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BQ24770: Inconsistent charge current

Donovan Sydow
Intellectual 300 points
Part Number: BQ24770
Other Parts Discussed in Thread: BQ24773, BQ4050,

Hello, 

I am using a BQ24770 based upon the reference BQ2477x-EVM, which originally had a BQ24773. While we've implemented all the necessary logic to control a BQ24770 in addition to a BQ24773, we see inconsistent results with the charging current from a BQ24770. If we enable charging, and write the charge current values, if there is any glitch in charging input to the BQ24770, then the charge current will drop and latch down to around 130ma. This behavior doesn't happen on a BQ24773. We're measuring the charge current from a reference fuel-gauge design for the bq4050. We had similar behavior when we would initiate charging with the BQ24770, which was set or defaulted to 3A of charge current, and would consistently latch down to 130ma, we corrected this by changing our charge current value to 896, which is close to the 15W of power we expect to get out of our chargers.

Is there some sort of protection mechanism built into the BQ24770 that would force the charge current down to this lower limit, and is there a way to bypass it without constantly checking the charge current via the fuelgauge and resetting the charge current on the bq24770? 

  • 0
    TI__Guru 56650 points
    bq24773 and bq24770 should have same protections. bq24770 is a SMBus part. bq24773 is an I2Cc part. That is only difference to me.
    To debug this issue:
    1. Compare the register values (such as the charging current register...) before and after the failure event
    2. "If we enable charging, and write the charge current values, if there is any glitch in charging input to the BQ24770, then the charge current will drop and latch down to around 130ma" I would think the input voltage drops too much and the charge current register or other register values are reset. Please check the ACOK and VCC when you enable charge.
  • 0 in reply to Wang5577
    Intellectual 300 points

    Thanks for confirming that the behavior on the bq24770 and bq24773 should be identical. unfortunately we are seeing this discrepancy in behavior from our manufacturered versions of your EVMs. Is there anything material that should be different than the TI produced EVMs, and the reference designs you provide for the EVM that would cripple the charging behavior? Even according to the documentation it states that the POR charge current should be 0ma, which isn't 130ma, which is what we measure using the bq4050, this is also less than the precharge current of 384ma. VCC is 18.5v, and ACOK is 2.5v when on the charger.

  • 0 in reply to Donovan Sydow
    TI__Guru 56650 points
    For both bq24770 and bq24773, at POR,
    a .if the Vbat>Vsys_min threshold, the charge current is zero.
    b. if the Vbat<Vsys_min threshold, the auto wakeup function is default enabled which allows 128mA charging current.
    What are the Vbat and CELL pin setting when you see that issue?
  • 0 in reply to Wang5577
    Intellectual 300 points
    Cell is pulled up high >3v (3.3v) in this case. BAT is at 15.85v, even though we have the max charge current set to 16800.
  • 0 in reply to Donovan Sydow
    Intellectual 300 points
    Sorry I misunderstood. Measuring the Battery voltage at the moment it is at 15781mv.
  • 0 in reply to Donovan Sydow
    TI__Guru 56650 points

    To debug this issue, we have to find which loop controls the converter.

    Right now, the charging current is 130mA. We have to check the charge current register value and compare it with the charging current.

    If the charge current register value is much higher than 130mA. Let us step by step check other loop:

    a. Charge voltage loop: charge voltage register value vs battery voltage (at charge enable)

    b. input current limit loop: Vacp-acn vs input current register value and ILIM pin value

    c. input voltage vs Vindpm register setting

    d. read charge current register value back to compare it with 130mA.