BQ25708: Issue with CHRG_OK signal for 2 Cell configuration.

Hello,

Can you measure VBUS when the I2C interface is not working? If you have a valid voltage here (and 5V is well above UVLO), then you should have I2C communication.

If you reference EVM User Guide Table 2, enabling the inrush circuit requires installing JP7 in the bottom position, which connects VIN directly to VBUS pin on BQ257xx. This allows the BQ257xx to be powered even though ACFET/RBFET are blocking. Then the device will drive CHG_OK, which will power the system. Is this how your system is configured? Are you able to post schematics for your design to the forum?

Regards,
Steve

Hi Steve,

The VBUS is low when I2C interface is not working. The challenge is that we are using CHRG_OK for enabling the adaptor voltage to flow to the circuitry only after validation of any failure in the system. 

Using JP7 would automatically bring in the input adaptor voltage to the circuitry, which we do not want. The case that I explained in the query happened when the battery went low or dead at an voltage of 4.5V (2 cell). But the CHRG_OK would not kick in till about the 3.4V range. So, the charging would not happen and we are unable to override the LDO on state as I2C is not working in this condition. 

Is it possible to configure the CHRG_OK on the basis of number of cell in battery we are using, as the datasheet states, it is common setting. 

Regards,

Suraj.P

Dear Steve,

My apologies for overlooking in the previous reply. 

The case is that when we bypass the Vin to the VBUS, the Vbus is high, we are getting the input voltage to VBUS, but at this point, the I2C Connection is not up and running. VDDA is not coming up as battery is discharged to 5.2V (Deep discharge). Is there an other voltage that is needed for I2C to work other than the VBUS. Also, additional note is that pin 5 is not connected as per the datasheet.

Please advise on if anything needs to be done to get the I2C up and running to turn on LDO. 

Regards,
Suraj.P

Hi Suraj,

The BQ25710 can run off of a 1S (3.5-4.2V) cell, so the level of discharge on the battery should not be an issue.  I set up the EVM with 5.0V on battery, no adapter, and JP2-4 set for 2S, and there was no issue with I2C communication.

As you mention, the REGN / VDDA LDO is not turned on when booting from battery only by default, because the low power mode is disabled by default.  But I can communicate with the device.  In fact, I was able to communicate with the VBATT set to 3.5V (again with CELL_BATPRES set to 2 Cell).  In this case VBUS = 0V (measured at JP7 with jumper removed), but VSYS = 3.49 V. 

So you should be able to communicate over I2C with VSYS only present.  If you have VBUS as well, then either of them should be able to power the IC for the IC to communicate.

Perhaps there is an issue with the I2C signal or I2C pullup?  What powers the pullups for the I2C lines?  Is there any chance that the pullups are unpowered in this state?  Can you send a scope capture of the I2C SCL/SDA lines.  (If you have a logic analyzer to decode it, that is even better, but we should be able to decode by hand if all you have is a scope.)

Also, just to confirm -- if you have the battery at a level of say 6.0V, then you are able to communicate with the device with no problem whether the adapter is present or not, but once you see the battery drop to around 5.2V, you are seeing the issue?

Regards,

Steve