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BQ25504 : VBAT_OV , VBAT UV

Other Parts Discussed in Thread: BQ25504

Hi TI Community

Recently I have been testing my Design which includes a BQ25504 as a Battery charger. 

During calculations I set VBAT_UV = 3.0, VBAT_OV=3.9,however when I measure the waveform on VBAT_UV and VBAT_OV pins, I get the following result:

Why does it happen ? When the scope is on these pins do I must get the programmed thresholds ?

Thanks in advance 

  • The VBAT_OV and VBAT_UV feedback resistors are only connected to VSTOR via the VRDIV pin every 64ms.  This is done to reduce battery leakage current and/or IC quiescent current.

  • Yes, I read that in the EVM datasheet . But my question is why do the VBAT_OV and VBAT_UV pins have the same value given that I programmed diferent values?

  • Does the VSTOR voltage change when you place the scope probe on VBAT_OV? If so, then your probes (like the one I used) are a 10MegOhm/10pF.  Placing the probes on VBAT_OV and VBAT_UV pins with >1 MegOhm resistors connected to them alters the value of those resistors, changes the VSTOR regulation level and therefore gives the wrong values at the VBAT_OV and VBAT_UV sense pins.  If you lower the resistor values or use a HiZ scope probe (e.g., 100MegOhm/1pF), you would measure the proper divided down values at each pin.

  • I have been doing some tests stated in the EVM datasheet and I have had some troubles with the one where the thresholds voltages are checked. 

    As the datasheet says I simulated a battery with a lab power supply and a series diode. Then I swept the input voltage (simulated battery)  from 0 to VBAT_OV.

     The next picture shows the result  (Yellow-VSTOR ; Blue; VBAT)

    As you can see if the Battery voltage is slowly risen VSTOR doesn't follow VBATvoltage, although if the rise is made fast both voltage are the same. Why VSTOR is stuck at this voltage ?

    Also I have this big problem, when VSTOR is between VBAT_UV and VBAT_OV  (Charger Enbaled), LBST voltage is stuck at VIN_DC Value, so the Boost converter is not working.

    Thanks in advance

  • When a battery is attached, the switch between VSTOR and VBAT only turns on for 50ms if VSTOR < ~200mV in order to get VSTOR>1.8V.  If there is a load on VSTOR pulls VSTOR below 1.8V in 50ms or VSTOR isn't fully depleted, then you have to rely on the input power to boost VSTOR up to VBAT_UV in order for the switch between VSTOR and VBAT to close again.  

    When simulating a battery with a power supply and diode, you also need a resistor to sink current from the charger. Otherwise the voltage on the power supply will rise.

    When not boosting, what is the voltage at VIN_DC?

    Please note the datasheet caution statement below:

    If VIN_DC is higher than VSTOR and VSTOR is higher than VBAT_OV, the input VIN_DC is pulled to ground through a small resistance to stop further charging of the attached battery or capacitor. It is critical that if this case is expected, the impedance of the source attached to VIN_DC be higher than 20 Ω and not a low impedance source.