LOVE THIS STUFF!!! Alot of my ideas never make it much further than design either. I’ve been wanting to learn USB/Charging circuitry for awhile and this is :+1: :+1: :+1: !!!
Andrew_Debbie, I believe the people here is more concerned with what this can do and not so much with any particular optimizations. I'd stick a BIG ASS inductor in there and let users enjoy unprecedented charging flexibility and features (like fully adjustable charge voltage).
Added a few more parts. The Screw terminals at Vin are rated 15A nominal. Should be fine at 8.
R4 is a 10k NTC thermistor. It will end up mounted off board on Rev 0. On the schematic now mostly for BOM. More decisions later. I’m thinking about sacrificing a $4.99 charger for the battery holders.
R7 and R8 are the current sense resistors. The 4015 sets the current by measuring the voltage between CSP and CSN in 1mv steps up to 32mV. Maximum current is at 32mV. 8mΩ == 4Amps and 4mΩ gives 8A.
I’m going to set this up for 2 resistors in parallel. Put both of them in if you want 8A. Only put in 1 for 4A. — Of course you can use different values… The resistors are going to dissipate about 1/8 W. I’m going to use 2 or 3W resistors so that they don’t heat up too much.
CSN and BATSENS both connect to the battery positive terminal. At 8A or even 4A there could be enough voltage drop across the wire from CSN to the battery to matter. I haven’t checked. I’ll also have to model the voltage drop on the negative side. All of that will happen later on, when I start too look at layout and packaging.
There are people on here that have a lot more experience with small power FETs than I do. I'm happy to have suggestions. There are lot of things to check. The main one is to not over load / over heat the internal gate drivers and the LDO inside the LTC4015. it is possible to use an external LDO to power the gate drivers.
LT's example designs use an On-Semi (Fairchild) FDMC8030 Dual N-Channel MOSFET for the top and bottom switch. The FDMC has good numbers -- rDS on < 14mΩ at 10A , VGS = 4.5V
The FDMC8030 packs two FETs into a tiny 3x3mm MLP package. I like the specs but not the challenge of getting the layout right or being able to solder the part.
Going through the product selector at On-Semi, I found a couple of older Fairchild parts that should work.
FDS 8949 Dual N-Channel At VGS = 4.5V rDS on = 36mΩ 6A.
I'm not sure if this one will work at 8A. Should be ok at 4A but I'd have to do some thermal estimates.
FDS 8447 Single N-Channel At VGS = 4.5V it is 12.3mΩ and 11.4A.
It is single transistor. Trade off of size for better specs and easier layout.
For the first pass, I'm going to use the FDS 8447. We will see...
The battery is a dummy to show the complete circuit. There should be a capacitor in parallel with the battery.
I’ve left out the FETs that are supposed to switch between Vin or battery power for the load. SYS is suppose to be the greater of Vin or Vbat. Tying SYS to VIN without a switch could be a problem if you pull the power with a battery installed. I may have to add two FETs. I may do it anyway.
I did first order sizing on the inductor and some of the capacitors. The data sheet has equations and recommendations. I haven’t done any thermal calculations yet.
For those following along, see page 43 of the data sheet -
L = (VBAT •(1– VBAT / VIN (MAX))) / (0.25• f •ICHG(MAX))
In other words, higher switching frequency and higher charging currents make the inductor smaller. A larger different between VBat and Vin makes the inductor larger.
The inductor must not saturate at Imax. So we need an 8A + 60% * 8A inductor at a minimum. I’m probably going to limit mine to 4A but will leave room on the PCB for 8A.
I made an Excel spreadsheet to help size the inductor.
VBat = 4.3 Volts
f = 476kHz
IChg = 8A
Vin = 8Volts —> 2.09 µH - 500kHz is 2 µH
IChg = 4A
Vin = 12V
L = 5.8µH
IChg = 1.5A
Vin = 8V
L = 11µH
10µH should be enough for most of us. I may use a 15 because I don’t care about board size or inductor cost. I’ll have a look at parts later.