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TIDA-010208: Confirmation

May
Genius 9880 points
Part Number: TIDA-010208
Other Parts Discussed in Thread: BQ76952

Hi Team,

Good day.

Customer want to confirm regarding about the device. Below is the inquiry of the customer

"

I was wondering if I could get more information on TIDA-010208, which uses this IC. On the schematics document, page 6, there is a comment about the LD pin. It says "Charger wake up from shutdown" and "Predischarge detection". I get the charger wake up part but would like to know more about the "Predischarge detection".

If I understand correctly, in order to wake up from SLEEP mode, one of the ways is if the current crosses some threshold. This implies that the DSG FET is already on, which defeats the purpose of pre-charging. So I was wondering if I could get some clarification about the working principle of this detection, through which of the operational modes the BMS IC switch and what settings are used in order to achieve this.

I am interested in waking the device from SLEEP mode by detecting the load on the battery. The referenced TIDA design seemed to do so, but it wasn't explained how so I asked this question. That is all.

"

Regards,

Maynard

  • 0
    TI__Expert 3145 points

    Hi Maynard,

    Regarding "predischarge detection", when PACK+ has a big cap load, we will need the predischarge path to charge the cap slowly to avoid inrush current. One way to judge if predischarge finished or not is to monitor PACK+ voltage, if PACK+ voltage is equal or almost equal to BAT voltge, we can say the output cap is fully charged and it is ok to turn on DSG MOSFET. LD is used to monitor PACK+ voltage for predischarge judgement. 

    You can expect to wake up BQ76952 from SLEEP mode by adding a load (means there will be current flowing through current sensing resisters), but you need to turn on DSG MOSFET or enable "source follower" mode of DSG MOSFET driver (in TRM 5.2.2). We have several ways to wake up bq76952 from sleep mode (in TRM 7.3). You can configure the wake up current threshold through "Power:Sleep:Wake Comparator Current" register. 

    thanks.

    Best Regards,

    Ryan Tan

  • 0 in reply to Ryan Tan
    TI__Genius 9880 points

    Hi Ryan,

    Good day, I just received response from our customer regarding this. Please see below.

    "

    I already realized most of the things you explained here but I would like to know how they tie together and confirm something with you:

    If the device that uses predischarge is in SLEEP mode and DSG fet is ON, as you said, would connecting a capacitive load still result in a large inrush current because the DSG was already ON?


    Or does the following happen:
    Device enters SLEEP mode and wants to turn on DSG fet, which firstly results in turning on the PDSG fet and the device waits for a load. When a load is connected then the DSG turns on only after voltage on PACK pin crosses some threshold and device switches to NORMAL operation mode only when current crosses a certain threshold and it continues working in NORMAL mode.

    Am I understanding this correctly? Because it isn't clear to me that the DSG fet can be off while the device is in SLEEP mode.

    "

    Regards,

    Maynard

  • 0 in reply to May
    TI__Expert 3145 points

    Hi Maynard, 

    If your system enters sleep mode with DSG MOSFET on, you will see a inrush current when connecting a cap load. I think your second idea is a good way, you could use MCU to detect when a load is attached and then turn on pre-discharge MOSFET, after the cap load is fully charged, turn on DSG MOSFET. Otherwise, keep both pre-discharge and DSG MOSFETs off. I think that depends on how you design the system. 

    Best Regards,

    Ryan Tan