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BQ76925: help me check the BQ76925 application schematic

cickey xu
Intellectual 615 points
Part Number: BQ76925

hi,

my customer used bq76925 to manage 6s li-ion battery,they refer to our SLUA707 ,weblink is  

now they had designed theirself schematic,but they cant ensure whether the circuit can run normally. so they need us to help them to check the schematic.

in addition ,they dont how to design schematic for waking up chip. can you give some suggestion?

5545.Sheet1.pdf

  • 0
    TI__Guru 54715 points
    Hi Cickey,
    It can be difficult to tell if a design can meet the customer's requirements. As for connecting the device:
    C18 lower side should connect to GND as in SLUA707 rather than to VC0.
    The VIOUT capacitor is 200x larger than the SLUA707, use something in range shown in the data sheet table 27.
    Consider if your MCU needs a bulk capacitor.
    The charge FET symbol looks the same as the discharge FET, be sure the polarity is correct.
    The Rgs resistor on the charge FET is rather large at 10M.

    For a wake up circuit, see the SLUA707 Q2 and Q3.
  • 0 in reply to WM5295
    Intellectual 615 points
    Hi,WM5295,
    thank for your help and guidance；
    I had asked for customer modify the schematic by what you point。
    about the wake up circuit，i want to reach below target:
    1.when DC adaptor is plugged in,our bq76925 need to be waked up;
    2.when load is inserted,and the load terminal run normally;bq76925 need to be waked up,if the load dont run(such as in sleep mode),our bq76925 dont need to be waked up;
    can you tell me how to achieve these function?how should i design the schematic.
    thank you.


    best regards.

    Cickey Xu
  • 0 in reply to cickey xu
    TI__Guru 54715 points
    Hi Cickey,
    We can not design the customer product for them, but some suggestions:
    The bq76925 ALERT is an open drain output with a Wakeup detector attached. Wakeup is accomplished by a pull up of ALERT when the bq76925 is off. ALERT must go above 2V to wake up, if ALERT is also connected to the MCU be sure the voltage does not exceed the MCU input voltage. If connected by pull up to V3P3 be sure it does not exceed the ABS MAX or connected circuit ABS MAX voltages.
    Detect the DC adapter connection and drive the ALERT pin high.
    The bq76925 does not control FETs, these would be controlled by the MCU in the system. If the MCU is on while the bq76925 is off it could detect the load by some signaling from the load and wake the bq76925. If the MCU is off some circuit would need to detect the presence of the load and activate the ALERT function. If the battery voltage is 0 because it is off, and the load voltage is 0 because it is not connected to a working battery, there will not likely be any signal available. The system would need some control or additional connection to the battery to trigger the wakeup. If the FET driver biases the battery output when off, the load can provide some movement in the PACK terminals which can be detected. While it is for an un-related part and not directly applicable, you might see the application note www.ti.com/.../slua769 for pack triggered circuit ideas.
  • 0 in reply to WM5295
    Intellectual 615 points

    hi,WM5295,

    thank for your suggestion and help.

    i had thought up another ways for waking up bq76925.

    by the way ,in slua707 application.why we need Q4? can we delect it?

    because i think,if our cell discharge completely,the voltage of pack+ and vc3 is almost 0v ,in this case, Q4 will never be on;

    and our discharge and charge FET also never can open; if we dont have precharge circuit, i think the  battery pack will cant be charged. is it like this?

    so i think, if we dont have precharge circuit,Q4 must be delected.

  • 0 in reply to cickey xu
    TI__Guru 54715 points
    Hi Cickey,
    Q4 provides a lower voltage for the FET drive. Without Q4 in the www.ti.com/lit/slua707 design, if PACK+ were to go to 24V or higher with transient, the gate voltage for Q10 and Q11 would be too high and damage the FETs. A regulated voltage would be good for the FET drive limiting the gate voltage to near the specified RDSON value and providing maximum available voltage during low voltage conditions but it may require a minimum load and the designers chose a simpler circuit. With fewer cells or lower voltage cells the same simple circuit may not be necessary or as suitable.
    Check with your cell supplier on safe operating conditions for the cells, some cells may not be recommended to be recharged if the voltage is too low. Also note the POR conditions of the bq76925, it is not recommended to operate below 4.2V and may require 7.5 to wake. In these ranges as you indicate the gate voltage from Q4 may be too small to drive the FETs
    You will know your system requirements best. Select a gate drive voltage appropriate for your design.
  • 0 in reply to WM5295
    Intellectual 615 points

    Hi,WM5295 ,

    i see what you said,and i will suggest customer to  design more suitable circuit;

    thank for your strongly support.