Part Number:BQ40Z60
Hello,
Project Background
I'm working on the power portion of a quadcopter design. Currently we are powering our electronic speed controllers (ESCs) using 4 cell LiPo battery packs that are charged by external chargers. We are using a very simple fuel gauge module to measure the charge of the battery. Basically when it beeps, we need to land immediately.
As an example here is what one of the battery packs look like: https://hobbyking.com/en_us/turnigy-nano-tech-1000mah-4s-25-50c-lipo-pack.html
I would like to design a battery module that we permanently attach to a 4 cell quadcopter battery that would allow us to plug in a barrel jack to charge the batteries (not when they are on the quadcopter). We would also like to use the SMBUS to monitor the battery's level and health using a kernel module we are planning on writing (When the battery is installed on the quadcopter, a SMBUS would connect). I appreciate that the module would add some weight to the board but the convenience of this design is more important to my client.
It seems as though the BQ40Z60 is the correct solution for our design but if anyone has a better idea I would be grateful for any suggestions.
I have purchased an EVM module and the EVM seems to be a great fit except for our current requirement (~40Amps max for this design).
Problems
After reading through some other posts and reviewing the datasheet and TRM it seems I have three main issues I need to solve.
1. I need to size the battery sense resistor to allow me to draw the high current levels required while still taking advantage of the discharge over-current detect. I think 1mOhm might be the correct fit.
2. I read in one of the forums the current sense values may need to be scaled in order to properly use the fuel gauge.
3. I need a very low RDS for the CHG/DSG FETs. Since these FETs will not be switching on or off the same way as the two charging FETs I think I can use FETs with large gate charge and low RDS. For example two of these FETs in parallel:
http://www.ti.com/lit/ds/symlink/csd17570q5b.pdf
Based off of the graph on the first page and the fact that the VGS would be on the order of 8V - 10V we are looking at 1mOhm per FET or 0.5mOhms with two in parallel and at 40 Amps there would be 0.8 watts over the two FETs lost to heat which we can mitigate with a small heat sink.
I'm concerned about the gate charge requirements for these four FETs (2 for charge, 2 for discharge), at 8V. each FET has a 180nC gate charge. Would the BQ40Z60 be able to drive these FETs or is there an issue that I'm overlooking?
Could I possibly use 3 FETs for CHG and DSG.
Maybe I can design the system so that it doesn't use the CHG/DSG FETs as long as charging and discharging are mutually exclusive.
Any thoughts or feedback would be very helpful. Thanks in advanced.
Dave
