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Original question:

BQ25170: Dropout voltage at 100 mA charging current?

BQ25170: BQ25170

Fukasawa Taro
Prodigy 90 points
Part Number: BQ25170
Other Parts Discussed in Thread: BQ25180

Hello.
What is the relationship between VIN and VDO?
On the datasheet, for example, it is stated that when IOUT is 500mA, the dropout will be about 425mV.
For example, if the input voltage is 4.5V and the battery voltage is 4.1V, what will the operation be if VOUT is 4.0V due to internal drop?
I believe the condition VIN > VOUT + VSLEEPZ would be met, but I do not believe charging operation is possible.
Thank you in advance.

  • 0
    TI__Genius 12485 points

    Hi Fukasawa-san,

    The VDO specifies the drop out voltage as a function of the charging current. In this case the RON (VDO/ Test current) comes out to be about 0.85ohm. 

    For calculation, you can use the RON spec and determine headroom you need on VIN.

    Your VIN has to be higher than VOUT + (charge current*RON) and VIN has to be higher than VOUT + VSLEEPZ thresholds for proper operation. 

    Regards,

    Gautham

  • 0 in reply to Gautham
    Prodigy 90 points

    Hi Gautham.

    Thank you for your answer.
    If the following conditions are not expected, what will be the operation?

    VIN > VOUT + (charge current*RON)
    VIN > VOUT + VSLEEPZ

    In our system, VIN is accurate to 5V±10% and we would like to design the charging current at 600mA.
    In the worst case, the above conditions may not be met.
    What are the events that may occur in this case?
    Is there a way to handle this?

    Thank you in advance for your help.

  • 0 in reply to Fukasawa Taro
    TI__Genius 12485 points

    Hi Fukasawa-san,

    If either of the conditions are not met, the device will not be able to fully charge the battery as the device will need to enter CC-CV transition point. If you reduce the charging current, the VIN > VOUT + (charge current*RON) can help as the VIN > VOUT + VSLEEPZ is a fixed threshold and your VIN seems to be meeting that requirement. 

    What is the battery charge voltage in your application? Also is the VIN a proprietary adapter?

    Regards,

    Gautham 

  • 0 in reply to Gautham
    Prodigy 90 points

    Hi Gautham-san,

    The charging voltage of the battery is 4.1V and the capacity is about 3000mAh.
    The VIN is a custom adapter.
    We would like to design the battery to take less than 5 hours to fully charge.
    VIN may drop to about 4.5V when the load on the system side is under worst-case conditions, but it holds about 5V under normal conditions.
    Is there a function to reduce the charging current in response to a voltage drop?
    Is it possible to introduce this feature in another device?

    Best regards.

  • +1 in reply to Fukasawa Taro
    TI__Genius 12485 points

    Hi Fukasawa-san,

    Some of our devices such as the BQ25180 has a VINDPM feature which you can turn ON and will reduce the current as a function of VIN voltage. 

    Will that device work for your application? The function of VINDPM (which is configurable) will reduce the current into the battery (and the system) as the voltage approaches 4.5V. 

    Regards,

    Gautham