BQ76952: BQ76952

John DeCesare

Part Number: BQ76952
Other Parts Discussed in Thread: BQSTUDIO

Hi,

I have a design using the BQ76952 that I've been working with Luis on. The prototype came in and everything seems to be working fine but we are seeing two odd issues.

1. With the a 4S of batteries which is about 15V in our system and nothing connected to the PACK, we are reading the pack voltage at 16.5V and sometimes even higher via the ADC in the BQ76952. When we measure it with a voltmeter, the voltage snaps back to 15V. How can this be happening?

2. CELLs 0-2 when reading the voltages in bqStudio seem to be right on. I'm measuring then with a voltage meter as well. But the ADC reading of the CELL16 is much higher than the actual cell. The cell is about 3.9V but we are reading close to 4.3V via software. We did change the series resistor on Cell 16 from 100 ohms to 20 ohms which was suggested by TI since it powers the Cell balancing but this is really the only difference between CELL0-2 and CELL16 between the batteries and the BQ76952. Is their a reason for this?

Thanks!!!

John

over 1 year ago

0 Luis H. S. over 1 year ago

TI__Mastermind 42060 points

Hello John,

I am curious, do you have any of the MOSFETs enabled? Is load detect enabled?

If PACK+ is floating, then it can easily be pulled up by leakage currents or other sources. If DSG is ON, the voltage of PACK+ may rise due to the DSG pin voltage. When the voltmeter is connected, it may be pulling PACK+ down slightly causing the voltage, due to current leakages, to drop slightly.

That sounds strange for Cell 16. The larger voltage drop usually would only happen with cell balancing on. Do you see any voltage drops on any of the resistors along the resistor (You may want to measure this with a scope to see transients).

If you measure on the pins, does it match what you see? What is the Cell 15 voltage?

Best Regards,

Luis Hernandez Salomon

0 John DeCesare over 1 year ago in reply to Luis H. S.

Intellectual 310 points

Hi Luis!!

Thanks AGAIN for the help. We are looking at the PACK+ voltage. I don't see it on my system but my customer does so we will table that for the time being. As for Cell 16, we actually see the ADC reading back Cell 16 at a higher voltage that the actual cell and even what is presented at VCELL 16. I have attached a document that describes the issue in gory detail with all my measurements. Thanks!!!! John

VCELL 16 measurements.docx

0 Luis H. S. over 1 year ago in reply to John DeCesare

TI__Mastermind 42060 points

Hello John,

Looking at the bqStudio values, it seems very similar to an open-wire condition, you can see "Cell 4 Voltage" measures -489-mV, while Cell 16 Voltage measures 4116-mV, if you add these two together you get ~3627-mV, which sounds like it is roughly what it should be.

Looking at the schematic, I believe that R231 needs to be populated, C143 and C127 needs to be unpopulated and C151 needs to be populated. In order to get a 4s system.

You need a capacitor between VC16 to VC15, VC15 to VC2, VC2 to VC1 and VC1 to VC0 to get a 4s system.

If you notice, right now you actually have 5 filter capacitors (C131/C129/C127/C143/C118) for just 4 cell inputs!

I think the image I shared with you previously may help visualize this:

As for the PACK+ voltage, it is likely just the leakage currents rising the voltage up. Any sort of load should pull down the voltage to Vss quickly (If the FETs are disabled).

Glad to help .