BQ79656-Q1: Design Support

Abhiram Tiwary

Prodigy 135 points

Part Number: BQ79656-Q1

Dear TI support,

We are testing on 4-Layer design with BQ76596 AFE IC. Our observations are following.

Once in a while we are getting cell balancing voltage dips on 1^st 8 cell voltages connected in series once in a while during Charging or discharging. As shown in Pic-1.

# Note: GND_S = CELL-1_-VE + AVSS + CVSS + DVSS + REFHM

2. We concluded it as communications error so we balanced our Daisy chain. We mitigated this issue by putting Y-Capacitor of 0.1uF 100V capacitor externally between the Low voltage ground (12V) and High Voltage ground GND_S (48V).

We request you to kindly check our layout, is it or is it not according to your given layout guidelines in the datasheet ?

Please let me know if anything more require from our side

Best Regards,

Veto C.

over 1 year ago

0 Anthony P Pham over 1 year ago

TI__Mastermind 23410 points

Hi Veto,

I'll take a look at this. Are you able to monitor the voltage dips on an oscilloscope as well or provide some data that shows the voltage dip?

Best Regards,

Anthony Pham

0 Abhiram Tiwary over 1 year ago in reply to Anthony P Pham

Prodigy 135 points

Hi Anthony,

We solved the issue by adding a 2.2uF 100V MLCC Y-capacitor between MCU and AFE's Grounds. Please let me know if you have any suggestions on that

+1 Anthony P Pham over 1 year ago in reply to Abhiram Tiwary

TI__Mastermind 23410 points

Hi Abhiram,

It sounds like then that there may be some current pull that's causing the voltage between the grounds to be not negligible. What we've done on our EVMs that may help here is adding a digital isolator to separate the grounds. The digital isolator helps keep the voltage potentials relative to the AFE ground without influnce from the MCU ground.

The capacitor method works but I'm unsure of how robust it is across all levels of current consumptions.

Best Regards,

Anthony Pham

0 Abhiram Tiwary over 1 year ago in reply to Anthony P Pham

Prodigy 135 points

So, What you think, Is this could be a PCB layout issue or something else ?

0 Abhiram Tiwary over 1 year ago in reply to Abhiram Tiwary

Prodigy 135 points

Dear Anthony,

I have some Dilemma in the following points

1. As we have used transformer Isolation between both the Grounds (MCU & AFE's), do we still require any digital Isolator ?

2. Are you suggesting the digital Isolator ISO7342CQDWRQ1 between both the grounds ?

3. If we use a digital Isolator like the above, exactly where to use them, I guess after the Bridge IC BQ79600 ?

I am attaching our Circuit diagram for your reference

Please let me know If you have any doubts regarding the queries as well as the schematics

0 Anthony P Pham over 1 year ago in reply to Abhiram Tiwary

TI__Mastermind 23410 points

Let me spend some time with the schematics and let oyu know.

Best Regards,

Anthony Pham

+1 Anthony P Pham over 1 year ago in reply to Abhiram Tiwary

TI__Mastermind 23410 points

Hi,

Apologies for the delay. My comment regarding separate GNDs was between the MCU GND and the AFE ground but as it was problems on the VIF communication, the capacitor looks to be a good solution.

Layout looks good.

Best Regards,

Anthony Pham

Power management

Power management forum

BQ79656-Q1: Design Support