8.2.3.11 NFET Selection

The choice in NFETs for a single-cell battery pack design will depend on a variety of factors including package type, size, and device cost as well as performance metrics such as drain-to-source resistance (r_DS(on)), gate capacitance, maximum current and power handling, and similar. At a bare minimum, it is recommended that the selected FETs have a drain-to-source voltage (V_DS) and gate-to-source (V_GS) voltage tolerance of 12 V. Some FETs can are designed to handle as much as 24 V between the drain and source terminals and this would provide an increased safety margin for the pack design. Further, the DC current rating should be high enough to safely handle sustained current in charge or discharge direction just below the maximum threshold tolerances of the configured OCC and OCD protections and the lowest possible sense resistance value based on tolerance and TCR considerations, or vice-versa. This ensures that there is sufficient power dissipation margin given a worst case scenario for the fault detections. In addition, striving for minimal FET resistance at the expected gate bias as well as lowest gate capacitance will help reduce conduction losses and increase power efficiency as well as achieve faster turn-on and turn-off times for the FETs. Many of these FETs are now offered as dual, back-back NFETs in wafer-chip scale (WCSP) packaging, decreasing both BOM count and shrinking necessary board real estate to accommodate the components. Last, one should always refer to the safe operating area (SOA) curves of the target FETs to ensure that the boundaries are never violated based on all possible load conditions in the end application. The CSD83325L is an excellent example of a FET solution that meets all of the aforementioned criteria, offering r_DS(on) of 10.3 mΩ and V_DS of 12 V with back-to-back NFETs in a chip-scale package, a perfect fit for battery pack designs.