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BQ25185: Suggestion to use it in our wearable device

zunuran nasrullah
Prodigy 120 points
Part Number: BQ25185
Other Parts Discussed in Thread: BQ25180

Tool/software:

We are currently working on a wearable device. We want to use BQ25185 for our single cell 30-60mAH battery with it. Power source for BQ25185 is going to be a NFC wireless power system. So basically NFC wireless power receiver output will be connected to input of BQ25185. 

Lithium-ion or Lipo battery charge current will be between 10-30mA max and max charge voltage will be 4.2V. 

My main reason to ask for assistance is to figure out whether BQ25185 is suitable for our system or is there any better option in the product range of TI?

Actually the size of PCB is very small so we really have to use small formfactor options available. Power saving is also very crucial as we cant afford large Quiescent currents from our components. 

I am summarizing my question below

1- Is this IC suitable for my application or we can have better options?

2- Will I get efficient performance for charge currents of 10-30mA range?

3- Can you guide us about the value of  resistor/capacitor combination for ISET Pin? Do we have any formula for that ? Because as per datasheet less than 50mA charge current setting requires an RC circuit for better results.

4- If Load is running and battery reaches 3V then will system will shutdown? because no input power is available?

Regards

  • 0
    TI__Intellectual 1390 points

    Hello Zunuran,

    zunuran nasrullah said:
    1- Is this IC suitable for my application or we can have better options?

    Based on your requirements, a linear charger, such as the BQ25185, would be suitable for your application. The BQ25185 is a standalone (RC-settable) linear battery charger with power path. Are there are any other features that you are looking for?

    zunuran nasrullah said:
    2- Will I get efficient performance for charge currents of 10-30mA range?

    For a charge current of 10-30mA, I would definitely recommend using a linear charger due to the reduced cost and size compared to a buck converter, for example. At these low charge currents (10-30mA range), the efficiency loss in a linear charger is minimal, especially if the input-output voltage difference is small. I recommend checking out the entire BQ2518x family of chargers. Again, the BQ25185 is great if you want a standalone battery charger. However, we also have BQ25180 which is configurable via I2C.

    Additionally, please refer to section 8.3.7.7 of the BQ25185 datasheet for power dissipation equations.

    zunuran nasrullah said:
    3- Can you guide us about the value of  resistor/capacitor combination for ISET Pin? Do we have any formula for that ? Because as per datasheet less than 50mA charge current setting requires an RC circuit for better results.

    For charge currents below 50mA, we recommend adding an RC circuit in parallel with RISET to help stabilize the charge current. A common RC circuit that we recommend for the ISET pin is a 2kΩ + 4.7nF series combination, which should be in parallel with the ISET resistor.

    zunuran nasrullah said:
    4- If Load is running and battery reaches 3V then will system will shutdown? because no input power is available?

    VBAT must be above the battery undervoltage threshold, VBUVLO (typically 3V, falling), or else the BATFET will disconnect the battery from SYS to protect the battery. However, if an adapter is present when VBAT < BUVLO, then SYS will receive power from the adapter.

    Please feel free to reach out if you have any more questions.

    Best regards,

    Alec Lehman

  • 0 in reply to Alec Lehman
    Prodigy 120 points

    I like the Option of BQ25180 as its I2C Controlled and we can have more control over the charging system. I will let you know if I find any issue in it. Thanks

  • 0 in reply to zunuran nasrullah
    TI__Intellectual 1390 points

    Hello Zunuran,

    The BQ25180 is a great choice. If you have any questions about it, I'll be here to help.

    Best regards,

    Alec Lehman

  • 0 in reply to Alec Lehman
    Prodigy 120 points

    Can you tell me what's the consequence for not using TS/MR for hardware resetting? if I left this pin unconnected then I can still use hardware reset function via software?

  • 0 in reply to zunuran nasrullah
    TI__Intellectual 1390 points

    Hello Zunuran,

    zunuran nasrullah said:
    Can you tell me what's the consequence for not using TS/MR for hardware resetting?

    First, please refer to section 8.3.8 "Pushbutton Wake and Reset Input" of the BQ25180 datasheet.

    There are three main functions of a push button on the TS/MR pin:

    1. Wake the device from ultra-low power modes like ship mode
    2. Send an interrupt to the host when the button has been pressed for Wake1, Wake2, or long press durations (this allows for different functions in the end application, such as menu selection)
    3. Send the device into ship mode or reset the system by performing a hardware reset

    However, even if a push button is not used, a hardware reset, ship mode entry, or shutdown mode entry can all be performed via I2C by writing to Reg0x09<6:5>.

    zunuran nasrullah said:
    if I left this pin unconnected then I can still use hardware reset function via software?

    Yes (see above), but if the External NTC Monitoring (TS) function is also not needed, then please tie a 10k resistor to the TS pin.

    Best regards,

    Alec Lehman

  • 0 in reply to Alec Lehman
    Prodigy 120 points

    Thanks. My plan is to use NTC so I will connect NTC with this pin and i will use hardware reset/ship mode etc via I2C as i don't have space to incorporate push button in PCB. Can you suggest some suitable NTC in small SMD package? 103AT-2 is TH package which is not suitable for our system. Thanks

  • 0 in reply to zunuran nasrullah
    TI__Intellectual 1390 points

    Hello Zunuran,

    I recommend checking out this NTC or an equivalent:

    https://www.mouser.com/ProductDetail/TDK/NTCG063JF103FTB?qs=Mv7BduZupUhnshILNLOdEw%3D%3D

    It is important to consider the thermistor's beta value, tolerance, and resistance at 25°C.

    Best regards,

    Alec Lehman

  • 0 in reply to Alec Lehman
    Prodigy 120 points

    Thanks I am checking it. 1- Can you explain bit more about what's your criteria of selecting NTCs for particular ICs?

    2- in BQ25185 we have an RC circuit connected parallel to Iset pin to improve efficiency of low charge currents <50mA.  But in BQ25180 we don't have an external charge current control Pin. Will we still get good efficiency and precision for low charge currents of 20-20mA?

  • 0 in reply to zunuran nasrullah
    TI__Intellectual 1390 points

    Hello Zunuran,

    The following Application Note provides useful information about using NTCs with BQ25180:

    Maximizing the Full Potential of NTC Using BQ25180 Linear Charger

    zunuran nasrullah said:
    2- in BQ25185 we have an RC circuit connected parallel to Iset pin to improve efficiency of low charge currents <50mA.  But in BQ25180 we don't have an external charge current control Pin. Will we still get good efficiency and precision for low charge currents of 20-20mA?

    Just to clarify, the RC circuit on the ISET pin is only relevant to BQ25185 because charge current is set by an external resistor on this device. This additional RC circuit helps to stabilize the current signal. With BQ25180, there isn't an ISET pin because charge current is programmed through I2C. Additionally, the charge current can be set as low as 5mA with BQ25180.

    Best regards,

    Alec Lehman