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I want to build a NiMH battery charger using an LTC4010. The datasheet doesn't give any hints on how to choose the two FETs.

Here is what I think is important - please correct me if wrong:

  1. R_DS(on) should be low to limit losses while the FET is conducting.
  2. Q_g should be low to allow for faster switching, to limit switching losses.
  3. V_GS must be lower than the drive voltage of the controller
  4. V_DS must be higher than the input/output voltage
  5. I guess Input Capacitance should also be low?
  6. The Continuous Drain Current should be high enough

Any other parameters to watch out for?

To provide an example: My charger will have a 18V power supply and charge 8 cells in series with about 2A. I did a search on DigiKey and looked out for FETs that I thought fit the above criteria and found that the TSM8568CS stood out because Qg and R_DS(on) both were low. Would this FET-pair be a good fit for my application?

Low power/heat dissipation is higher on my priority list than saving a few cents on the FETs.

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    \$\begingroup\$ That pretty much narrows it down for you. Perhaps 4 should state “maximum peak voltage, transients included”. \$\endgroup\$
    – winny
    Commented Apr 1, 2021 at 22:14
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    \$\begingroup\$ A common mistake is to control using the Gate Threshold Voltage. This is where the FET barely turns on. Use the Vgs in the Rds table, at least 4.5V, higher if you want lower Rds. \$\endgroup\$
    – Mattman944
    Commented Apr 1, 2021 at 22:33
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    \$\begingroup\$ Another factor: after you have calculated how much power it will dissipate, check the thermal resistance of the package to be sure it won't heat too much. \$\endgroup\$
    – bobflux
    Commented Apr 1, 2021 at 22:50

1 Answer 1

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You're on the right track, except:

V_GS must be lower than the drive voltage of the controller

The V_GS used to specify R_DS(on) should be lower than the drive voltage of the controller. At the same time, the maximum allowed V_GS should be higher than the drive voltage of the controller.

And while we're at it, if for any reason you're driving V_GS negative (assuming NMOS), there's a spec for how negative you can drive it, which shouldn't be exceeded.

Usually this is pretty easy to achieve, but pay attention!

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