bq2970x FET

Hi Jamaal,

There are a lot of FETs available, even so there is often some compromise between cost, size and characteristics.  In my opinion there does seem to be a big jump in capability and size between the small signal FETs and medium power FETs and for low power applications the FET options seem oversized.  Pulse current capability of a FET is typically much higher than the constant current capability, check the specifications on the specific FET.

The basic design procedure in the datasheet for selecting the FET is good, realize you are working with 1 cell and will need a logic level FET.  Determine the maximum RDSon you can have, then select a FET which meets that, has a suitable voltage margin, will handle the power.  For low current batteries you may need also to check that the RDSon is high enough to allow SC and/or OCD detection.  You can add a resistor to adjust the detection levels but this adds space and cost to the system.  Find suitable FETs and select one based on the other characteristics: voltage, current, power, size, cost... 

For FETs, see www.ti.com/mosfet,  questions about FETs should go to the MOSFET forum http://e2e.ti.com/support/power_management/power_stage .  We certainly hope you can find a suitable part here, but your favorite distributor will have other choices.

Thank you for your help,

Would this new FET work?  Dual, looks like the right configuration, and very small.

CSD87501L - CSD87501L 30-V Dual N-Channel Common Drain NexFET™ Power MOSFET

Thanks again

Just to clarify...

The data sheets recommends a couple specs to hit with the FET that we don't understand.  It asks for a Vdss of at least 25V, why is that? 

The FET I suggested doesn't give a Vdss, but a V source to source of 30V.  Do we really need 50V V source to source to match the spec.  I can't believe we need that kind of voltage for a 4V battery, but I'm not sure what I'm missing here?

Also, the design example asks for a continuous current capacity of more than 50A.  Other places it just calls out 50A for 350us.  Which is correct?  The FET we are hoping to use has a pulse current of 72A for 100ms, which I think is ok, but wanted to verify.

Thanks for your help

The 25V Vdss limit is somewhat arbitrary.  Many power FETs will start at 20V or 25V for a max Vds rating, but there are lower voltage options such as 12V which may be suitable.  I don't know why the design example jumps to a 25V.  There is a UN test for overcharge where for packs under 18V the lower of 2x the maximum battery voltage or 22V should be used.  An aggressive tester may want to apply 22V anyway and a 25V FET would stand that off.  Or it may be a more generic design example. A lower Vds voltage limit FET may have a lower Vgsth also which could be a benefit on single cell cell applications, you may want to consider that.  With the lower voltage FETs also watch for the maximum Vgs limit.

With back to back FETs such as with the bq29700 power path, with both FETs off one FET will act as a diode and the other FET will see most of the voltage, so two 25V FETs will not give you a 50V standoff but perhaps 25.5V.  With a dual common drain FET the common drain may not come out of the package and the limit may be an internal test or design feature.  The external voltage measurable is source to source.

I did not read the design example as 50 A continuous but may have missed it. Certainly the FET should be able to handle what looks like a 7A continuous without trip, as well as some comfortable margin for OC and SC.  The FET should have a pulse current rating for the SC current expected.  Normally a FET also has a safe operating area chart and possibly a power chart which should help understand if the part can take the pulse. Without checking carefully I would think your selected FET would work, but you will want to check carefully and if there are questions perhaps check with the FET experts.  When your system gets to low voltage the Rdson will start to increase, generally it would look like the FET you have selected would work, but should be checked.  It you may need a lower Vgsth FET(s) like those on the EVM, or you may want to search for one now to give more margin.