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BQ27000: Mosfet choice and schematic Review

Thomas Durey
Prodigy 40 points
Part Number: BQ27000
Other Parts Discussed in Thread: CSD13202Q2, BQ2970

Tool/software:

Hello there,

I’m trying to design an All-in-One USB-C PD Charger. (See this post for more details.)

As explained in the previous post, I need to add a battery protection IC.
I have chosen the BQ29700 because I have only one cell in series and 2 or 3 in parallel (1S2P/1S3P).
I have started my schematic inspired by the typical application in the datasheet and the BQ29700 EVM.

Here are my input parameters:

Design Parameters Value at 25°C
Input voltage range 4V to 5V
Maximum operating discharge current 3A
Maximum charge current for battery pack 5.5A
Overvoltage Protection (OVP) 4.275V
Overvoltage Detection Delay Timer (OVP Delay) 1.25s
Overvoltage Protection (OVP) Release Voltage 4.175V
Undervoltage Protection (UVP) 2.8V
Undervoltage Detection Delay Timer 150ms
Undervoltage Protection (UVP) Release Voltage 2.9V
Charge Overcurrent Detection (OCC) Voltage -100mV
Charge Overcurrent Detection (OCC) Delay Timer 9ms
Discharge Overcurrent Detection (OCD) Voltage 100mV
Discharge Overcurrent Detection (OCD) Delay Timer 20ms
Load Short Circuit Detection (SCC) Voltage, BAT to –V ≤ Threshold 500mV
Load Short Circuit Detection (SCC) Delay Timer 250µs
Load Short Circuit Release Voltage, BAT to –V ≥ Threshold 1V

Q1: Can you provide me with a step-by-step method to select the appropriate discharge and charge MOSFETs?
I’m having trouble understanding which parameters are important and what calculations I need to perform.

Q2: Can you perform a quick review of the schematic?

Q3: In the current schematic, I have chosen the CSD13202Q2. Is this MOSFET suitable for my application?

Thanks a lot for your help!

Thomas.

  • 0
    TI__Mastermind 42060 points

    Hello Thomas,

    Thomas Durey said:
    Q1: Can you provide me with a step-by-step method to select the appropriate discharge and charge MOSFETs?
    I’m having trouble understanding which parameters are important and what calculations I need to perform.

    The number one parameter to check is your current requirements. Current protections is based on the Rdson of your MOSFETs. So you have to make sure that it meets your OCD/OCC/SCD requirements. For example if your MOSFET is 8-mOhm Rdson, then between DSG/CHG you will have 16-mOhm. With your OCD/OCC of +/- 100-mV, you would trigger these protections at 100mV/16mOhm = +/- 6.25-A nominally, without taking into account FET or BQ2970's tolerances. 

    Thomas Durey said:
    Q2: Can you perform a quick review of the schematic?

    There needs to be a 330-Ohm resistor between Cell+ and the BAT pin. Additionally, you should not have a 100-Ohm in series with C38. Remember to place the capacitor as close as possible to the IC.

    Thomas Durey said:

    Q3: In the current schematic, I have chosen the CSD13202Q2. Is this MOSFET suitable for my application?

    It seems okay to me.

    Best Regards,

    Luis Hernandez Salomon