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BQ25895: Device with strange behaivor

Ivan Santanna
Intellectual 265 points
Part Number: BQ25895

Hello Jeff, long time since our last contact... just to notify you, I had to redesign the PCB to reach 2A of charge.

For your reference, our fisrt conversation is on the link: 

e2e.ti.com/.../622771

Now, the doubt is: I cannot reach the VSYS fixed on 3.5V as configured when the battery drops below this voltage. The VSys voltage is following the VBat voltage even when VBat is below 3.5V.

The register configuration is bellow:

Reg00 = 0x26; //  - 2A de carga
Reg01 = 0x0A; // Reg01 - VINDPM = 1V
Reg02 = 0xF0; // Reg02 -
Reg03 = 0x1A; // Reg03 - VSys 3.5V, Charger Enabled,
Reg04 = 0x20; // Reg04 - Fast Charg Current LImit to 2048mA
Reg05 = 0x33; // Reg05
Reg06 = 0x5E; // Reg06 
Reg07 = 0xCD; // Reg07 
Reg08 = 0x03; // Reg08 
Reg09 = 0x44; // Reg09 
Reg0A = 0x93; // Reg0A 

Reg0D  = 0x12

The circuit is:

Could you please give me a tip?

Thanks in advance...

Ivan

  • 0
    TI__Guru**** 166826 points

    Hi Ivan,

    Can you send the remaining registers with the status/fault bits?  Those will tell me if the device is INDPM, VINDPM, Thermal Reg, NTC fault, etc.  What is the load on VSYS?  If VSYS=VBAT then the device is in supplement mode, meaning the load on SYS is greater than the input power can provide.

  • 0 in reply to Jeff F
    Intellectual 265 points

    Hello Jeff, thanks for your quick feedback, and I'm sorry for my delay, but I was on a business trip.

    The status registers requested are below:

    1) VBat > VSysmin (3.7V)

    Reg 09 = 0x44

    Reg 0A = 0x93

    Reg 0B = 0x02

    Reg 0C = 0x00

    Reg 0D = 0x0E

    Reg 0E = 0x45

    Reg 0F = 0x46

    Reg 10 = 0x3E

    Reg 11 = 0x00

    Reg 12 = 0x00

    Reg 13 = 0x26

    Reg 14 = 0x39

    2) VBat < VSysmin (2.8V)

    Reg 09 = 0x44

    Reg 0A = 0x93

    Reg 0B = 0x02

    Reg 0C = 0x00

    Reg 0D = 0x0E

    Reg 0E = 0x00

    Reg 0F = 0x00

    Reg 10 = 0x00

    Reg 11 = 0x00

    Reg 12 = 0x00

    Reg 13 = 0x26

    Reg 14 = 0x39

    In both cases, the VSys = VBat (not respecting the VSysmin = 3.5).

    The VSys load has around 60mA constinuously and some peaks of 1ms of 1A~1.3A. The load is a Sierra's GSM module only... I have two tantalum capacitors of 100uF each on this line to minimize the peaks impact.

    Another important information is that I intend to use BatFet to reset the VSys power supply if the module freezes. It's not implemented yet, but I'm thinking to use BATFET_DIS bit on Reg09 bit 5 to do that. Should I take care of HiZ mode also? To turn off VSys, I must make sure that HiZ is enabled and then turn off the BatFet. To turn on VSys back, I must disable HiZ and then allow BatFet to turns on. Is it?

    If you need any further information, feel free to request, ok?

    Thanks in advance...

    Best regards

    Ivan

  • 0 in reply to Ivan Santanna
    TI__Guru**** 166826 points
    Ivan,

    The registers show no input voltage. The IC can only maintain minimum system voltage when input power is applied at VBUS. Without input voltage applied, BAT=SYS. Also, when V(BAT) < MINSYS register setting, the ADC does not function. For the ADC to be able to function down to V(BAT)=3.0V, please set the MINSYS register to 3.0V, its lowest setting.