,

Chuchen

  • 0
    TI__Guru 54715 points
    Hi Chuchen,
    The bq77915 RCTRx resistors are shown in the data sheet recommended operating conditions table 8.3 as nominal values. These resistors are large to limit the current into the clamps for the CTRC and CTRD pins from the upper device CHG and DSG pins which are low resistance drivers. The RHIB and RCB resistors between the upper and lower devices are smaller since the drivers are current limited.
    The data sheet does not specify a maximum current into the pins. The nominal current with the upper device FET drivers in regulation will be less than the 12V/10M ohm or < 1.2 uA. The data sheet shows the CTRx pin voltages with a 600 nA test current. As more current is put into the CTRx pins, it will offset the supply current of the lower part sending the supply current through the upper cell group as well as the lower group. This could help offset the extra FET load on the lower part from the FET RGS resistors, but in general the clamps were not intended to take high currents, matching the loading with external resistors would be preferred. See section 3 of the application note www.ti.com/.../slua774, the CTRx architectures are similar between the 2 parts.
  • 0 in reply to WM5295
    Prodigy 20 points

    Hi,

    Thank you for the info. I read it through and realized that we can just lower the resistance based on the calculation to adjust the current. However, it still doesn't provide a range or a lower limit which is essentially what we need.

    On a side note, I also realize that we can add another resistor to match the load. However, the current is still going through the additional resistor so the net current is not saved base on my understanding. Am I missing something? How does it save the current and what is the benefit on matching the resistances of the top and the bottom? I also want to point out that we are connecting the CHG and DSG on the bottom stack to the BQ76200 CHG_EN and DSG_EN without any resistors. In this case how would we do the matching, and how does this may change the resistance needed between CTRC/CTRD and CHG/DSG.

  • 0 in reply to Qiwang Zhang
    TI__Guru 54715 points

    Hi Qiwang,

    There really is not a minimum resistance for RCTRx, the abs max for the pins is the voltage shown in the specification.  But without a resistance you won't have a current limit.

    Adjusting the external resistance to the clamp voltage may be hard to adjust the currents to match. VSS of the upper device is a more predictable value. The bq77915 has balancing so there may not be as much concern with matching the loads as with the bq77905.  The current into CTRx will return out the VDD and back to the same cells, and offset the current from the cells into VDD.  If more current flows into CTRx than into BAT the BAT pin voltage will rise above the cell, a little odd and improbable but would be OK.  I did not intend to cause confusion.

    The bq76200 has 1M nominal input resistances for the enable pins, it does vary significantly as shown in the data sheet.