Bit of a leap since my last query, I know. I finally got exasperated with tying components to loose trailing wires, and luck threw me some spare time so I cratch built a PCB fabrication facility at home from a ruck of recycled materials (where it was safe to do so), and have since made a few successful, though fairly small PCBs.
Also had my head buried in datasheets too, though some parts still don't quite make sense yet, but I'm getting there.
Right, enough babble (sorry for that), on to my query.
I'm looking at making one of my holy grails, an integrated lithium charge circuit. I went for the MCP73833 as it seemed to have the best features, and its small enough to drop into any design I can come up with
The componant/track layout of said circuit didnt take too long for me to work out, but its choosing the right values for some of the componants I'm having a little difficulty in getting my head round.
The datasheet provides this example circuit:
The batteries I intend to use are GTL RCR123a 3.7v 2300mAh (with protection circuit), the charger they came with is rated at 4.2v 800mA. If my math is correct, according to the datasheet I'd need a 2k2ohm resistor across Vss and PROG to match the charger specs, though I'm given to understand that lower ratings only make the charge sequence slower, which is fine by me if it's a safer bet on my end.
I've also included setting the NTC therm at 0-50°c using the suggested series/parallel resistors.
The bit I can't quite work out is the status leds. The diagram shows 470ohm @6v, which seems massively conservative to me, from the math the leds would hardly show up at all.
I've stuck a fixed 5v regulator prior to the circuit in my layout, and even running them at only 15mA (in case the pins are low output), my calculations put the resistors at 150ohm (unless I've missed something somewhere).
Any help here would be most appreciated.
Anyway, here's how my layout pretty much looks like, though I've made a few revisions since this was pic was grabbed.
