BQ76940: Loading of VC5X and VC10X Pins

Hi Rohan,

Do you have the recommended filter components connected to the VC5X, VC10X, and BAT pins? These are the Rf (1k) and Cf (10uF) components shown in Figure 14 of the datasheet. Also, around the 7:00 mark of this video, it explains some of the critical connections for these pins: https://training.ti.com/zh-tw/how-create-schematic-bq76920-bq76930-and-bq76940

Regards,

Matt

Hi Matt,

We are using Rf = 1k and Cf = 4.7uF for our design. These components are included. As informed, we see a drop across the diodes connecting VC5 to VC5X. The current is approximately 500uA. Even if ALERT pin was set to HIGH, the current into VC5X shall be 25uA. This is leading to incorrect cell measurement. What can be the cause for so much current flowing into VC5X or VC10X net?

Can a low REGSRC voltage have this effect. By low, I mean REGSRC is approximately 7V. Is it possible that the part draws current from VC5X to support REGSRC demands?

Regards,
Rohan

Hi Rohan,

The ALERT pin consumes about 15-25uA of current when it is enabled (see the dIALERT parameter in the device datasheet). However, the host should be clearing the ALERT pin by writing to the SYS_STAT, so it should only be high a small percentage of the time. If it is never cleared, it can slowly drain the lower cell group.

Are you seeing the issue on the lower cell group? Also, do you see the dip at all times or only when cell balancing is enabled?

Regards,

Matt

Hi Matt,

The fluctuations are random. Our hypothesis was that alert was causing this. But the current would not tally to 500uA so we think it's something else.

We are not powering charge discharge fets using afe and not doing coulomb counting as well.

Is it possible that vc5x supplies current to regout in an event that regsrc cannot supply?

Do you recommend a higher value of CF for 9S applications? We are using 4.7uF and rf of 1k.

Regards,

Rohan

Hi Rohan,

The voltage fluctuation may be related to the thermistor load. The thermistor turns on every 2 seconds (timing is not synchronized between the cell groups though). So it will drift with respect to each group. If the thermistor is hot, the resistance will be smaller. I don't know if it could be getting small enough on your board for the diode to conduct. Are you using standard diodes with 0.6-0.7V voltage drop?

We generally recommend 1k and 10uF for Rf and Cf, but the values you are using should work okay. I do not think these values would be related to this issue.

Best regards,

Matt

Hi Matt,

We are using 1N4148WS diode in our application which shall have a Vf of 0.7V at 5mA. As per the datasheet the forward drop at 0.5mA shall be 0.4 to 0.6V. Do you recommend a change in this diode?

Can you suggest any other pins/nets to check so that we can understand the cause for this issue? 

Can you comment on our hypothesis that VC5X net supplies to REGOUT if REGSRC is too LOW or cannot supply the current?

Regards,

Rohan

1N4148WS.pdf

Hi Rohan,

I do not think VC5X will supply REGOUT if REGSRC is too low. Are you using VC5X to control a FET gate like the datasheet example? Do you mind sharing your schematic?

I think this diode is okay. Do you know how often the voltage dips occur? This frequency may provide clues.

Regards,

Matt

Hi Matt, 

We are using it to power an FET. As mentioned, its a little random, but we ll try our best to give you some more details.

Attaching the VC5X schematic for your reference. R61 is not mounted. R87 is 100R and not 100K. Let me know if you need any more information.

Regards

Rohan

Hi Rohan,

This circuit looks correct. I don't see an issue here. 

Maybe you can send me an E2E connection request and then you can privately share the complete schematic for this IC?

Matt