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BQ24074: Custom design draw 7mA from the battery consistently

Danilo A.
Mastermind 25675 points
Part Number: BQ24074

Hi Team,

We would like to ask your help regarding the high current draw of the customer's custom board. According to our customer,

I am using the BQ24074 in an application and am finding that with the Vin connected to a 5V adapter, after charge termination it seems to draw 7mA from the battery consistently when the battery voltage drops to 4.1 V, it appears to trickle charge the battery at 3mA in pulses

I have attached the schematic for our application and a plot of a charging profile. This is charging from a USB 5V source. The current is measured between the battery and the module, and the voltage is the battery voltage.



It shows, as expected, fast charge up to ~4.2V followed by tapering charge to 50% fast charge rate. This is expected with ITERM set to 15K.

However, after termination it seems to *draw* 7mA from the battery, until the voltage drops to around 4.1V.

Then, it occasionally turns on charging again at around 3mA, perhaps until the voltage goes above 4.1V again?

Disconnecting Vin stops the 7mA draw and returns to 15uA expected quiescent draw.

The LED on power_good should be driven off the power path from Vin, and indeed, removing the LED does not affect this behavior.

Pulling !CE high stops the draw and returns to 15uA quiescent. Releasing !CE to low again resumes the behavior (so it's not a timer problem as that should reset timers).

I wondered if this was caused by my ammeter, so I replaced that with a scope and a 1 ohm sense resistor, but got similar results

I posted a picture of the scope showing the voltage diff across the ammeter resistor (my 1ohm looked similar). It shows a periodic change in current from 7mA out of the battery to maybe 3mA going in, at about 650ms intervals

Here is a trace from the ammeter. negative values are going into the battery. it shows 7mA coming out, and then periodically looks like it's maybe entering fast charge mode very briefly? fast charge should be limited to 500mA

Regards,

Danilo

  • 0
    TI__Mastermind 23200 points

    Hi Danilo,

    Would you be able to share the a close up picture of the IC? Would you also to be share the waveforms of voltages at VIN and VOUT? Something else that would be helpful is the current being pulled from IN.

    Do you have a load on the OUT pin?

    Best Regards,

    Anthony Pham

  • 0 in reply to Anthony P Pham
    Prodigy 50 points

    here is a photo of the board.

    I am away from my setup so I can't get you the waveforms right now, I will be able to do so on Wednesday.

    VIN is a 5V USB adapter, and I'm pretty sure it's clean but I will check.

    VOUT is only loaded by the "power good" LED, through a 10K resistor, should be under 0.5mA. Removing the LED does not affect this. Note that the 7mA draw only occurs while !CE is pulled low.  If I pull it high, the current drawn from the battery drops to around 15uA. So I don't think anything is drawing from OUT.

    I'll get the rest of the measurements you asked for on Wednesday.

    Here is the layout:

  • 0 in reply to lewis girod
    TI__Mastermind 23200 points

    Hi Lewis,

    Awesome thanks for the update. I look forward to your update. 

    Best Regards,

    Anthony Pham

  • 0 in reply to Anthony P Pham
    Prodigy 50 points

    Voltage waveforms:

    Yellow: Vin

    Cyan: Vbatt

    Magenta: Vout

    Note that if I disconnect Vin, the voltage dips go away. These dips are just down to 5V nominal voltage

    In the process of measuring the current from Vin I found something odd.

    I put an ammeter in line with Vin and found a draw of 5.2mA; when I did this, the Vbatt began showing a steady current into the battery of 3.8mA.

    When I removed the ammeter from Vin and replaced it with a wire, it went back to drawing 7mA out of the battery with periodic bursts of around 50mA in.

    Another thing I observed is that if I short the ammeter on Vbatt momentarily, the charger will shut down properly, and stop this behavior. It stays well-behaved until I remove Vin and re-enable it, at which point the bad behavior returns.

    Note that in our application, we will have a current sense resistor on the Vbatt line so we can measure current into or out of the battery, so this will be more or less the situation in our application as well.  We could in principle use the current mirroring on the ISET pin instead to measure charging, but that is not our current plan.

  • 0 in reply to Anthony P Pham
    Prodigy 50 points

    So, I think I've come to a hypothesis that the ammeter is causing the battery sense routine to act up. 

    The ammeter series resistance on my bench meter is about 2 ohms, and I had another problem with the leads that may have increased the resistance further. Note that our application will be using a sense resistor of 0.02 ohms, which should be negligible in comparison.

    However I still need to figure out a way to measure the behavior on the bench

  • 0 in reply to lewis girod
    TI__Mastermind 23200 points

    Yeah I would suspect the ammeter being 2 ohms could affect it it not in a sense mode configuration. 

    When the device falls below 4.1V and the device begins to "pulse" charge between trickle, what is the amount of voltage increase on the batter? I can see it on the VBATT waveform.

    My suspicion is that, due to termination being set to 50%, it's seeing termination early. Would you be able to lower this termination percentage and see if the pulse charge disappears?

    As for the 8 mA draw, do you know if this happens only on this particular battery? Have you tried on a different battery?

    Best Regards,

    Anthony Pham

  • 0 in reply to Anthony P Pham
    Prodigy 50 points

    It would terminate early with the level being 50%, that's to be expected. 

    I built a new setup that doesn't use the off the shelf ammeters, instead uses 0.05 ohm sense resistors, and now the problem is no longer happening.

  • 0 in reply to lewis girod
    TI__Mastermind 23200 points

    Hi Lewis,

    It sounds like in that case that the resulting voltage drop from the 2 Ohm setup was influencing what the BAT pin saw, possibly even discharging it to the point of passing the re-charge threshold. The 3 mA could possibly be the current that was seen but then the device immediately terminates.

    Glad switching to the other multimeter seems to have fixed it.

    Do you still see the 7 mA current draw or has that also disappeared?

    Best Regards,

    Anthony Pham

  • 0 in reply to lewis girod
    TI__Mastermind 23200 points

    Hi Lewis,

    Are you able to comment on my last reply?

    Best Regards,

    Anthony Pham

  • 0 in reply to Anthony P Pham
    Prodigy 50 points

    Hi Sorry for the delay.

    After implementing a 0.050 ohm current sense resistor, no further issues are observed. The anomalous behavior has gone away.

    Thank you for your assistance!

  • 0 in reply to lewis girod
    TI__Mastermind 23200 points

    Hi Lewis,

    No worries! Thanks for the update, I'll go ahead and mark this as closed. 

    Best Regards,

    Anthony Pham