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BQ25015 Rset resistor

Other Parts Discussed in Thread: BQ25015

Hello All,

 

I am using the BQ25015 in our system to charge a li-ion battery and power up our board. The Rset resistor (R34 in the attached schematics) is originally

a 2.2K resistor and that worked fine but the charge current was too high for our needs and so I have changed this resistor to a 4.3K and it seemed to work fine

and give us about half of the charging current.

 

The problem is with deeply discharged batteries, where the device enters the battery pre conditioning  mode. In the original design (Rset=2.2K) that worked fine but

now, with the 4.3K resistor, this feature does not seem to function any more.

 

More ever, in the original design, I could connect the charging adaptor to the board without a battery, and the board would turn on and work. Now, when I connect

the charger without a battery, the board does not power up.

 

I know Rset is involved both in setting the charging current and also in setting the precharge rate but I cannot seem to figure out why changing it from 2.2K to 4.3K made

the impact above.

 

Thank you very much,

 

Nir.

 

 

 

8535.POWER _ PAGE1.pdf

  • Hi Again,

    TI - I would love to get a response.

     

    Thank you,

    Nir.

     

  • Hello Nir,

    Sorry for the delay.

    My first though it that you might be fighting against the input selection subsystem in the device.

    Have you tried letting either the USB or the AC pin float therefore only having one connected? There should be nothing wrong with the ISET resistor you have chosen so lets work to isolate what else could be causing the problem.

  • Hello Nir,

    I was just talking with another member of the BMS team and he brought up a great point.

    What is your system load current? If you are trying to start the system in precharge mode the current will be only 10% of the ISET value.

     With a Rset of 4.3k you will have a precharge current of approximately 19mA. Is that enough to power your system allowing it to turn on?

  • HI Ryan,

    That is a great point!  do see a 20mA reading on my power supply and yes, i am afraid that is not enough for my electroncis to power up. Is there a way to disable the precharge mode on this device?

     

    Thank you,

     

    Nir

     

  • Hello Nir,

    Unfortunately for your case the Precharge is not a feature that can disabled.

    What voltage are you seeing on the BAT/OUT pin? Since you are most likely stuck in precharge I would expect under 3.0V?

    One thing that you could try would be to pull FPWM high, while it is not recommended in precharge it would be good to test if running the dc-dc converter will manage this situation. If your system is being powered off the buck converter this action will force it on which might solve the problem since it is likely that it is off while in precharge mode. I would recommend maintaining control over this pin since once you have passed out of the start-up state you might improve your efficiency by pulling it low again (See Pg. 6 of the datasheet).

  • Hi Ryan,

    Yes, i am seeing voltages under 3.0V when in precharge. Thank you for the note regarding the FPWM pin - i will give it a try.

    Another question: I am trying to save battery power and when my system goes to 'sleep' i still see about 1mA drain

    from the battery. I have checked all other components on my board and they are not to blame. Is it possible that the BQ is consuming this, even if no external power (charge) is applied? If it is, can i do something to reduce it?

    Thank you again,

    Nir.

     

     

  • Hello Nir,

    I can not think of any reason why the BQ25015 would consume that much current in a sleep mode. It should be as low as 15 uA.

    Does puling the EN pin & FPWM pin low reduce this consumption?

    Do you have a schematic that you can share on the thread?

  • Hi again Ryan,

    Please take a look at the attached schematics and let me know if you see anything which can cause this high current consumption when the external power is disconnected and the device is only running on battery mode.

    Thank you,

    Nir.

    3264.Around Handset - POWER.pdf

     

  • Ryan,

    One more thing i found out : When i increse the battery level (i am not connecting a battery, it is a power supply simulating the battery), the current goes down, and vice versa. The regulated voltage does not change, which means that all other components on the board do not see this voltage change. Does this tell us that the BQ is consuming this current?

    Thank you,

     

    Nir.

     

  • Hello Nir,

    Looking over the schematic you supplied I am curious where the BATT- connects to ground.

    The other potential reason is that the BQ25015 still has the EN pin high since it is tied to the battery. This will increase the quiescent consumption of the device. Are you able to move the enable pin control to the on board controller so that when it goes to sleep it can also "sleep" the BQ25015?

    Also how has the testing of the start-up with the FPWM pin controlled been going?

  • Hi Ryan,

    sorry, i was only sharing part of the design. The BATT does connect to GND.

    But will pulling EN low force the DC/DC to off state, by that killing the voltage to the host controller?

    Still having a hard time playing with the FPWM pin since the layout of the device is tiny and i do not have an eval board with me , just our hardware.

     

    Nir.

     

     

     

  • Hello Nir,

    You are correct that the EN pin will disable the dc-dc converter.

    What is the power budget of your system when you have things in low power mode? The BQ25015 has a low power mode quiescent current of approximately 15uA so trying to understand how much power is being drawn through the converter is important to understanding what is causing your battery current.