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BQ25798: Output voltage drop to zero

Part Number: BQ25798
Other Parts Discussed in Thread: , BQ24650

Hello TI e2e forum

I am working In Agam-Control company, we are manufacturing solutions for irrigation 

We have products which operate from Solar Panels 6V, and 12V, using the sun energy to charge Li-ION or Li-Polymer battery -one cell

Vin can vary from 3V to 24V, we found TI BQ25798 new buck-boost charger with MPPT for solar panel

We are testing The BQ25798 -Evaluation module (BQ25798EVM) trying to confirm the chip operating modes

One of our operating conditions, is working from external Voltage, without the internal Battery

We are working with12V input source It is a lab power supply, the max current from the LAB PS consumes 0.8Amp, the battery is disconnected, this mode is written as a valid mode in spec, we are loading the output, signal system with changing resistor Load to control the current and consume 2Amps 

When we do it, we have voltage drops to 1.5V every few seconds (Question: Is this normal by design?) , when we try to consume more current from the system output, at 2.5Amps, we see that the output voltage is disconnected, and the output voltage drop to zero. Our operating BQ25798 is 1.5Mhz

The output voltage does not return at all, even if we disconnect the load, or wait enough time in case we think we had a thermal shutdown (we waited 4 hours )

We understand that there is a current limit protection , but when this condition is ending , we expect that the output voltage will return back

Our CPU and all digital circuits are operating from the SYSTEM voltage , this mean ,that all our power source to our CPU is turned off , and board is dead , we can’t disconnect power ,it is in filed , we can’t control the I2C or reset the chip , no CPU , product is dead

I want to confirm that when I have a valid input voltage, the system will recover and output voltage will return back to work in no time, how can I confirm it?

Adduitional questions:

  1. I would like to know if the voltage drops are legitimate situation?
  2. How can I be sure that these voltage drops will not cause output voltage shut down?
  3. How can I calculate My voltage drops, related to my current consumption? 
  4. How can I cause that the output Voltage will return to work?
  5. Will interrupt signal from the BQ25798 to the CPU report on such a problem?
  6. If yes, can I control such a situation with the I2C, to prevent system voltage drop to zero, before it happens?

  • Hi Oleg,

    Without a battery at BAT and charge enabled, the SYS and/or BAT output may overshoot at power up, causing the converter to temporarily shutdown.  The charger should recover from this automatically.  To prevent his, either add more capacitance at SYS and/or BAT to absorb the overshoot or disable charge via the /CE pin or I2C at power up.

    The FAULT registers should report what has happened after an INT pulse.  After being read, the FAULT registers clear.  The STATUS registers may also help identify the issue. 

    Toggling HiZ bit should return the charger to normal operation.



  • Hello, Jeff

    Thanks for the quick response

    You write: ""

    Jeff F said:
    The charger should recover from this automatically

    How long will the recovery process take? 

    I was waiting few minutes after the overshoot removed, but SYS voltage does not restored

  • Hi Oleg,

    Can you read the fault and status registers one time in order to determine what fault occurred?  If the IC is shutting down, then the fault is usually something serious like a short on a pin.


  • Excuse me Jeff, but it looks like we don't understand each other

    I know exactly, what a fault occurred - the SYS output is shorted

    And you wrote in previous post that "The charger should recover from this automatically"

    I repeat (and rephrase) my question: 

    After how many seconds the  charger should recover and restore power on SYS output?

    As you can see in my first post, I have a lot of questions about this charger,  but first it is desirable to come to understand with a basic things

    Thank you


  • As I can see in spec in " System Short Protection (VSYS_SHORT)"
    "The latch off is cleared when EN_HIZ bit is set to 0"
    "If the hiccup mode for system short is disabled by setting DIS_VSYS_SHORT = 1, the converter continues running in forced PFM operation when the SYS voltage is lower than 2.2 V."

    So, a more specific question is: 
    How to disable System Short Protection feature (DIS_VSYS_SHORT = 1) on startup, before the main MCU powered up?
    Or how to reset EN_HIZ bit or whole charger (also before the main MCU powered up)?

    Thank you for assistance



  • Oleg,

    To protect the charger IC from damage, there is no way to completely disable sys short protection.  If you disable hiccup mode (which you are currently entering and then the charger shuts down), the converter still runs in forced PFM operation until the SYS voltage rises above 2.2V.  Why does the system appear to be shorted to the charger?  What is the load at SYS on startup?  Is your input power, computed as input voltage times input current limit, not high enough for 2.2V * sys load current?  If so, can you add a soft starting FET in series between SYS output and the load? 



  • The system appear to be shorted because our system has a big capacitor (1.5F) connected to SYS
    In case the capacitor is totally discharged, he act as short circuit

    If I disable hiccup mode (via external i2c connection), (converter still runs in PFM mode), its take 3-4 second to carge the capacitor, and then the system work normally

    But the problem is that I cant not disable hiccup mode on start-up from my MCU, it is still not powered 

    Actually, once charger shuts down on startup, it is no way to automatically return to normal operation except power cycling (or I2C command from external device - remember - main CPU is powered from the charger)

    Is it right?

  • Hi Oleg,

    Unfortunately, yes.  You will need to add a soft start circuit which can be as simple as a PFET between SYS and the capacitor.  I wrote an appnote on how to do that many years ago.



  • Understand, thank you

    Can you suggest another charger for:

    1s LiIO or LiPo battery 3.7V 8000mAH
    input range 5-26V
    Dynamic power-path management (work with/without battery)
    MPPT (small solar panel 6V or 12V, 10W)

  • Oleg,

    You can try BQ24650 standalone controller with external FETs but it is optimized for >1s Lithium charging.   



  • Thank you

    If I have additional questions about BQ25798, can I continue in this thread or you prefer to close this "issue" and open another one?

  • Hi Oleg,

    I would prefer that you started a new thread.