FS312F-G lithium cell protection IC - selection of depletion-mode MOSFETs

[seanspotatobusiness]

I was thinking of using a lithium-cell protection IC called FS312F-G for a very low-current application where the high overdischarge protection voltage (3 V) is needed.

It requires some external components including depletion-mode MOSFETs and a polarised capacitor. Anyone know why it must be a polarised capacitor?

Also, is it just me or are depletion-mode MOSFETs with low Rds(on) values (~150 mohm) very, very expensive? They also seem to get physically bigger and give higher Vds but I don't need a high current or Vds. The schematic shows n-channel and p-channel depletion-mode MOSFETs but I can't even find p-channel depletion-mode MOSFETs with an Rds(on) in the milliohm range. It's actually just a backwards n-channel.

[uer166]

The capacitors' symbol is just a generic decoupling capacitor, it doesn't need to be polarized. The FETs look like generic enhancement mode nFETs, not depletion (same as all all-in-one BMSs of this type).

[seanspotatobusiness]

Thanks. Are you sure about the mosfet mode though? The symbols I'm looking at have solid lines in the middle of the symbol for depletion mode and broken lines for enhancement mode.

[SeanB]

Just use the mosfets specified in the data sheet, they are cheap, and will work with the device, and easily will handle low power, up to around 2A with no problems. Capacitor is a cheap ceramic chip capacitor, any value from 100n to 1uF will work, it is just there to reduce noise, and handle the drop in voltage when the battery supplies a sudden load, and the voltage drops briefly, so the chip will not lock out the battery. You can get a complete board for almost cheaper than the parts retail, with all parts already on it, and 4 terminals to use. Look at the DW01 datasheet, and you buy the boards for 10 for $1, delivered. Thus the board is under 3c in bulk.

[seanspotatobusiness]

The datasheet doesn't specify a particular MOSFET - it leaves selection up to the user.

The reason I can't use an off-the-shelf module is because the current draw in my case is very low (~1 mA). This will not cause the voltage of the battery to sag to trigger the 2.4 V threshold of typical DW01 modules. At ~1 mA, my cells would be brought down to 2.4 V with no rebound after the protection IC kicks in and would be permanently damaged.

[seanspotatobusiness]

Look up symbols of depletion mode versus enhancement mode mosfets. The thing that differentiates them is a solid versus a dotted line. The line in the schematic is solid, not dotted. The symbols in the datasheet schematic are for depletion mode mosfets. I think it's a badly done schematic if they are supposed to be enhancement mode.

[wraper]

I really doubt they should be depletion mode. It would make no sense for protection device, nor are they cheap or common. I think manufacturer just used incorrect MOSFET symbol in their datasheet.

[ifonlyeverything]

They make breakout boards by the millions with these ICs and TP4056 for charge control. They commonly use FS8205A MOSFETs, which are not depletion mode.

[Peabody]

GreatScott ended up using this part in the final version of a charger board, and I'm sure he didn't use depletion mode mosfets.  About 8:25 into the video.  He says it's the same as the DW01 except for the higher overdischarge voltage.

[seanspotatobusiness]

Thanks for the video link. I accept that they are enhancement mode; it was just a misleading schematic symbol. When I look at multiple images of capacitor symbols via Google Images, some say it's polarised and some don't. I'll just use a 0603 ceramic.