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TPS65218: About the pin of CC

Hayashi
Expert 1840 points
Part Number: TPS65218

Hi teams,

My customer wants to know the pin of CC.
Would you tell me about follow questions?
1. About the resistor
What is the purpose the resister to connect from Coin-cell?
And, why is it the value of 10ohms?
2.On the possibility of reverse current
If the Vcc deliveried to the TPS65218, is it problem the reverse current from VCC to CC?
As the intent of this question, when the voltage of Vcc is 5V, I think that it is no compliant with UL standard it cause the current flow to the Coin-cell.
For that, does it build in the protection diode of reverse current?
Would you tell me that there is no problem even if there is no external diode to protect the UL standard?
Best regards
Hayashi
  • 0
    TI__Expert 8730 points
    Hi,

    I have assigned your request to responsible Applications Engineer and we will get back to you as soon as possible.

    Regards,

    Murthy
  • 0
    TI__Mastermind 30280 points
    If you refer to the TPS65218 (or TPS65218D0) datasheet, the Functional Block Diagram on page 22 shows that IN_BU and CC body diodes both point towards SYS_BU. VCC (or VSYS) is applied at IN_BU, and because of the back-to-back FETs from IN_BU to CC it is not possible to see 5V from VSYS at CC pin. Reverse current is not a problem. An external diode is not required.

    The 10-Ohm resistor is simply a current-limiting resistor between the coin cell and the rest of the circuit. This is a requirement of UL, and is not needed for the TPS65218D0 device.
  • 0 in reply to Brian Berner
    TI__Expert 5445 points
    Hi Brian-san,

    About the 10-Ohm resistor between Coin Cell and CC pin,
    a) Could you let me know the reason why UL require that? What kind of requirement does UL have for that?
    b) Are there any guidelines of setting that resistor value?
    Thank you.

    Best Regards,

    Koshi Ninomiya
  • 0 in reply to Koshi Ninomiya
    TI__Mastermind 30280 points

    Ninomiya-san,

    I am not a UL expert, but I was able to find this link which discusses the topic of Coin Cell batteries in UL-certified applications on another site.

    The resistor is used instead of adding a "fail-safe" diode in addition to the body diode of the CC to SYS_BU FET.

    The article above links to another paper by a battery manufacturer that lists the maximum abnormal "charging" current that can be fed in to a reverse-biased coin cell battery. The most common value is 25mA.

    The way to calculate the ideal current-limiting resistor depends heavily on the battery's internal series resistance.

    I've made a few assumptions to calculate this number, but here is one way the 10-Ohms will work:

    VSYS (input to IN_BU) = 4.2V (from the main Lithium Ion battery)

    VCC (coin cell battery) = 3.0V (ideal voltage)

    VFD (Diode drop across body diode from IN_BU to SYS_BU) = 0.6V

    Diode from SYS_BU to CC has failed shorted.

    RS (Internal series resistance of coin cell) = 15 Ohms

    IREV,CC (reverse current into coin cell) = (VSYS-VFD-VCC)/(RS+10Ohms) = 0.024A < 25mA

    This is just one example, and is only a 1st-order approximation. You can follow a similar procedure to get started determining the series resistance in your design.