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[FAQ] BQ2575X Central FAQ Page

Ethan Galloway
Mastermind 18590 points
Part Number: BQ25758
Other Parts Discussed in Thread: BQ25750, BQ25756, , BQ25820

  • What type of control do I have over the charging with hardware only?
    For charging with hardware only, you can change the charging current, IAC DPM current, VAC DPM voltage, battery voltage, disable/enable charging through the CE pin, change the gate drive voltage by increasing voltage on DRV_SUP, and change the switching frequency

  • Will there be a problem if there's no battery connected to the output of the BQ25750 or BQ25756?
    There will not be a problem if the BQ25756 or BQ25750 is charging without a battery attached. The charger will charge the output capacitors, reach the charge voltage and then terminate. The capacitor voltage will then fall and reach the recharge threshold of the charger. At recharge, the charger will start the process over again.

  • How do I set the charging current higher than the ICHG resistor setting with I2C?
    You'll need to set EN_ICHG_PIN=0. This disables the ICHG Pin. This will allow you to set the ICHG current higher than the hardware setting. This will also allow you to set the termination current lower than the hardware setting.

  • Why is the max charging voltage 70V, but the datasheet says the BQ25756 can only support 14S batteries?
    We listed the BQ25756 as being able to charge a 14S battery to leave room for transients from hot plug events. The BQ25756 can charge 16S Li-Ion batteries, but you may need to add protection for transients.

  • Is the IC pin spacing good for 85V?
    Yes, the IC pin spacing works all the way up to this voltage.

  • Why are the D4 and D5 blinking alternatively?
    Most likely, the charger is going into charge-recharge oscillation. The charger will charge the output capacitors, reach the charge voltage and then terminate. The capacitor voltage will then fall and reach the recharge threshold of the charger. At recharge, the charger will start the process over again.

  • How does the MPPT function work?
    MPPT works by sweeping VAC_DPM and the charger finds the max power point from the sweep.
    • How do I test BQ25756 MPPT function simply?
      You can use a normal power supply. When using a power supply, I recommend turning down the power supply current limit to below the BQ25756 input current draw and setting EN_CHG=0. Turn the power supply ON and set EN_MPPT=1 on the BQ25756. Then, you can enable charge on the BQ25756. The BQ25756 should find the MPPT to be power supply's voltage setting.

  • Does the BATSNS resistor have to be 5mΩ?
    Yes, the BATSNS resistor has to be 5mΩ.

  • What do I need to know about the input sense resistor for USB SPR/EPR contracts?
    We recommend using a 5mΩ sense resistor when using the part for USB power contracts

  • What do I need to know about the gate drive voltage?
    As the gate drive voltage goes up, the VGS ON and OFF swing of the FET is larger and the switching losses will increase while the RDS(on) goes down. Also, the gate current will go up as the gate drive voltage increases. In testing, we found 7V to be a good comprise between the RDS(on) and the switching losses.

  • Will the BQ2575X IC remember the register when the IC is powered off?
    No, the BQ25756's registers are not stored in memory and the register values will need to be rewritten when the BQ25756 starts up again. Alternatively, the BQ25756 can operate in standalone mode. Resistors can be used to set the charge current, input current limit,

  • How do I operate 2 BQ25756's in parallel?
    The BQ25756 ICs can operate in parallel with themselves without communicating between each other.

    • Can 2 BQ25756 ICs operate in MPPT mode from the same power source like a solar panel?
      2 BQ25756s can't operate in parallel in MPPT mode from the same power source. If the BQ25756 ICs have different solar panels, this would be possible.

    • Can BQ25756s operating in parallel have different input power sources?
      Yes, both BQ25756s will work at their specified charge currents to charge the battery.

  • What happens when the IC starts up and starts switching?
    The IC does the inductor detection first. A series of current pulses are sent through the inductor at startup to determine the time constant of the system. Furthermore, the system will power up assuming the DCR of the inductor is around 11mΩ. When the converter output current exceeds about 1.9A, this assumption will be reevaluated. This reevaluation occurs slowly with small changes taken every 3.5ms. The time-constant calculations are done from the input supply in forward mode and from the output side in reverse mode depending on how start up happens. The time constant calculation happens one-time each power up whereas the DCR adjustments are always running.

  • Does the BQ25756 by default enable termination?
    Yes, termination is enabled by default.

  • Do I need a battery monitor or battery gauge?
    While charging the battery, you will need some way to provide protection and cell balancing for the battery. If you are charging bare battery cells, you will need a battery monitor or battery gauge. However, some battery packs have built-in hardware to provide cell balancing and protection. You will not need a battery monitor/gauge in this case.

  • Can I use REGN to power an external device?
    I do not recommend doing this. There is very little spare power left over after powering the IC and the gate drive.

  • How do I set EN_PRECHG mode on startup without I2C to charge super capacitors? 
    This is not possible with the BQ25756. You'll have to use the BQ25758 to do this. On startup, the BQ25758 will try to regulate the VO_SNS to 5V. You can connect a voltage divider to VO_SNS to get the right output voltage. This will only work for voltages above 5V.

  • For the buck-only chargers/converters, how do I prevent voltage on the output from being seen on the input? 
    You will need a diode to prevent reverse current.

  • How do I select the switching MOSFETs?
    You can refer to the Power MOSFETs Selection in the datasheet for information about how select MOSFETs for your design.

  • Can the BQ2575X work with GaN FETs?
    No, the BQ2575X parts can't work with GaN FETs.

  • Common battery voltages and FB BOT resistors:

    Chemistry

    Li-Ion = 4.2V/cell
    LiFePO₄=3.6V/cell

    		 Cell Counts Desired Cell Voltage

    FB Bot E192 Resistor

    FB Top = 249KΩ

    		 Voltage with FB=1.536V FB Voltage adjusted to get the battery voltage exact
    Li-Ion 1S 4.2V 143kΩ 4.21V 1.532V
    LiFePO₄ 4S 14.8V 29.8kΩ 14.38V 1.538V
    Li-Ion 4S 16.4V 25.8kΩ 16.38V 1.538V
    LiFePO₄ 8S 28.8 14kΩ 28.92V 1.53V
    Li-Ion 7S 29.4V 13.8kΩ 29.32V 1.54V
    Li-Ion 10S 42V 9.53kΩ 41.81V 1.542V
    Li-Ion 15S 63V 6.26kΩ 62.96V 1.536V
    LiFePO₄ 16S 57.6V 6.9kΩ 57.6V 1.546