BQ77915: Is BSS214NH6327 the right MOSFET?

Hi Art,

With cell balancing, the inputs for the cell pull together as shown in figure 9-1 of the BQ77915 data sheet.  This gives about 1/2 the cell voltage for a gate voltage for the external FET when balancing with a circuit such as figure 9-3. For a balance FET you would like to have RDSON specified at half the cell voltage when you will start balancing, or less. For balancing at 3.4V a FET with RDSON at 1.7V would be desired.  If you can't find such a FET you must consider operation below the thresholds and predict operation from the typical values testing to be sure you have adequate performance from the circuit.

The FET proposed shows RDSON specified at 2.5V and 4.5V, so it would not be an immediate choice.  Looking at the typical performance curves, figure 6 Typ. drain-source on resistance shows a curve at 2.2V Vgs.  Figure 6 Typ. drain-source on resistance shows the FET starting to turn on at about 1.5V and that voltage dropping as the FET heats.  For low current it might be suitable with good heat sinking of the FET, but a lower RDSON FET may be specified. 

TI has some small MOSFETs, as examples the CSD17483F4 and CSD17382F4 have RDSON specifications at 1.8V Vgs, again not ideal if starting  balance at 3.4V, but closer than 2.5V Vgs.  Looking at Figure 3. Transfer Characteristics in the data sheets shows both are typically turning on below 1.5V, the CSD17483F4 shows more detail at lower current.  The Figure 7. On-State Resistance vs Gate-to-Source Voltage curves both show RDSON increasing rapidly below 2V, here the CSD17382F4 would seem to have an advantage.

Finding an ideal FET may be a challenge.  Use the search tools at your favorite distributor or FET supplier to find good options.  Since many only show RDSON at certain common Vgs values, you may have to search for low threshold voltage devices in the current range and package type you would consider to find possible parts before inspecting more carefully for RDSON at low voltage.

Hope this helps.