• State
  • Locked Locked
  • Replies 2 replies
  • Subscribers 63 subscribers
  • Views 590 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

bq77905 CHG Pin

Lucas Medeiros
Prodigy 40 points
Other Parts Discussed in Thread: BQ77905

I'm going to build a custom 4S LiPo BMS. Looking at the available devices on TI I found the bq77905, that looks great to my reach what i want.
But I just have 2 questions

- In my project I just need to monitoring the cells under voltage, over voltage, over current and over temperature, at discharge operation, because I'm going to use a purpose built LiPo charger. So, can I just use the discharge Mosfet to reduce cost? If yes, what do I need to do with the CHG pin at the schematic?

- At the datasheet I have seen that the over voltage is from 3 V to 4.5 V, what does this mean? Do I need to buy a specific IC, that comes with a specific over voltage level? Maybe I have not seen this information, if so sorry.

Thanks any help.

  • 0
    Prodigy 40 points
    I forgot to write before, but I have seen a app note about separated current paths. But there is nothing about use just one mosfet, if I don't need to charge the batteries.

    www.ti.com/.../slua772a.pdf
  • 0 in reply to Lucas Medeiros
    TI__Guru 54715 points

    Hi Lucas,

    The bq77905 CHG pin is an output, if it is not used it can be left disconnected.  A system such as that may look like Figure 7 of the www.ti.com/.../slua772a.pdf application report but without Q2, J2 and the related passive components.  LD is an input and should still be connected.  However even when not using a charge FET most battery designers want some check of the individual cell voltages during charge, either by the protector in the battery or by the charger monitoring the individual cell voltages.  When not using a charge FET they may generate a signal from the protector CHG pin for their charger to indicate that the battery detects some abnormal condition.  These designs are usually unique to the system and I can not think of an application report with an example.

    The protection levels are set at TI.  Table 3 summarizes the possible configuration options. Tables 1 and 2 in the bq77905 datasheet show the available and some anticipated configurations.   The OV and UV thresholds which might typically be used with LiFePO4 cells are not in a listed configuration today.  For availability of other configurations contact your TI sales office.