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I would like to use the BQ2970 as a protection cuircuit for my litium battery (Varta CP1254 A3). It has a max. continuous discharge current of 120mA and also max. charge current of 120mA. How do I have to choose the external transistiors? I read that I need to take the overcurrent detection voltage (Uodv = 100mV) and devide it by the maximal current (I): Ron= 100mV/2*120mA = 0.41 Ohm. This restistance has to be the Ron restistance of the transistor. Is this correct?

I could not find any transistor which has such a high on-resistance. Will I get problem regarding heat, if the transistor has such a high resistance? Usually the resistance is in the mOhm range.

Did I make a mistake?

I think that I am not the only one who is using a battery with such low current limit. Is there a way to choose a transistor with low On-resistance by for example using a voltage devider?

Circuit


Added:

Thanks a lot for your answers. So could I do it like on this picture?

Two transistor and a resistance to meet a disconnection current of 150mA

enter image description here

Is it better to choose a high resistance and low Ron or like I did in the picture?

Will I lose a lot of battery capacity for my application when choosing high resistances because of low efficiencies? Is there a problem regarding heat?

One easy way to get more on resistance is to select a higher voltage part that has the same package .>

What do you mean by that? Can you give an example?

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    \$\begingroup\$ It is the total resistance FET + resistor + FET that matters. You can arrive at a desired value using low Rdson and high resistance or higher Rdson and and lower or no resistance. For a given total resistance the result is the same. \$\endgroup\$
    – Russell McMahon
    May 14 '20 at 7:53
  • \$\begingroup\$ Power = I^2 x R. Or Vsense-max x Imax . Both give the same result. You can calculate power loss and required resistor dissipation. At Vsense = 1V and Imax = 120 mA dissipation is about 120 mW = minimal. \$\endgroup\$
    – Russell McMahon
    May 14 '20 at 7:55
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You can add series resistance between the two MOSFETs if you need a minimum effective Rdson value.

In this case, if Vsense is 0.1 V and Imax is 120 mA then if you set Ioverload at say 150 mA then total Rseries + 2 x Rdson = Vsense/Ioverload = 0.1 / 0.15 = 0.66 ohm.
Choose values to suit.
Subtract 2 x Rdson from that.
Usually Ioverload will be set somewhat above I continuous as this is meant to be a failure protection device rather than a current limiter or regulator. Actual Icontinuius charge or discharge would usually be the function of other circuit elements.

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  • \$\begingroup\$ Is it better to choose a high resistance and low Ron or like I did in the picture? Will I loose a lot of battery capacity for my application when choosing high resistances because of low efficiencies? Is there a problem regarding heat? \$\endgroup\$
    – user252265
    May 13 '20 at 16:12
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Series resistance can and does work .+1 Russell McMahon.I have used channel DS voltage sensing for many applications .The RDs on of mosfets has come down a lot as I have got older.One easy way to get more on resistance is to select a higher voltage part that has the same package .This is cost effective these days.The power that is wasted depends on the voltage drop for the chip to sense and the current .So whether you run a high resistance probably cheaper mosfet or a low resistance fet and series resistance the efficiency will be the same. RDs on of mosfets has a positive temp co .This means that max current will be higher when cold and lower when hot .For many applications this is good because it makes the power system more idiot proof .The RDs on of mosfets also varies from device to device and batch to batch .The typical spec is significantly lower than the max spec and the min spec tends to not exist .So if you need accurate current trip the resister will give more predictable results .It has been said that on resistance of mosfets of the same part number can vary by up to 30 % at room temperature .

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  • \$\begingroup\$ Thanks. I have a few more questions, see post below. \$\endgroup\$
    – user252265
    May 13 '20 at 11:00

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