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BQ25886: HELP, DON'T WORKS IN THE PRODUCTION PCB

Part Number: BQ25886

Hello

Please we have a problem in the production version of our product.

We used the same schematic in the board we designed for internal tests, it worked correctly, but when we redesigned the PCB, the IC stopped to work.

WHEN WE CONNECTED THE BATTERY, THE SYSTEM WORKS PROPERLY, WHE WE CONNECT THE USB CONNECTOR, THE CHARGER DON'T WORK AND THE SITUAZIONE IS AS FOLLOW:

- The /PG is low, but every 250msec return high for about 10 msec.

- The state output is high fix.

This is the schematic.

Note:

- C33 is in reality 44uF, R8=R7=470 ohm, OTG is floating (we tried also with a 10k pull down, but nothing changes, and in the first PCB for internal test worked with OTG floating).

- we have tried with D+ & D- connected to USB connector, connected with 200 ohm together end connected with 0 ohm together, same result

- In the PCB for internat tests, L2 was 4,7uH, and it worked, but didn't work in the new PCB, so, given that you didn't suggest to use inductors greater than 2,2uH, I passed at 1uH in the new PCB

Herewith are some screen shots of signals in different zoom views.

Vbat and vsys seems to be 5 V cause limitation of my digital oscilloscope, but really they are 8V as they had to be.

If you can't see exactly the labels of the screen shots, they are (from top to down) VREGN, VBUS, VBAT, VSYS, VPMID, VSW

I read that if the VREGN doesn't start (as I suppose happens in my circuit) it can be cause the source NOT VALIDATED, but I don't see any problem in the VBUS signal.

Moreover, I tried to give with a power supply directly 5V near to the circuit, but nothing changes.

If I could I 'd return to the PCB made for internal testing, but we are limited by the plastic enclosure of the product.

Please can you tell me which can be the problem???

Thanks and best regards.

  • Hi Enrico,

     

    I have a few questions to investigate this further:

     

    1) Is there any external load that draws current from REGN, PMID, or SYS? If so, please remove the load for now so that we can isolate the BQ25886 charger circuit for debugging purposes.

     

    2) Can you please capture a high resolution oscilloscope waveform that shows VBUS, PMID, REGN, and /PG on the same graph? The conditions that must be met in order to turn on REGN are shown below. Based on your digital captures, which show that VBUS is being pulled down significantly, it looks like poor source qualification may be failing. What is the voltage and current capability of your input source?

    3) Please confirm any schematic differences between the working prototype board and the failing production board. You mentioned that L2 was changed, but are there any other component value differences?

    Best regards,

    Angelo

  • Hi Angelo

    I'll give you the signal you asked for, but before this I would like to tell you following informations:

    1) No loads on REGN and PMID, only loads on SYS, but I already tried to cut the trace that bring VSYS to load and it has the same behaviour

    2)The significant pulled down on the VBUS signal, arrives at minimum at 4,41V, if you give a look to the the range of the oscilloscope you see is from 4,5 to 5V.

    3)The input source is a USB 2.0, but I did tests also with supply at 5V and limit on 1A.

    4)I had a pre-series of 10 pieces of the prototype board, all them worked. I did another pre-series of 10 pieces of the production board with exactly the same components of the prototype board (that is the schematic that I had annexed to this form) and no one of them worked, so I started to modify the following components values:

    - C33 to 44uF

    - R8=R7=470 ohm

    A last observation I think is important: I DIDN'T connect the power pad on the bottom of th IC to GND (the prototype worked well without this connection) may this bring to the behaviour of my board and the prototype board worked only for a lucky combination????

    Thanks and best regards.

     

  • Sorry but I cannot get the screenshot as you asked for cause limits of my oscilloscope.

    By the way, I can give you the immagine of the PG pin after the connection to the USB BUS.

    I tried to put 4,7uF on the REGN pin replacing the 10uF PIN, but nothing changes.

    Waiting for your help.

    Thanks and best regards

  • Hi Enrico,

     

    The thermal pad on the back of the IC must be soldered to GND. Please refer to the layout guidelines in Section 11 of the datasheet.

     

     

    Please also follow the schematic checklist document below. For the PMID capacitance, at least 10 uF is required, and we recommend no more than 40 uF. Therefore, please change C33 back to 22 uF.

     

    7318.BQ2588x Schematic Checklist.pdf

     

    Can you capture an oscilloscope waveform (not a logic analyzer capture) showing VBUS, IBUS, VBAT, and IBAT? I want to see how much current is being drawn from the adapter when VBUS drops from 5V down to 4.4V. Also, I can't see how much VBUS capacitance you have in your schematic.

     

    Lastly, what is your desired charge current? I want to confirm whether your ICHGSET resistor value is correct.

     

    Best regards,

    Angelo