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BQ24725A: Switching from Cell Pack to Adapter supply delay issue

DudeFromRiga
Intellectual 305 points
Part Number: BQ24725A

Hi,
We've ran into issue with switching supply from Cell Pack to Adapter+ .

Initial Setup: "System" is supplied by "Pack+". At some moment "Adapter+" is connected.

Problem description: There is significant delay (~1.1s) between the moment when voltage to "Adapter+" node is applied and moment, when we see voltage increase on "System" node. (all names for nodes are taken from datasheet, for naming compatibility)

After research of problem, we noticed, that same delay exists on ACDRV pin.
According to datasheet, : ACDRV is "Charge pump output to drive both adapter input n-channel MOSFET (ACFET) and reverse blocking n-channel MOSFET (RBFET). ACDRV voltage is 6V above CMSRC when voltage on ACDET pin is between 2.4V to 3.15V, voltage on VCC pin is above UVLO and voltage on VCC pin is 275mV above voltage on SRN pin so that ACFET and RBFET can be turned on to power the system by AC adapter. Place a 4kΩ resistor from ACDRV to the gate of ACFET and RBFET limits the in-rush current on ACDRV pin.

Our measurements show that before/after (after = some micro- or milliseconds later) "Adapter+" is applied, Voltages are following :

1) Voltage on ACDET  is 1.2 / 3.0V  (1.2V is due to circuit, discussed in e2e.ti.com/.../561130 , which is required, to retain I2C communication, even when there is no "Adapter+ voltage applied"

2) VCC is equal or higher than Pack+ voltage

3) SRN is equal to Pack+

Interesting observation: when the circuit is launched the most first time (i.e. first - cell pack is attached, and then - adapter+), the delay is around 150ms ; later, if we turn off adapter+ and the turn it on again, all next attempts give 1.1seconds delay.

What could be the reason of this delay, and how can we eliminate it?

P.S.> If schematics is required, please let us know - I'll e-mail it.

  • 0
    TI__Mastermind 25800 points

    Hello,

    Please refer to the following excerpt from the BQ24725A datasheet, section 8.4.1 Adapter Detect and ACOK Output:

    The first time after IC POR always gives 150ms ACOK rising edge delay no matter what the ChargeOption register value is. Only after the ACDET pin voltage is pulled below 2.4V (but not below 0.6V, which resets the IC and forces the next ACOK rising edge deglitch time to be 1.3s) and the ACFET has been turned off at least one time, the 1.3s (or 150ms) delay time is effective for the next time the ACDET pin voltage goes above 2.4V. To change this option, the VCC pin voltage must above UVLO, and the ACDET pin voltage must be above 0.6V which enables the IC SMBus communication and sets ChargeOption() bit[15] to 0 which sets the next ACOK rising deglitch time to be 150ms. The purpose of the default 1.3s rising edge deglitch time is to turn off the ACFET long enough when the ACDET pin is pulled below 2.4V by excessive system current, such as over current or short circuit.

    The behavior you are seeing sounds exactly like the following. Can you verify what you're seeing is now what is above?

     

    Regards,

    Joel H

  • 0 in reply to Joel Hernandez II
    Intellectual 305 points

    Hi Joel,
    Yes, this is what we're seeing. I haven't been expecting, that this behavior is applicable not only to ACOK but also ACDRV pin.
    Thank you!

  • 0 in reply to DudeFromRiga
    TI__Mastermind 25800 points

    ACDRV depends on the state of the ACOK pin.

    I hope this resolves your problem.

    Regards,

    Joel H