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BQ40Z80: LEDCTNLA Pin Question

Daniel Brust
Prodigy 25 points
Part Number: BQ40Z80

Hello Guys,

i am currently designing a custom 24V Lipo battery pack. I am planing to use a touch-button ic to enable the LED Display functionality.

The Touch IC which was selected is the AT42QT1011 from Microchip. This IC needs a power supply of 1.8V to 5.5V and about 80µA.
As i see in the evalboard you use the input voltage of cell 2 to power the "first" BQ771807 for the secondary overvoltage-protection.
Is this generally a good design practice? When i use cell balancing, the power of the touch IC, which is drawn from cell one would also result in discharging the other cells? 

But i think the power draw from the touch IC is not really much and an additional LDO would also consume (probably more) power...
On which additional current draw it is better to use an additional switching regulator/LDO?
By the way: which GND net should i use for the device? VSS or the PGND net? I am not sure about this...

Another Question regarding the LEDCTNLA pin:
We use some additional LED´s to show some operating states. Is it possible to use the LEDCTNLA pin and connect it to the base of an NPN transistor which than switches the other LED´s on for the same amount of time as the SOC-LEDs are lighten up?
I am not quite sure about this., because as i see the LEDs are connected anti-parallel between the LEDCTNLx pins, so there musst be some switching/multiplexing going on?
LEDCTNLA is connected to LED1 on the evalboard, which should (for me) present the lowest possible SOC?


So could this implementation work?
Is it possible that one of your engineers could short check the schematics before layout design starts?

Kind Regards

Daniel

  • 0
    TI__Mastermind 42535 points
    1. The reason we used the two protectors was because they were stackable. Secondly losses at higher voltage would be lower which is why we decided to split it 2-5. The touch IC dsg the cell 1 would not really affect other cells. Yes, the cell will be slightly imbalanced depending on how much power is used. However, the cell balancing on the gauge would be able to compensate for that esp during chg.

    2. Please explain what you mean by "On which additional current draw it is better to use an additional switching regulator/LDO?"

    3. Please use PGND, Vss is supposed to be low power and low noise GND.

    4. Ah, sorry no. The LEDs are charlieplexed. I don't think your setup would work.
  • 0 in reply to Batt
    Prodigy 25 points
    Hello,

    1: I got that. But how much power should be maximum taken out from the singe cell 1?
    This is also what i mean with "On which additional current draw it is better to use an additional switching regulator/LDO?"
    I could implement an extra LDO or switching regulator which regulates the hole battery stack voltage down to 3.3 volts to power the touch-IC, instead of using just cell 1 voltage.
    But i assume that an extra regulator would draw more power / has more power loss than using only cell 1 power?

    Generally our plan is to implement an USB DCP charger, which should be always active - even if the battery pack is not inserted in the system.
    Currently we are using the same voltage as the BQ40Z80 is using on its VCC pin to supply the 5V buck regulator.
    The current would in this case flow through the body-diodes of the CHG-FETs, which would result in unnecessary power loss?
    So should we better power the buck regulator direct from the battery stack voltage right after the fuse?


    3: Makes sense. But what happens here with the coulomb-counter? The current on PGND is sensed trough the shunt sense resistor - the current on VSS not...
    What happens, when i only draw power from cell 1 and return the current over the shunt resistor?


    4: We will check that again.
    But this question seems to be not necessary anymore, because i read in the technical reference manual, that the LED´s are only active, while the DISP-button is pressed/hold?
    I recently thought, that after a single press on the DISP button, the LED´s are lighten up for a configurable period of time and than turn off automatically?
    If this is not the case, we need some additional timing device, which holds down the DISP line low until n-seconds passed.
    So we could also light up the additional LED´s in this way.

    The plan was to light up all necessary LEDs (incl. SOC-LEDs) for about 5s after a single touch on the DISP touchbutton.


    Kind regards

    Daniel
  • 0 in reply to Daniel Brust
    TI__Mastermind 42535 points

    1. It doesn't work like that. I can't give you a percentage of power that you can use to modify battery behavior like that. The answer depends on chemistry, how many cells are in parallel and what is the individual cell mAh rating. Naturally if you use a 100mAh cell, you'll dsg it pretty soon and have a large change in terminal voltage with your touch panel.

    Yes, an LDO will draw power but equalize it across your pack. Any current passing through the FETs will cause a power loss. So, better to power it from the stack.

    3. Both are tied via net ties anyway. So it doesn't matter. Effectively gauging on this device is done using DOD and OCV, so as terminal voltage changes the gauge can estimate power loss accurately.