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BQ25570 VBAT_OK Problem

Baptiste Hoffmeyer
Prodigy 50 points
Other Parts Discussed in Thread: BQ25570

Hello,

I am using a BQ25570 (Figure 1), and I have a problem with VBAT_OK digital signal. 

I set VBAT_OK at the following thresholds :

VBAT_OK_HYST = 3.0V

VBAT_OK_PROG = 2.1V 

VBAT_OK, is toggled correctly at the threshold points, but when the VSTOR reach the maximum VBAT_OV (Set to 5.0V). VBAT_OK signal start to have a strange behaviour, in fact, VBAT_OK toggles randomly (Oscillogram 1). It appends only when solar cells are lighted, this mean only when the system is charging.

On the oscillogram, VBAT is not shown but VBAT is stable,but not VSTOR. We can see, that VSTOR signal peak to 5.4V.

BQ25570's datasheet explains VBAT_OK digital signal operation, maybe I miss something, but I didn't expect this kind of behavior.

Datasheet mention that BQ25570 does a "short-circuit" on VIN_DC in case if VIN_DC > VSTOR and VSTOR == VBAT_OV. (Oscillogram 2) show VBAT_OK and VIN_DC. I think is not what append in my case, because VIN_DC is never higher than VIN_DC.

It may be the threshold which does not work properly, so I have done a measurement (Oscillogram 3) : Show VBAT_OK and VRDIV (PIN 8). I don't know why VRDIV signal behave different when VBAT_OK toggle low.

Any suggestions ?

Best regards.

Figure 1

Oscillogram 1

Oscillogram 2

Oscillogram 3

  • 0
    TI__Guru**** 166626 points

    This is not expected.  To clarify one point, VBAT_OK monitors VSTOR not VBAT.

    What is the loading on VSTOR and VOUT?

    Does this happen on every board/IC or just one board?

    Does the same thing happen on a TI EVM?

  • 0 in reply to Jeff F
    TI__Guru**** 166626 points

    Can you retake an oscope plot with VIN_DC, VBAT_OK, VSTOR and VRDIV on the same plot?

  • 0 in reply to Jeff F
    TI__Guru**** 166626 points

    How well was your board cleaned of flux?  When I modified my first EVM to match your circuit, I didn't see the problem but when I modified a second EVM and didn't clean off the flux as well, I see a similar problem (VSTOR overshoots and BATOK drops out momentarily).  If I blow on the board, the problem gets worse (does it more frequently) due to the mositure in my breath.  There is a warning in the datasheet about how residual flux can form parasitic resistors/capacitors that resize the >1Mohm voltage setting resistors.  Can you try re-cleaning the board?  If that doesn't work, then reduce the size of your resistors by a factor of 100 and see if you still have the problem (assuming your input source can supply the current for those resistors).   

  • 0 in reply to Jeff F
    Prodigy 50 points

    Hello Jeff,

    Thanks you for your reply.

    Solar cell characteristic : Voc = 1.8V, Icc = 15mA
    C31 and C34 are not mounted.
    C33 : 220mF, Rs = 25 Ohm

    Questions / Answers :
    1) What is the loading on VSTOR and VOUT?

    VSTOR -> Unconnected
    VOUT (1.8V) -> ~4mA

    2) Does this happen on every board/IC or just one board?

    I did many measurements on 4 different PCB, same problem for each.
    If no storage element are connected, VSTOR peak to 10V.

    3) Does the same thing happen on a TI EVM?

    Unfortunately I haven't a TI EVM yet to do some tests.


    4) How well was your board cleaned of flux? / Can you try re-cleaning the board?

    PCBs have been cleaned by the assembly manufacturer.
    I'll do a re-cleaning and tell you some news.

    5) Size of your resistors by a factor of 100 and see if you still have the problem (assuming your input source can supply the current for those resistors).
    Do you mean change R23/R27, R21/R25, R20/R22/R26? The datasheet recommend a value range of MIN=11Mohm to MAX=15Mohm.
    I don't understand exactly why a minimum is required, but I'll do the modifications on Friday to verify if the problem still persist.


    I observed that the problem occured only when VSTOR > ~4.5V.
    This mean that the system do not need to be charged at VBAT_OV to start toggling but it must need to be lighted to toggle. If I set VBAT_OV to 4.3V, VBAT_OK work properly.

  • 0 in reply to Baptiste Hoffmeyer
    TI__Guru**** 166626 points

    Regarding 2, VBAT needs to have at least 100uF equivalent capacitance on the output to prevent overshoot.

    Regarding 4, most board manufacturers are not accustomed to cleaning boards with resistors >>1M.  When developing the TI EVMs, I had to prove to my EVM supplier that they needed to perform a special cleaning to remove parasitic resistors.

    Regarding 5, I was not clear.  I meant to suggest that you lower your resistors by a factor of 100 (i.e., instead of 1Mohm you use 10kohm).  Parasitic resistors formed by solder flux are in the 1Mohm+ range.  If your resistors are smaller, the parasitic flux resistors will not have an effect and that will prove my theory. 

  • 0 in reply to Jeff F
    Prodigy 50 points

    Hi Jeff,

    Regarding to my schematic, I changed resistors values to a factor 10 (Circuit C).

    R23 = 820k

    R27 = 470k

    R20 = 390k

    R22 = 390k

    R26 = 570k

    Always the same problem.

    The problem occur only when VSTOR is greater than ~4.5V and if the system is charging, below this voltage it is hard to do toggle VBAT_OK. I'm not sure it's a parasitic resistors problem.

    I do a test with 3 different (A/B/C) circuit at the same time under different temperature (-5° ; 25° ; 50° ; 80°) and each lighted.

    Circuit A : R27 = 4M7 (VBAT_OV @ 5V)

    Circuit B : R27 = 5M9 (VBAT_OV @ 4.3V)

    Circuit C : See above.

    Circuit A & C VBAT_OK toggling.

    Circuit B VBAT_OK work well. 

     

    I tested another circuit (D) with a capacitor of 1.5F Rs < 0.1Ohm, and I noticed that VBAT_OK is more stable @ VSTOR 5.0V than circuits with a capacitor of 220mF Rs = 25Ohm. I added manually a resistor of 100Ohm to simulate a Rs of 100Ohm for the 1.5F capacitor, after the modification, VBAT_OK start to toggling frequently.