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Original question:

BQ2970: Question about consumption current and MOSFET requirement

BQ2973: Picking a MOSFET in a very low current system, Vdss still 25V ?

Steven VALSESIA
Prodigy 30 points
Part Number: BQ2973
Other Parts Discussed in Thread: BQ25175, CSD13381F4,

Hi,

I'm using a BQ29733 (-100mV / 120mV for Vocc / Vocd) to protect a VARTA CP1654 li-ion 155mAh cell battery.
The system use a 150mA charging current, and system need only 10mA (mean value).
The PACK+ will be connected to a dedicated charger, the BQ25175, connected to a USB port.


VARTA CP1654 A4 MAX DISCHARGE = 3.5CA = 542mA
VARTA CP1654 A4 MAX CHARGE = 2.5CA = 387mA

So I first selected a CSD17483FA 250mOhm FemtoFET as specified by VARTA which gave me:

With RDSon = 210mOhm (25°C, VGS 3.5V)
-> max discharge = 286mA (1.91C)
-> max charge = 238mA (1.59C)

With RDSon max = 280mOhm (25°C, VGS 3.5V)
-> max discharge = 214mA (1.43C)
-> max charge = 179mA (1.19C)

As you can see, the max value of RDSon make the max allowed charging too far from the cell capabilities and too close from the used charging current.
Am I overthinking it?


So I've moved on the 180mOhm FemtoFET CSD13381F4:

With RDSon = 150mOhm (25°C, VGS 3.5V)
-> max discharge = 400mA (2.58C)
-> max charge = 333mA (2.15C)

With RDSon max = 200mOhm (25°C, VGS 3.5V)
-> max discharge = 300mA (1.94C)
-> max charge = 250mA (1.61C)

It's better, but VDSS is 12V when the BQ2973 datasheet say 25V.

Here is my questions:

  1. Why it's so important to choose a MOSFET with VDSS at 25V as specified?
  2. Why the PACK- and V- input are given to minimal value of "BAT – 28" Volts ? Where do this "28" comes from?
  3. If BAT is very low, let's say 2.7V, V- can be set at 25.3V? How?
  4. What happen to RDSon at -10°C or 10°C? Why the datasheet only give 2 curves for RDSon at 25°C and 125°C? It would be great to have curves for at least 3 temp + all VGS rated.

Don't hesitate to ask if you have questions,
Thank you for your time.

Best regards,
Steven Valsesia

  • +1
    TI__Mastermind 27670 points

    Hello Steven,

    1. Why it's so important to choose a MOSFET with VDSS at 25V as specified?

    The MOSFET selection is based on your own system's design parameters, the MOSFET with VDSS at 25-V is specific to the example design values given in Section 12.2.1 of the datasheet. Since your system is low-current, I don't see the need for VDSS to be 25-V. 

    2. Why the PACK- and V- input are given to minimal value of "BAT – 28" Volts ? Where do this "28" comes from?

    This is the absolute minimum than the V- pin could take. It is the physical threshold of the device. This is probably a characteristic of the IC design.

    3. If BAT is very low, let's say 2.7V, V- can be set at 25.3V? How?

    V- would not be set at 25.3-V, this would just be the absolute minimum voltage that the V- pin can receive, but it can be any value between the min and max.  Any voltage outside this range could damage the IC.

    4. What happen to RDSon at -10°C or 10°C? Why the datasheet only give 2 curves for RDSon at 25°C and 125°C? It would be great to have curves for at least 3 temp + all VGS rated.

    This is something you would have to ask to the team in charge of the MOSFET.

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Prodigy 30 points

    Thanks Luis,

    It makes more sense to me now.

    How to you understand the "4.5 V to 7 V" input voltage in the Table 10.1 considering the 4.275V over voltage protection?

  • +1 in reply to Steven VALSESIA
    TI__Mastermind 27670 points

    Hello Steven,

    I imagine these input voltages would be in reference to a charger connected to the system. The input voltage of the  BAT pin can take up-to 8 Volts. Your cell voltage would not get that high.

    Best Regards,

    Luis Hernandez Salomon