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BQ25505 VBAT_OV

Gustavo Quintano Vacas
Prodigy 20 points
Other Parts Discussed in Thread: BQ25505

I am using BQ25505 to charge 1.3mF on VBAT_SEC from an indoor solar cell (3V ocv and 42 uA scc at indoor lighting). 

I have been testing during days VBAT_OV threshold. It has been fixed to 3.209 V with 1% resistors ROV1=7.5MOhm and R0V2=5.76 MOhm.

The problem is that values on VBAT_SEC reaches 3390 mV and sometimes rise up to 3480 mV.

How is posible so much desviation?

  • 0
    TI__Mastermind 20570 points

    The regulation accuracy on bq25505 is +/- 2%. When you mention the VBAT_SEC reaches 3390 mV or 3480 mV, is the IC still in boost charge mode? Or do you mean the VBAT_SEC is regulated 3390mV or 3480mV.

    Are you testing on our EVM? if you installed resistors>1Mohm, most of the new resistors using solder with flux, the residual flux can create parasitic, varying resistors in the 1Mohm range that are in parallel with your voltage setting resistors. If extreme cleaning with alcohol doesn't work for prototyping, try standing the resistors on their side (tombstone). 

    Another possible cause of the IC not regulated properly was due to switching noise into the VSTOR pin causing the part to think it is in unregulated cold start. You can confirm if this is the problem by looking at the VRDIV node. To prevent this, you need to have a 0.1uF or 0.01uF capacitor as close as possible between VSTOR and one of the IC GND or NC pins.

  • 0 in reply to Jing Zou
    Prodigy 20 points
    I am using two PCB prototypes and i understand that it is still in boost charge mode.

    I have verified VSTOR and VRDIV waveforms and both of them look the same as in documentation on figure 20, pag 22 ( I can't attach oscilloscope views).
    Value of "2/3 x VBATOV" is 2.131 V which corresponds to 3.196 V VBATOV real setting (ideal value is 3209, so -0.4 % desviation).

    About capacitors on VSTOR, there are 4.7 uF and 0.1 uF on VOC_SAMP pin that is tied to VSTOR, as documentation.
    I have also tried cleaned with alcohol and standing resistors on their side and normal again without success.