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BQ25120: Issue with circuit design

Lance Norman
Prodigy 10 points
Part Number: BQ25120
Other Parts Discussed in Thread: , BQ25125, BQ25798

I am using the BQ25120YFPT in a custom circuit design, the schematic is included as an attachment. The device is a smart wearable that accepts tactile input from the user, which then broadcasts a message over BLE to a smartphone, receives a BLE confirmation message, provides haptic feedback (vibration motor M1), and then goes back to sleep. The main BLE SoC is Silicon Labs BGM220SC22WGA2, the BQ25120YFPT is used for regulating coin cell (BT1) battery voltage to system Vcc (1.8V) used to power the BLE SoC. It is also used to charge the coin cell from input charge contacts. I also connected the vibration motor to the BQ25120YFPT as a load on LS/LDO.

I got the custom board fabricated (x10), I've included the gerbers and BOM if it helps. On most of the boards the battery voltage is below 2V and I am not getting anything on Vcc. When I connect Vin to 4V from a power supply I still get nothing out and the battery does not charge. I assume that I did not design the circuit correctly for using the BQ25120YFPT with these other components, for instance I connected the I2C, RESET and LSCTRL lines to the Si Labs SoC for control, but this SoC is powered by the system out so I'm not sure if that is correct. 

Could you please take a look at the schematic and see if there is anything wrong with the circuit around part U2? If I removed the I2C pullup resistors and/or cut some traces would I be able to get this board to to start charging the battery and bringing Vcc to 1.8V to power the MCU?

Please let me know if I can provide more details. Also please do not circulate attached documents as they are confidential and contain proprietary information.

Thanks!

Lance

Opal Rev E2 v50_2023-03-02 (2).zip

BOM Rev E (1).pdf

Opal Rev E2 schematic (2).pdf

  • 0
    TI__Genius 17725 points

    Hi Lance,

    Just a quick question, is there a reason you've selected the BQ25120 rather than the BQ25120A? Typically we recommend BQ25120A for newer designs. After reviewing your schematic the only things I can think may be improved is grounding the ISET, ILIM, and IPRETERM pins so that those default values are known and equal to the default register values.

    In regards to your issue I believe there is a couple things that may be taking place here. The BQ25120A has a start-up battery voltage of 3.2V. So for initial power up the device requires 3.2V on BAT in order to start up. After start up the device only requires V(BAT) > V(BUVLO) which can be programmed as far low as 2.2V.

    From the input side of things, the device can technically start up at V(IN) > V(UVLO) (3.6V typical). However, the current which the input can provide is limited by VINDPM protection. This protection stops any additional current from being drawn from Input if V(IN) < V(INDPM), which by default is 4.6V. Since the source you are applying is at 4V the device does not pull the current necessary to power the SYS rail or otherwise. This is meant to protect the input from being overloaded and falling below safe levels. Once the device does start up, V(INDPM) can be changed and the VINDPM protection feature can be disabled, allowing charging at V(IN) > V(BAT) + V(SLP).

    If you are expecting to use low voltage input source, you may consider the BQ25125. This device has VINDPM disabled by default and a 1.8V default SYS voltage. It does have DPPM disabled, however, so it would be important to confirm if that is important to your application.

    On your current set up I believe raising BAT above 3.2V or Input above 4.6V should allow the device to turn on as expected.

    Best Regards,

    Juan Ospina

  • 0
    Prodigy 10 points

    Hi Juan,

    Thanks so much for your reply.

    There is no particular reason I selected this one. It may have been the case that it was the only one of that size that was currently in stock. Does the BQ25120A come in the same small package?

    So it seems all of my boards w/ batteries are under 3V. When I put 4.7V from a benchtop supply on VIN and gnd, I get up to 500mV on VCC/SYS and the battery isn't charging at all.

    Can you see anything tied to the SYS rail on my schematic that could lead to this behavior? Or should I try to somehow get the battery charged to 3.2V off the board?

    I wont be able to ground the ISET, etc lines as they aren't accessible on this board. 

    I have attached the datasheet of the battery I am using, if you look at the nominal voltage (3V) and IV curve does it look like this is too low to use with this BQ25 chip?

    I have also attached a picture of the physical board to give you a sense of what I'm dealing with. Any help would be appreciated to try and get this board up and going, which I guess would be at least turning on the BLE SoC with 1.8V on SYS rail, even with a benchtop charger connected. And then I would look forward to working with you on a next revision using more suitable hardware.

    Thanks!

    Lance

    ML414H_E.pdf

  • 0 in reply to Lance Norman
    TI__Genius 17725 points

    Hi Lance,

    Lance Norman said:
    Does the BQ25120A come in the same small package?

    Yes, it is the same package

    Lance Norman said:
    When I put 4.7V from a benchtop supply on VIN and gnd, I get up to 500mV on VCC/SYS and the battery isn't charging at all.

    Hi Norman, this is interesting. With a full 5V are you able to get any further behavior? What is your supply's current limit?

    Lance Norman said:
    Can you see anything tied to the SYS rail on my schematic that could lead to this behavior? Or should I try to somehow get the battery charged to 3.2V off the board?

    A very big load on SYS could result in SYS or PMID collapsing to a lower voltage. How much current are you expecting your SYS load to be pulling? It's hard to tell exactly what is in the picture but it looks like your SOC is likely the highest current draw, I can't make out the chip reference.

    Lance Norman said:
    Or should I try to somehow get the battery charged to 3.2V off the board?

    If possible I would attempt this to confirm the device is starting up as expected, ideally this would be above 3.2V to ensure the threshold is crossed but it looks like it is best not to charge your battery above that point.

    Lance Norman said:
    I have attached the datasheet of the battery I am using, if you look at the nominal voltage (3V) and IV curve does it look like this is too low to use with this BQ25 chip?

    This looks to be too low for voltage for the BQ2512x family of devices. The battery seems to have a maximum voltage of 3.2V but the 12x charger family does not have a setting for regulation voltages below 3.6V. There isn't too many chargers that are designed to go that low in our portfolio. You might check out BQ25798 which may be configurable down to 3.2V but it is a much larger charger.

    Best Regards,

    Juan Ospina