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BQ24078: bq24078

Part Number: BQ24078

Hi Sir,

WOuld you pls help questions for bq24078, thanks.

a)What would be the maximum fast charge current allowed to be set under USB500 mode?

 b)When the Riset of EVM is set to be 1800ohms, the max fast charge current is about 470mA,

Is it because the limitation of the Kiset(max)?

c) by using the BQ24078 evm to charge a 2500mAH battery,(Riset =2K ohms to achieve 450mA fast charge current)

the fast charge process took about 5.5hrs,

and the constant voltage charge process took about 1hrs to reach Iterm to trigger the /CHG(red led) to be turned on.

Is it common to have a 1hrs+ constant voltage charging process by using BQ24078 charger ic?

Thanks, Ian.

  • Hi Ian,

    What would be the maximum fast charge current allowed to be set under USB500 mode?

    In USB500 mode, the maximum charge rate would be about 500 mA. This very much depends on the setup of the device as some of the current will be used for the system load and so we generally provide the equation:

    Input Current = Charge Current + Load/Out Current

    When the Riset of EVM is set to be 1800ohms, the max fast charge current is about 470mA,

    With 1.8-kOhm resistor, this would result in the following calculations:

    Fast Charge [mA]
    Typical 494
    Max 541
    Min 442

    The measurement you're seeing can be due to a few factors: fast charge current factor variance (as seen above), resistor tolerance (if 1800-Ohms is nominal value w/o tolerance), or possibly DPPM (this can be verified with output voltage). To answer your question, it would most likely be the factor variance.

    Is it common to have a 1hrs+ constant voltage charging process by using BQ24078 charger ic?

    Ian Wang said:
    and the constant voltage charge process took about 1hrs to reach Iterm to trigger the /CHG(red led) to be turned on.

    I'm a little confused, is the device entering a new charge cycle? If the device is configured similarly to how we show in the typical application circuit, the LED will turn on when the device is charging. 

    How is the fast charge process being measured (how was the 5.5hr measurement taken)? 

  • Hi Anthony,

    Thanks for answers.

    For Item#A & #B,

    -Based on the bq24078 evm, set R2 as no pop, by reducing the R11 variable pot, the max fast charge current could be achieved is 470mA

    -Could you suggest the value of the Riset to be set at BQ24078 EVM in order to achieve 500mA fast charge current in USB500 mode?

    -Below are the screenshot of the schematic of bq24078 evm

     

     

    For Item#C,

    -Based on the captured results during charging process, it didn’t enter the new charging cycle after /CHG is H

    -The Red led is turned on during charging process, and stop after it reach termination current.

    -Below is the charging process been captured by datalogger with below setup

    Battery = 2500mAH

    Riset = 2000ohms (set by adjusting the R11 Variable pot)

    Operation mode = USB500

    Thanks, Ian.

     

     

  • Hi Ian,

    Ian Wang said:
    Could you suggest the value of the Riset to be set at BQ24078 EVM in order to achieve 500mA fast charge current in USB500 mode?

    Information for how to program the fast charge current to a desired value is discussed in the Battery Charging section of the datasheet. To program 500mA, the resistor should be the closest resistor value to.1780 Ohm. Do note that when the device is configure for USB500 mode (EN1 = HI, EN2 = LO), the typical input current limit is 475mA with a maximum of 500mA. 

    Ian Wang said:
    Based on the captured results during charging process, it didn’t enter the new charging cycle after /CHG is H

    The /CHG pin is not a control method for beginning a new charge cycle. The BQ24078 will begin a new charge cycle when the battery voltage falls under the recharge detection threshold, which is VBAT(REG) - 100mV typically. Once the battery falls under this value, the device will charge again. 

  • Hi Anthony,

    For item#c,

    Understood that /CHG pin is the output pin of bq24078 to show the charging cycle status.

    The main concern is, 

    While charging a 2500mAH battery with fast charge current about 440mA(Riset = 2K ohms),

    the bq24078 took about 1hrs+ to exit constant voltage charging stage.

    Is it common to have a 1hrs+ constant voltage charging process by using BQ24078 charger ic?

    Thanks, Ian.

  • Hi Ian,

    Ian Wang said:
    Is it common to have a 1hrs+ constant voltage charging process by using BQ24078 charger ic?

    Yes, this can be an observation seen with the charger.