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TPS65218 Series Resistor value on CC pin

Nicky Lim
Genius 3195 points
Other Parts Discussed in Thread: TPS65218

Hello,

According to TPS65218 datasheet and its EVM user's guide, there is 10ohm series resistor between TPS65218 CC pin and Coin Cell Battery.

But in attached AM437x reference schematic 1.4Kohm  is connected.

Which resistor value is recommended?

And what is the purpose of this series resistor?

Regards,

Nicky

tidraw1.pdf

  • 0
    Genius 3195 points
    Hello,

    Could you let me get your answer ASAP?

    Regards,
    Nicky
  • 0 in reply to Nicky Lim
    TI__Intellectual 1990 points
    Hi Nicky,

    Sorry for the delay. The purpose of the series resistor is to limit any reverse charging on the coin cell battery if it were to occur in the system. Each coin cell battery vendor has a specification on what the maximum allowable reverse current is for that type of coin cell battery. For instance, a Panasonic CR2450 has a maximum allowable reverse current of 30mA. To calculate the series resistance that is needed, the following equation could be used.

    R_series = (VIN_BU/I_REVCUR) - R_IR

    R_series = series resistance
    I_REVCUR= maximum allowable reverse current
    R_IR = internal coin cell source impedance
    VIN_BU= voltage that will be seen on the IN_BU

    Some important things that need to be taken into account when calculating the series resistance is the internal source impedance of the coin cell that is being used and the loading on DCDC5 and DCDC6 when on coin cell.

    TPS65218 supports up to 1kohm of (source impedance + series resistance ) when the loading on DCDC5 and DCDC6 is less than 10uA and 10uA, respectively. If the loading and series resistance were to exceed these numbers then the coin cell battery may not be fast enough to replenish the CC cap and SYS_BU cap and DCDC5 and DCDC6 would not be able to regulate due to SYS_BU dropping too low.

    Are you planning to use these rails for RTC for AM437x? What voltage will you be supplying to IN_BU?

    Let me know if there are any questions. Thanks!

    Regards,

    Paul Kundmueller