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BQ25700A: Dead Battery support (Instant-On With No Battery or Deeply Discharged Battery)

Kristaps Ragausis
Prodigy 140 points
Part Number: BQ25700A
Other Parts Discussed in Thread: , BQ25703A

Hello,

Wanted to ask for correct implementation of BQ25700A  for Dead Battery support.
There are few points that I would like to check.

I am going to use back to back FET's like in bq25700AEVM-732 for inrush current control and USB port Vbus separation from large capacitance:

But I have one problem here:
Before connecting input voltage I don't have 3.3V to pull up CHG_OK. 3.3V will be available later after device is turned ON. But I get around 1.19V on Q9B base that comes from 3.3V bus where it is being injected most probably from REGN over JP5. This makes impossible to allow current to flow through back to back FET's and start Wakeup charging.
Could someone help with correct solution for proper initialization of powered off device charging?
I had idea to pull up Q9B base and CHG_OK from separate 3V LDO that gets power from REGN and 3.3V through diodes so this LDO will be powered only after Vbus connected.

From datasheet ("8.6.5.1 System Voltage Regulation") I read that if I remove BATFET then configuration over I2C is needed. This means if battery is fully depleted and BQ25700A registers are reset to defaults due to power off and I can't charge battery because I can't configure BQ25700A.
So if I need to design product that needs to have "dead battery charging" I can't remove BATFET? Is it correct?

Maybe there are some more tricks that I need to consider for Dead Battery charging?

Thanks,
Kristaps

  • 0
    TI__Guru 56650 points
    Kristaps,

    One the EVM, 3.3V LDO voltage can be powered up either by the input source or by the battery. So, "Before connecting input voltage I don't have 3.3V ". It is not true.
    When you want to use CHRG_OK control, please move VBUS jumper connection to the input side.

    BATFET cannot be removed. BQ25703A has auto-wakeup function for dead battery which provides 128mA charging current to the battery.
  • 0 in reply to Wang5577
    Prodigy 140 points

    Thanks for reply Wang!

    I already did moved Vbus jumper to input side and it's clear now regarding BATFET.

    Regarding 3.3V I mean I will not have 3.3V LDO that is being powered all the time from battery in real device not to drain battery. EVM is just for simulating real device now and I disconnected LDO to simulate real world device as it will be.

    So I wanted to solve this situation correctly and had idea of using 3V LDO for CHG_OK and transistor base pull up and power it from:

    1. System's 3.3V that is ON when system is ON. So it will be OFF when system is shut down or battery dead.
    2. REGN that is 6V whenever I get voltage on Vbus. So it will be powered when I attach power input to device that is OFF.

    Would that be best solution?

    Thanks,

    Kristaps

  • 0 in reply to Kristaps Ragausis
    TI__Guru 56650 points
    Kristaps,

    Yes. That should be the solution.