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Charger & Fuel gauge configuration based on bq20z95 and bq24745

David Rotenberg
Prodigy 115 points
Other Parts Discussed in Thread: BQ24745, BQ24707A, BQ24707, BQ24725, BQ24747

Hello,

I need to design a full battery management configuration (charger, fuel gauge etc.) based on the bq20z95 fuel gauge. I'm using Li-Ion 4-cell batteries. I'm thinking of using the bq24745 charger and bq294xx as the 2nd level protection IC. A few questions:

1.Is that charger recommended? If not, which charger should I use?

2. Is there an application note/block diagram for that kind of configuration?

3. The fuel gauge and the charger will share the SMBus communication. Who actually controls the charger? Is it the host or can the fuel gauge control the charger independently?

 

Thanks in advance.

  • 0
    TI__Guru 56650 points

    1. We have few new SMbus charger ICs those were released after bq24745. You can check bq24707A or bq24725. The bq24707 and bq24725 are added more protection features: inductor short; high side buck FET and low side buck FET short protecton.

    2. There is an bq24745 application note putting bq20z95 and bq24745 together. http://www.ti.com/litv/pdf/slua533  

     3. Do you have another micro-controller in your system? So, if the battery is fully discharged, the micro-controller still can send a command to wake the charger or gas guage up.

  • 0 in reply to Wang5577
    Prodigy 115 points

    Hi Wang,

    Thank you for your reply. Per your recommendation I will use the bq24707A.

    About the configuration: I do have a micro-controller in my system. If I understand correctly, during the system's steady state (after wake-up and when the battery isn't fully discharged), the bq20z95 controls the charger independently via SMBus, without any intervention of the micro-controller. The micro-controller is required only during wake-up in order to start the system. Also, the micro-controller communicates only with the fuel gauge and the fuel gauge configures the charger independently. Am I correct?

     

    About the charger: In the datasheet (bq24707A, page 2) there is a "system" connection - is that the same signal as the "system present" signal (pin 15) of bq20z95? If not, what is it?

     

    Thank you

  • 0 in reply to David Rotenberg
    TI__Guru 56650 points

    If set bq20z95 in broadcast mode, it can send out the charge voltage and charge current commands. But, it can not set the input current for the charger. The charger, gas gauge IC and micro-controller can share the same SMBus. The micro-controller can commumicate with both gas gauge IC and charger IC. So, in general, bq20z95 doesn't need to be in broadcast mode if the application has a mirco-controller. The mirco-controller can control both the charger and gas gauge.

    The bq24707A datasheet and bq20z95 datasheet are independent.  The system of bq24707A is the system load connection. The "system present" of bq20z95 may be different. The gas gauge may check "present" terminal impedance to sense the system load present or not. You have to find it in bq20z95 user's manual.

  • 0 in reply to Wang5577
    Prodigy 115 points

    Hi Wang,

    Another few questions:

    1. Is the charger capable of working without the micro-controller present, i.e. if the AC adapter is plugged in to the wall but the system is turned off? I must have a charger that supports that kind of charge (I do have the bq20z95 fuel gauge, maybe it's enough?).

    2. About the transistors that are used in the datasheet (page 2 - Q1, Q2, Q5): Are they mandatory? What are they for? I couldn't find a description in the datasheet. 

    3. Near the 1.5V (datasheet, page 2) it says: "If no adapter and Iout is needed, this rail is on". In my case, all the voltages come from the adapter (or the battery which is irrelevant), meaning that if there is no adapter, there will be no 1.5V either. Is that ok?

     

    Thank you

  • 0 in reply to David Rotenberg
    TI__Guru 56650 points

    If there is no micro-controller, the gas gauge IC has to run at boardcast mode. The charge has to be able to charge a deeply discharged battery up (http://www.ti.com/litv/pdf/slua533 ). So, the gas gauge IC can boardcast the commands to the charger.

    bq24747 can use 6V VDDP as wake-up source. But, the REGN of bq24707A is a reference for internal logic circuits. The REGN is not good to be a wake-up source. If you want to use bq24707A, you have to add a external wake-up source to handle a deeply discharged battery. 

    bq24747 default input current is 256mA with 10mohm input current sense resistor. But, the gas gauge doesn't send out the input current command. So, we have to short the input sense resistor to get more input current. But, we lost input current control on bq24747 application. So, in this case, an external micro-controller can set bq24747's input current limit.

    bq24707A default input current is 4096mA with 10mohm input current sense resistor. Even, the gas gauge doesn't send out the input current command. We still can get 4A input current. We also can change sense resistor value to control the input current limit.

    bq24707A datasheet doesn't have any relationship with bq20z95 datasheet. So, the "system" of bq24707A is the connection of the load. The "system present" of bq20z95 may define as other function.

  • 0 in reply to Wang5577
    Prodigy 115 points

    What do you mean by "you have to add a external wake-up source to handle a deeply discharged battery"? Is there an application note for that?

    Do you recommend to use the bq24747 instead of the bq24707A?

    Per my understanding, if the fuel gauge has certain voltage and current values, it will transmit them to the charger (either 247474 or 24707A) even without the host micro-controller  - am I correct?

    Do you have any comments about paragraphs 2,3 from my previous post?

     

    Thank you