early termination Max712

[dau]

Hello,
We've built a NiMH-charger with the MAX712 according to the circuit at page 1.

http://pdfserv.maxim-ic.com/en/an/AN936.pdf

The only difference is, that R4=0R2, R9=6R8 and a Schottky between Q1 and Battery.
(before we used NiCd with Max713 and because of that we added the discharge circuit)
We use 4 cells with 5000mAh and have programmed:
PGM0=V+, PGM1=Bat-, PGM2=PGM3=Bat-
According to the datasheet this should give 264min timer control and slope control.
Unfortunately chargeing stops quite early (30..60 min)?
What I also haven't found out is:
a) when does the timer start? at power up or/and current flow
b) how does the IC know whether to charge linearly or switch-mode
Thanks

 

[Comcokid]

Make sure your schottky is between Q1 and the battery, and not between BATT+ and the battery.

I'm not familiar with this particular chip, but a quick glance at the datasheet (on the

http://www.maxim-ic.com

website) indicates the BATT+ is where the chip senses battery voltage. Internally, after a op-amp stage, this signal is part of the 'control logic'. This signal is critical to setting the charge mode of the chip.