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BQ25790: BQ25790 power up without battery

Hollis Liu
Intellectual 2565 points
Part Number: BQ25790
Other Parts Discussed in Thread: BQ25700A, , BQ25700

Hi Team, 

Do we have any test picture of BAT pin during the start-up from Vbus without battery connected?

below is a test waveform on customer boards. And you can find the BAT voltage ramp up from 0 to ~10V(1.3s duration) to 17.6V. And till now, the battery is not connected to BAT pin. And i would like to know why there is a 1.3s@10V duration. In my understanding, the BAT voltage should keep rising during start-up as the BAT cap is charged continuously. What's your idea?

we did similar test on BQ25700A, and the output of BAT pin keeps rising from 0V to 17.6V as below. And there is no 10V transition voltage level. I am curious what introduces the difference. 

  • 0
    TI__Guru**** 157141 points

    Hi Hollis,

    When boosting, the BQ25790 family first turns on the FETs to pass the input voltage to the output voltage and then starts the boost converter once the voltage at SYS has settled.  You can confirm this by adding the SW waveforms to your plot. I do not why the settling time is 1.3s.  I do not support the BQ25700 but I know that it is a different core than BQ25790.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    TI__Intellectual 2565 points

    Thanks Jeff. 

    thanks for help. 

    in our test, even the input is 5V, we can also find the 10V @１.3s duration as below picture. ch1 is VBAT, ch2 is Vbus. 

    Jeff F said:
    When boosting, the BQ25790 family first turns on the FETs to pass the input voltage to the output voltage and then starts the boost converter once the voltage at SYS has settled. 

  • 0 in reply to Hollis Liu
    TI__Guru**** 157141 points

    Hi  Hollis,

    With no battery attached, VSYS rises and regulates to MINSYS voltage until the voltage at BAT reaches V(MINSYS) then they both rise together to BATREG. The time at MINSYS is a function of the capacitance on BAT, the charge current setting and input current limit setting.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    TI__Intellectual 2565 points

    HI Jeff, 

    There is a misunderstanding. The yellow line is the waveform of BAT pin, not SYS pin...

  • 0 in reply to Hollis Liu
    TI__Guru**** 157141 points

    Hi Hollis,

    I apologize. I got confused.  With no or deeply discharged battery, BAT provides 100mA trickle charge up to 2.2V then changes to a regulated output which is a function of the CELLS bit setting (1S~=2.6V, 2S~=5.0V, 3S~=8.0V and 4S=10.0) and then it charges to BATREG.  I have a question into design team about the length of time for this regulation.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    TI__Intellectual 2565 points

    HI Jeff, 

    Even the charge current is only 100mA, the BAT voltage should keeps rising until it is up to charge voltage in my understanding. Why does the 10V duration exist? 

  • 0 in reply to Hollis Liu
    TI__Guru**** 157141 points

    Hi Hollis,

    The 10V regulated BAT voltage should only last for about 1.5s and then the voltage should increase to the battery voltage or, if no battery attached, to BATREG.  The intent is the give the gauge+protector time to start up.   

    Regards,

    Jeff

  • 0 in reply to Jeff F
    TI__Intellectual 2565 points

    HI Jeff, 

    understand the 10V purpose. 

    So the 10V@1.5s level during the output is what we expected, right? How do we implement that in circuit and control logic? Can we remove the 10V@1.5s duration and let the BAT voltage rise up to be 17.6V directly as BQ25700A? 

  • 0 in reply to Hollis Liu
    TI__Guru**** 157141 points

    Unfortunately, no.  There is no way to disable it.

    Regards,

    Jeff