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BQ24170: Fast charge current 90mA?

Ray Vincent Aranzaso
Genius 13535 points
Part Number: BQ24170
Other Parts Discussed in Thread: BQ24232, BQ2432

Hi,

Good Day. I have a customer who is working BQ24170. Please see below his query for your reference. Thank you very much.

Hello, I want to know if it's possible to have max charge current of 90mA for a single cell 3,7V 90mAh li-po battery.
When setting up fast charge current (which is the max charge possible), per bq2417x datasheet,
I_charge = V_iset/(20 * R_sr)
V_iset minimum value for charger to work is 120mV.
So, putting in the values:
I_charge = 0.12/(20 * R_sr)
R_sr = 0.067 (67 mOhm)
However, the datasheet says that the full-scale differential voltage between SRP and SRN is 40 mV.
Now, I'm unsure what SW pin voltage is when input supply to BQ from adapter is 12V (like shown in Figure 19).
I guess it follows the input?
Anyhow, differential voltage between SRP and SRN limit throws me off. So what's the maximum current sense resistor R_sr can I use here?
I know I'd use a lower charge BQ, however, very few ICs from TI have an option to use external MOSFET (Q1 and Q2 in figure 19 in datasheet), which allows me to have 12V @ 3A at the System output.

Best Regards,

Ray Vincent

  • 0
    TI__Mastermind 30045 points

    Dear Ray,

    The minimum precharge current for the device is 120 mA with a 10 mOhm sense. This corresponds to 1.2 mV of sense voltage. A charge current of 90 mA would correspond to a precharge current of 9 mA. With a 67 mOhm sense resistor, the sense voltage would be 0.603 mV. This will be too low for the charger.

    Thanks,

    Mike Emanuel

    Please click "Resolved" if this answers your question.

  • 0 in reply to Michael Emanuel35
    TI__Genius 13535 points

    Hi Michael,

    Good Day. Please see below the response of our customer to your reply. Thank you very much.

    When you say 10 mOhm sense, do you mean Rsr resistor in Figure 19? Now you said if Rsr = 0.067 Ohm, then voltage across is 0.603 mV? Are you sure it's not 6.03mV? Vsr shouldn't exceed 40mV, right?

    Best Regards,

    Ray Vincent

  • 0 in reply to Ray Vincent Aranzaso
    TI__Mastermind 30045 points

    Dear Ray,

    I am indeed referring to Rsr. If you have a 0.067 Ohm Rsr with 90 mA charge current, you will also have 9 mA pre charge current. With this low current, you will have a Rsr voltage of 0.603 mV during precharge. But as I said before, the minimum sense voltage is 1.2 mV as detailed by the footnote, "(2) The minimum current is 120 mA on 10-mΩ sense resistor."

    Correct, the Vsr shouldn't exceed 40 mV.

    Thanks,

    Mike Emanuel

    Please click "Resolved" if this answered your question.

  • 0 in reply to Michael Emanuel35
    TI__Genius 13535 points

    Hi Michael,

    Good Day. Please see below the latest query of our customer for your reference. Thank you very much.

    Can you suggest a similar BQ charger with power path? The reason I have such a low mAh battery is just so it has enough capacity to allow for processor to shut itself down when main adapter input is disconnected. I really like the BQ24170, shame it can't charge at 90mA.

    Best Regards,

    Ray Vincent

  • 0 in reply to Ray Vincent Aranzaso
    TI__Mastermind 30045 points

    Dear Ray,

    Please look into the BQ24232. It has a power path, supports charge current between 25 mA  and 500 mA, and works for single cell batteries. If this will work for you, please click "Resolved" and open a new thread to ask questions about this part.

    Thanks,

    Mike Emanuel

    Please click "Resolved" if this answered your question.

  • 0 in reply to Michael Emanuel35
    TI__Genius 13535 points

    Hi Michael,

    Good Day. Please see below the response of our customer to your latest reply. Thank you very much.

    Hello, thanks for suggestion, however, it only has 500mA for its input pin? I was using BQ24170 with 12V @ 2A input and output, why is there even a limit on max input pin, it should all depend upon the Rds parameter of built-in MOSFET. And BQ2432 input voltage is up to 10.2V, 1.8 short of what'll be given.

    I looked at <1A chargers with power path, looks like you don't have any ICs with operational voltage up to 12V, and input current >1A, 1 cell battery charge current <90mA.

    Best Regards,

    Ray Vincent

  • 0 in reply to Ray Vincent Aranzaso
    TI__Mastermind 30045 points

    Dear Ray,

    Please help me to understand the customer charger specifications.

    Here is my understanding:

    1. 12 V 3 A input and output system node

    2. 90 mA Charge Current

    3. Power Path

    4. 1 Cell Battery

    Help me understand why you need a power path charger to achieve this solution? Why do you not route the 12 V 3 A outside of the battery charger and use a different charger to charge the small battery? Do you intend for the small 90 mAh battery to support the 12 V 3 A system?

    Thanks,

    Mike Emanuel

  • 0 in reply to Michael Emanuel35
    TI__Genius 13535 points

    Hi Michael,

    Good Day. Please see below the response of our customer to your reply. Thank you very much.

    That's because I need an uninterruptible power supply, I guess I'd make a logic with a microcontroller that would monitor adapter input and if there's a disconnect/voltage gets below certain threshold it'll enable the path to the system from battery.

    Power path is a must.
    And like I said, I need a small battery that would allow the i.imx8m cpu to allow itself to shutdown after main adapter input got disconnected.

    Best Regards,

    Ray Vincent

  • 0 in reply to Ray Vincent Aranzaso
    TI__Mastermind 30045 points

    Dear Ray,

    I looked into the portfolio again. None of our 1s and <1A charge current power path parts have inputs above 10.2 V. They are also all linear chargers, stepping down the voltage from the input. The best solution would be to use a smaller charger to charge the 1s battery and to allow connection of the 1s battery to the system in the event of a loss of power.

    Thanks,

    Mike Emanuel

    Please click "Resolved" if this answered your question.