You could consider current monitor chips that have a helper voltage and where common mode includes ground. But it's not as cheap. If each charge channel has its own switcher, can't you sense current there?
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Regards, Joerg
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If the battery terminals short out you do get a total loss of voltage. But it's no big deal if there's a helper voltage that, for example, provides a negative supply if the opamp CM range doesn't include VSS.
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Regards, Joerg
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Operate the sensing circuit from a separate supply that does not get effected from the main supply drop.
In your circuit, I assume you're using the +/- inputs of the opamp to form a dif shift via a shunt R ? The op-amp and Fet you're using can be operating from a secondary supply that can also share the same common.
of course, if you throttle back the drive using the current sense, you wouldn't get a total loss of voltage!.
In this application I am using National simple switchers so they don't help with the current measurement. They do provide an internal current limit which does provide short circuit protection.
I don't like the short circuit current being higher than the charging current. In the other job I've got to tidy up for production I'm using a sepic which does need a proper current limit.
I like to use common building blocks and MooseFET's circuit looks good at the moment.
I have battery good detection in the micro. The charger has to be safe even if the micro dies.
The customer checks short circuit by putting a switch across the output and a then a fuse in line to the battery. They then start charging and throw the switch.
Similarly for reverse battery protection they fit a reversing switch into the wiring. Again they start charging and throw the switch.
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> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0 =A0 =
=A0 =A0 =A0 =A0 =A0R1
> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0\
> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0/ R2
> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0\
> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0|
> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =3D=3D=3D
>
> > Two precision resistors instead of six.
>
> > (I used an LM324 for John. =A0He loves them.)
>
> If the load is shorted, the circuit doesn't work.
> This was the problem that my circuit was aimed at solving.
I guess I'm not visualizing Ravenhorde's setup. I'm assuming Vcc to
the op amp higher than the raw battery charging voltage, just as you
did...
Ah, I see. So use mine ahead of the regulator, if that's feasible.
If not, yours wins.
Nice work.
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Cheers,
James Arthur
It trades an op-amp for the transistor. By making the first stage increase the voltage on the sense resistor to a higher value and then using the simple diff stage, we can get a ground referenced current sense.
You could also make the measurement at the catch diode of the bucker if you don't need it to be accurate.
I'm using an LM2675 simple switcher in a buck configuration. Changing it to a sepic doesn't make sense as the extra cost would outweigh the cost of high side current sensing.
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