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BQ25751: How to design the BATFET drive circuit when the system power is off and how to design the buck boost charging FET if the charging only in buck mode.

sun qiyun
Intellectual 280 points
Part Number: BQ25751

Tool/software:

Hello， I am Sunny.

I have a question about BATFET control.

The pin BATDRV drives the BATFET gate with 10V relative to BATSRC to turn on BATFET. But in the system, the battery is used as the system power only when the system power is off. In this condition, BQ25751 will not have power as its power is from system power. Then, the pin BATDRV will not output and the BATFET cannot be driven, right? If so, could you help give me some suggestion to realize this function?

For buck-boost charging, if I only want to use its buck charging mode, how can I design this part? Can I not load Q3 and Q4 and load the R1003? Or any other advice?

Hope I can receive your reply soon. 

Thanks a lot.

  • 0
    TI__Mastermind 32016 points

    Hello Sun,

    sun qiyun said:
    The pin BATDRV drives the BATFET gate with 10V relative to BATSRC to turn on BATFET. But in the system, the battery is used as the system power only when the system power is off. In this condition, BQ25751 will not have power as its power is from system power. Then, the pin BATDRV will not output and the BATFET cannot be driven, right? If so, could you help give me some suggestion to realize this function?

    The BQ25751 will still receive power in battery only mode.

    sun qiyun said:
    For buck-boost charging, if I only want to use its buck charging mode, how can I design this part? Can I not load Q3 and Q4 and load the R1003? Or any other advice?

    No, Q3 and Q4 are required. If you are using buck-only charging mode, you will still need to use Q3 and Q4.

    Best Regards,

    Christian.

  • 0 in reply to Christian Moyer
    Intellectual 280 points

    Hello, Christian

    Thanks for your reply.

    Christian Moyer said:
    The BQ25751 will still receive power in battery only mode.

    Could you give a explanation or where does it illustrate in the datasheet?

    Christian Moyer said:
    No, Q3 and Q4 are required. If you are using buck-only charging mode, you will still need to use Q3 and Q4.

    Could you tell me why the two MOS are necessary? And from the EVM schematic, Q2 and Q3 could be replaced by the diodes D7 and D8, right?

    Thanks advance. 

  • +1 in reply to sun qiyun
    TI__Mastermind 32016 points

    Hello Sun,

    sun qiyun said:
    Could you give a explanation or where does it illustrate in the datasheet?

    There is a switch in the device. 

    sun qiyun said:
    Could you tell me why the two MOS are necessary?

    This has to do with the Inductor DCR detection, basically the device has to know that it's configure as buck-only charger, we don't have a way to tell the charger that it's in buck-only. If you removed the FETS, the device would not beable to correctly determine the Inductor DCR, and this could lead to loop instability.

    sun qiyun said:
    And from the EVM schematic, Q2 and Q3 could be replaced by the diodes D7 and D8, right?

    Yes, These are optional FETS and can be replaced with a diode.

    Best Regards,

    Christian.

  • 0 in reply to Christian Moyer
    Intellectual 280 points

    Thanks so much, Christian.

    In my design, the max discharging current will not exceed 1.5A. I want to design as below description which is different from the EVM. Could you please help tell me if BQ25751 can operate normally, if not , it will cause what?

    • Use diode Schottky to replace Q6 and Q7
    • Use a general purpose thick film resistor rather than an automotive metal resistor for R2

  • 0 in reply to Christian Moyer
    Intellectual 280 points
    Christian Moyer said:

    There is a switch in the device. 

    Hello, Christian

    From this diagram, it is showing that when the system power off, the battery power will input and the REGN LDO is also enabled, right?

  • 0 in reply to sun qiyun
    TI__Mastermind 32016 points

    Hello Sun,

    sun qiyun said:
    Use a general purpose thick film resistor rather than an automotive metal resistor for R2

    This should not be an issue, I recommend choosing one that has similar tolerance and power rating as the one used on the BQ25751 EVM.

    sun qiyun said:
    Use diode Schottky to replace Q6 and Q7

    Are you going to run this device in reverse mode? If so, the diode will prevent power on the VBUS output.

    If you are not going to use reverse mode, you could remove both Q5 and Q6.

    I recommend keeping Q7and Q8, battery is required to discharge to SYS load.

    Best Regards,

    Christian.

  • 0 in reply to Christian Moyer
    Intellectual 280 points

    got it,

    thanks Christian.