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BQ24166:I_Lim don't work

Thomas Obermeier
Prodigy 10 points
Part Number: BQ24166

Hello,

I use the BQ24166 in my Design.

   

Charging with a USB-Source works fine. Even if current is taken from the system at the same time. The USB-Current is always this what is configured with IUSB1-3 (1500mA).

This is not the case for the "In" Input:

- I Limited I_Lim to  max 1,76A (150 Ohm resistor)
- The Charge-Current is limited to max 1,63A (330 Ohm resistor)

=> I expect that my power supply will now not be loaded with more than 1.76A. But it will be 2,25A. The BQ24166 will get extremly hot.

what is still noticeable:
If it is charging via "In" and there is no system-load connected, it seems all good.

But if i connect the system, the "In"-Current is raising slowly, into the finite. The BQ24166 gets very hot. The faster it gets hot, the more the current rises.
 If I cool the Chip down, the "In" current decreases to the start value.
As long as the chip is "cold" it looks like the I_lim is working good.
Here a Video: 
(You can see me using cooler-spray)

Do anyone have an idea why?

  • 0
    TI__Guru**** 158711 points

    Hi Thomas,

    What is the voltage on the ILIM pin when this occurs?

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 10 points

    Hi Jeff,


    The Volltage on ILIM is 0,492V when this occurs.

    I'm a bit confused too. If I put a shunt (50mOhm) between the battery and the battery holder to track the current, I no longer have the problem. Then everything seems to work fine...
    The Voltage on ILIM is 0,452V when all works.

  • 0 in reply to Thomas Obermeier
    TI__Guru**** 158711 points

    Hi Thomas,

    That's odd.  The charger's feedback loops can regulate either current or voltage but not both at the same time.  I recall that there was an issue with these early devices that we called loop interference but don't recall a thermal runaway issue like you are seeing. 

    What was the battery voltage at which you saw this?  If it was below the MINSYS value (3.5V) then it is possible that the voltage regulating MINSYS loop wasn't releasing control to the input current regulation loop. 

    Regarding the device getting hot, the charger has thermal regulation loop which should activate when the IC die temp reaches ~120C and scale back the total output current with charge current decreasing first.   That loop turned on and trumped the input current limit loop. But that doesn't explain why adding the 50mohm series resistor fixes the issue.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 10 points

    Hi Jeff,

    What was the battery voltage at which you saw this?  If it was below the MINSYS value (3.5V) then it is possible that the voltage regulating MINSYS loop wasn't releasing control to the input current regulation loop. 

    The current Battery Voltage is 3,96V. Ilim is still not maintained and the current runs away with the temperature.

    Regarding the device getting hot, the charger has thermal regulation loop which should activate when the IC die temp reaches ~120C and scale back the total output current with charge current decreasing first.   That loop turned on and trumped the input current limit loop.


    The thermal regulation works. In the video you can see the temperatur and current raises to 120°C and then the current is regulated down.

    But that doesn't explain why adding the 50mohm series resistor fixes the issue.

    The 50mOhm phenomenon only occurs with fairly full batteries. Now I tested  a quite empty battery (~3.6V).

    The charging starts with Supply 2.4A@5.0V (1,64A goes to the Battery, ~0,8A to the system). // We remember, I Limited I_Lim to  max 1,76A

    After some seconds inductor starts singing (it doesnt do that by Supply currents <2.2A or higher input voltages). 99-100% PWM on SW-Output.

    When i reduce the system-currentent to ~0,4A, 1,82A goes to the battery. Supply-current was 1,88A@5.0V.

    It always ends in thermal regulation.

    At the moment i don't see a chance for this chip in my design :(


    Regarts
    Thomas

  • 0 in reply to Thomas Obermeier
    TI__Guru**** 158711 points

    Hi Thomas,

    I don't have an EVM in house to attempt to replicate your issue.  I have ordered one but it will not be here until early next week.  

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 10 points

    No problem, I'm looking forward to an answer, maybe the chip will still work for my project.

  • 0 in reply to Thomas Obermeier
    TI__Guru**** 158711 points

    Ok.  I will keep you posted.

    Jeff

  • 0 in reply to Jeff F
    TI__Guru**** 158711 points

    Hi Thomas,

    I got an EVM and replaced the RISET and RICHG to match yours.  I have tried various VBUS and VBAT voltages and can't reproduce your issue.  A few of those are attached.  BQ24166_RISET=324ohm_RILIM=150ohm.zip  Since the input current limit (IINDPM) and input voltage limit (VINDPM) and thermal reg loops work by clamping the converter cycle, I thought that maybe there was an issue with your input voltage drooping with high V(BAT) and hitting max duty cycle before hitting VINDPM but even that works on my EVM.  I am trying to locate a heat gun to see if artificially raising the die temp can reproduce the issue.  

    Regarding why adding the 50mohm resistor in series with the battery helps, I suspect it is because the additional resistance is pushing the voltage seen at the chargers BAT pin closer to VBATREG so the charge current is starting to taper down to ITERM.  

    I didn't mention before but placing an ammeter with the battery adds additional resistance and, if set to autorange, can cause oscillations.  When added in series with the input, the charger is likely running close to VINDPM.  To confirm, you can measure the voltages at BAT and VIN pins.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 10 points

    Hi Jeff,
    Thank you very much for your efforts.
    I will measure the days and also solder a new chip and test again.

    Regeards

    Thomas