BQ76942: CHG & DSG FETs wont toggle on

Rhys Phillips

Part Number: BQ76942

On start up of Battery management system, I am measuring voltage across the CHG FET and none over the DSG as I would expect. On start up of the BQ Studio Software after enabling FETs I'm seeing these registers set:

My circuit for my system is based off an 8 series configuration of the bq76942EVM circuit with a multiple parallel FET circuit with current loops for the CHG and DSG of the system (referenced from Multiple FETs with the BQ769x2 Battery Monitors).

The FET Enable pin seems to toggle fine but the CHG and DSG are permanently off and there doesn't seem to be anything I can do about it software wise. I wondered if there were any noticeable problems in the registers that are being set high as shown in the picture, possibly faults being triggered that I'm not aware of. I noticed SD_CMD was permanently high whether that restricts function of the chip or not?

Any advice would be great.

10 months ago

0 Maxwell Verboncoeur 10 months ago

TI__Mastermind 18880 points

Hi Rhys,

The SD_CMD bit indicates that the device received either a shutdown command from the host controller or the RST_SHUT pin was asserted high for >1s.

This sounds like the device is stuck in soft-shutdown mode. You can read this FAQ for more details and solutions: (+) [FAQ] BQ76952: What is the Soft SHUTDOWN state? - Power management forum - Power management - TI E2E support forums.

Regards,

Max Verboncoeur

0 Rhys Phillips 10 months ago in reply to Maxwell Verboncoeur

Prodigy 70 points

Hi Max, thanks a lot for the information on the soft shutdown state, very helpful. It led me to find a different issue of the LD and PACK pins being connected to the wrong place, they were connected to the top of the battery pack instead of the output PACK+. I have corrected this now and included the top half of my circuit below since it seems to have another issue now.

Previously when I had it connected incorrectly to the Battery pack I was able to talk to the chip over I2C but it was stuck in the soft shutdown state as mentioned in my original question. When It's moved to correct side I can no longer communicate to the chip at all. My TS2 and RST pins are currently flying at the moment. Are these the reason I can't communicate with my chip at all when the LD and PACK pins are connected to the right side of my CHG & DSG FETS?

Regards,

Rhys

0 Maxwell Verboncoeur 10 months ago in reply to Rhys Phillips

TI__Mastermind 18880 points

Hi Rhys,

RST_SHUT should have a 10k pulldown to VSS. If it floats up, then it could initiate a partial reset or a shutdown of the device.

Regards,

Max Verboncoeur

0 Rhys Phillips 10 months ago in reply to Maxwell Verboncoeur

Prodigy 70 points

Hi Max,

I've pulled the RST_SHUT pin down to GND now but I'm still getting no comms and the chip isnt acknowledging anything on the I2C. I did measure 1.8V across the REG18 pin. Does this mean my device is stuck in shutdown? I've tried applying 3.3V to the RST_SHUT pin for >1s but it doesn't seem to do anything. any ideas? Do I need to pull the TS2 pin and the RST_SHUT pin high at the same time?

Regards,

Rhys

0 Maxwell Verboncoeur 10 months ago in reply to Rhys Phillips

TI__Mastermind 18880 points

Hi Rhys,

Asserting RST_SHUT for >1s will command the device to enter SHUTDOWN mode. When the device is in SHUTDOWN mode, REG18 will be 0V, if it remains at 1.8V then that indicates that the part is in soft-shutdown. If there are no communications, then that indicates TS2 soft-shutdown.

The internal pullup on TS2 is very weak (~4.6Mohm to a 5V rail) and can be pulled low by as little as 1uA of leakage current. Do you have any external circuitry connected to TS2?

You might also try pulling TS2 low and then releasing it to see if the device is just in SHUTDOWN mode.

You may also check this FAQ to see if there are any faults that are sending the device into SHUTDOWN mode: [FAQ] BQ76952: How does SHUTDOWN mode work? - Power management forum - Power management - TI E2E support forums.

Regards,

Max Verboncoeur

0 Rhys Phillips 10 months ago in reply to Maxwell Verboncoeur

Prodigy 70 points

Hi Max,

I tried asserting the RST_SHUT pin for >1 and REG18 stayed at 1.8V so I can only assume that the chip is stuck in TS2 Soft-shutdown.
My TS2 pin is current left floating since it is not connected to anything but I have tried pulling it to GND and releasing it and pulling it high and nothing seems to bring back communications. There don't seem to be any voltage faults for the Stack and Bat. Are there any specific ways to check if the chip itself is faulty? other than that any other thoughts? Does the LD pin need to be held low/high whilst I pull TS2 low?

0 Maxwell Verboncoeur 10 months ago in reply to Rhys Phillips

TI__Mastermind 18880 points

Hi Rhys,

From the soft-shutdown FAQ I linked earlier:

To exit TS2 soft-shutdown, try initiating a partial reset by pulsing the RST_SHUT pin high for less than 1s. Make sure the TS2 pin is floating when you do this.

Regards,

Max Verboncoeur

0 Rhys Phillips 10 months ago in reply to Maxwell Verboncoeur

Prodigy 70 points

Hi Max,
So I managed to get the chip to shutdown and Reg18 was then outputting 0V. I then tried pulling the TS2 pin to GND and releasing it to wake it. I was still not able to communicate but this has now lead to my Resistor 35 which is the 100 ohm resistor on the input for BAT and the charge pump pin to start smoking from drawing excessive current. This did happen in a previous instance and I had to replace the BQ76942 chip to solve it.

Do you know any reasons why the input resistor for the BAT and CP1 pins might start experiencing such high current or whether this has happened in other designs before?

Regards,

Rhys

0 Maxwell Verboncoeur 10 months ago in reply to Rhys Phillips

TI__Mastermind 18880 points

Hi Rhys,

This sounds odd. Could you share your whole schematic for review?

Regards,

Max Verboncoeur

0 Rhys Phillips 10 months ago in reply to Maxwell Verboncoeur

Prodigy 70 points

Hi Max,

Here's the link for the circuit.

https://acrobat.adobe.com/id/urn:aaid:sc:EU:7cccbfb6-0534-410b-8002-9a925416dae0

I appreciate all the help.

Regards,

Rhys

0 Rhys Phillips 10 months ago in reply to Rhys Phillips

Prodigy 70 points

Hi Max,

To prevent any further time spent on this question, just letting you know I have now fixed the problem. There seemed to be no problem on the board and was infact the I2C line itself. I really appreciate all the effort and help you've given me.

regards,

Rhys