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BQ25798: Output voltage drops down periodically

Part Number: BQ25798
Other Parts Discussed in Thread: BQ24650

Test environment:

BQ25798 Evaluation module (BQ25798EVM)
The charger work in autonomous charging mode (without host connected) 
No battery (or total empty battery connected)
12V input source from lab power supply (3.5A maximum current)
1.5MHz 1S jumper connected (Vsys = 3.5V )
5.8OHM resitor connected as system load, 600mA SYS current (calculated and measured)

There output (SYS) voltage drops down each 1.5s:

each 1.5s the voltage fall to 0V during ~12ms

Is this a normal behavior?

I am trying to stabilize the output voltage with a capacitor connected to SYS
The small capacitor is not enough
Larger one (4700μF) prevents charger from starting (because of "System Short Protection"):

How to keep a stable output voltage?

  • Hi Oleg,

    With charge enabled, the minimum recommended BAT and SYS capacitors and no battery, the charger overshoots SYS and/or BAT voltage, resulting in OVP fault.  If you disable charge when the battery is removed or increase the BAT capacitors by at least 47uF, this will not happen.



  • Disabling charging is not an option

    I try to connect additional 47uF BAT capacitor (in place of NA C37 on evaluation board):

    Didn't have any effect:

    For 220uF:

    And only 1000 uF solve the problem:

    Why is there no expected effect with 47uF? What am I doing wrong?

  • Oleg,

    Odd.  Try adding it to the SYS output.  It is possible that it is a function of input current limit as well.  



  • The same. No effect...

  • Oleg,

    Do you have charge disabled?  If so, then the issue is your input power is not enough for your load.  What do the chargers fault and status register tell you is happening?



  • Charge is enabled. In a test environment, the problem can be solved by disabling the charge. But in the real application I can not disable the charge

    The input power is enough. The load is only 600mA, input power is only 250ma (from LAB power supply up to 3.5A on 12V)

    Tomorrow I will check the register data, but I don't see the point in this.

    I was expecting a stand-alone reliable charging device with MPPT function, that allow to supply the power 24/7 to unattended device with or without the battery.

    The I2C communication was planned to optionaly usage only for fine tuning

    Too many issues with this charger, may be because this is new development and only pre-production version

    thank you anyways

  • Hi Oleg,

    I am sorry to hear that this charger will not work for you.  One clarification I should have made previously.  The EVM artificially has the TS pin report that the battery is present and at a safe temperature for charging.  In your real application, if you remove the three terminal battery, with the third terminal connected the battery thermistor (which would connect to TS pin), the charger will report a fault and auto disable charge.  With charge disable SYS will not collapse.

    But you are correct in that this charger has limited functionality without I2C communication.

    BQ24650 is the only other charger capable of high input voltage and with MPP tracking capability.



  • Hi Jeff

    Using the TS pin as you suggest may be an option for me

    Actually, there are not too many critical points

    To summarize, there are:
    1. Enable MPTT (by default or by use one of existed input pins)
    2. Disable "hiccup mode for SYS short" by default

    These are both "software" things

    With those small change the charger may be a perfect solution for our product line
    May be it is is possible to exploit the fact that this IC is still in "pre-release" stage and make some changes to the default configuration (or make the custom build)

    Thank you

  • HI Oleg,

    Unfortunately, hiccup mode is a safety feature to prevent the IC from damage and cannot be disabled by default.  For TI to make a custom device requires at least 9 months for our development cycle and a sales commitment of several hundred thousand units per year.  If your production cycle and quantity allow for that, I can put you in contact with the appropriate people.     

    Otherwise, BQ24650 is your best option.


  • I don't mind safety features.

    But I need solution for an absolutely real situation:

    - The "device" connected to good input power supply (12V 10W)
    - It is total empty (or damaged) battery (with valid TS resistor) connected to the charger
    - It is no overshoots on SYS and/or BAT (the system load is equivalent to 5.8OHM - less then 1A current expected)
    - The charging is enabled by default

    I am agree that in this case the charger can activate some kind of BATTERY protection,
    but I expect that the charger will still be able to provide ~1A to SYS continuously, without latches off

    Why can't I achieve this behavior (even in the laboratory the conditions)?

  • Oleg,

    With EN_MPPT enabled, the charger periodically turns off the converter in order to determine the input source's open circuit voltage.  So, the charger cannot provide a load current to SYS during MPPT sampling unless a battery > SYSMIN setting is connected. 

    I have been copied in email by a TI field person on this issue.  Shall we move to email only?



  • MPPT is disabled in this case

    Send me direct mail to