• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Answers 1 answer
  • Subscribers 63 subscribers
  • Views 136 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

BQ25730: Input\Output Capacitors and Battery Discharge Protection

Eyal Rosenthal
Prodigy 140 points
Part Number: BQ25730
Other Parts Discussed in Thread: BQ25790

Hello,

BQ25730's datasheet shows that for 65W charging, you would need about 7*10uF capacitors on VSYS line. My application would require much less power usage - about 15W at max. Is there a lower input/output capacitance I'm allowed to work with in order to reduce the amount of components, or should I still use the same guidelines as with 65W?

Also, am I correct in saying that BATFET only protects the battery from the charger, but does not help in case of problems with the load? Would it work if I add another MOSFET to the BATDRV_n pin so that I would have 2 mirrored P-channels that could completely disconnect the battery in case of overcurrent\undervoltage when using battery only?

Thanks,

Eyal

  • 0
    TI__Expert 4780 points

    Hello Eyal,

    For 15W Load you should be able to get away with less capacitance. My recommendation would be to still go with at least 4x10uF but leave some DNP capacitors in the layout in case you find you need to increase capacitance after you get your PCB back.

    Regarding BATFET, the purpose is more for enabling the NVDC architecture. When the battery voltage is below the minimum system voltage, the FET is operated in LDO mode and the system can be directly powered by VBUS. When the battery voltage is above the minimum system voltage, the BATFET is closed and if the system needs to draw more current it can come from the battery (supplement mode).

    I included the description of the BATFET Below. Regarding OC protection, when BATOC is detected the converter just shuts down to reduce the current draw on the battery. It does not open the BATFET to isolate from the system.

    The device employs Narrow VDC architecture (NVDC) with BATFET separating the system from the battery. The minimum system voltage is set by VSYS_MIN register REG0x0D/0C(). Even with a depleted battery, the system is regulated above the minimum system voltage.
    When the battery is below minimum system voltage setting, the BATFET operates in linear mode (LDO mode), and the system is regulated at VSYS_MIN register value. As the battery voltage rises above the minimum system voltage, system voltage is regulated 150 mV above battery voltage when BATFET is turned off (no charging or no supplement current). When in charging or in supplement mode, the voltage difference between the system and battery is the VDS of the BATFET and the BATFET is fully on.

    Thanks,

    Khalid

  • 0 in reply to Khalid Bairuti
    TI__Expert 4780 points

    Hello Eyal,

    A couple more notes - the BATFET is not controllable by the user, it only switches on/off as described above depending on battery voltage and minimum system voltage.

    Also, since your load is lower (15W), have you considered an integrated FET solution i.e. BQ25790? This will not work for 5S, if that is your target.

    Thanks,

    Khalid

  • 0 in reply to Khalid Bairuti
    Prodigy 140 points

    Hi Khalid,

    Thanks for the answer. About using BQ25790 - my application needs to work for both Li-Ion and Lead Acid battery chemistries, which is why I skipped over BQ25790 since it doesn't state that it supports Lead Acid.

    About BATFET - its purpose is only relevant when a charger is connected. BATFET will either disable the battery charging or prevent the battery from supplementing current together with the charger. However if the battery is operating by itself (no charger or external source provided), the current can simply go through BATFET's body diode regardless of BATFET's on\off state.

    This is the situation that I'm concerned about because there's no protection on the battery in case of load short circuit. I think adding another P-channel - much like Q7 and Q8 MOSFETs in the EVM schematic (only on the battery side instead of charger side) - could help, I'm just not sure if BATDRV pin can drive 2 MOSFETs. Unless I'm missing something, adding another MOSFET on the battery path shouldn't hurt the NVDC architecture you pointed out.

  • +1 in reply to Eyal Rosenthal
    TI__Expert 4780 points

    Hello Eyal,

    I understand what you're trying to do now. I haven't seen anyone try it but I think it should work. Note that the p-channel MOSFET driven by BATDRV is recommended to have less than 5nF Ciss, so if you are connecting two FETs then you should consider the combined Ciss of the two FETs for it to be capable of driving them both.

    I would recommend putting a backup in your design to bypass the second FET in case you run into issues.

    Khalid