On Sun, Oct 13, 2013 at 11:53 AM, Werner Almesberger <[email protected] > wrote:
> I've now placed all the components and tested some more of them. > This is what the complete board looks like: > > http://downloads.qi-hardware.com/people/werner/anelok/tmp/brd0-fnt.jpg It look gorgeous!! > > - boost converter: works but I noticed that, when disabled, Vout = > Vin. This seems to be a low resistance path, which would mean that > high leakage of anything on the 3.3 V rail could drain the battery. > In this case, a FET to cut off that part or the entire rail would > be needed. > > The 3.3 V rail feeds the display and the memory card. The memory > card already has a FET (but see below). For the OLED, the design > relies on its own power-saving capabilities. Early tests with the > MCU-less board have shown a substantial standby current (114 uA) > into the OLED, well above the 1-3 uA the data sheet specifies. If > further testing confirms this, the FET may be needed. > > - speaking of FETs, when the battery is low, the voltage difference > between the MCU's supply and the 3.3 V rail should be large enough > to open the memory card's FET. In other words, it will stop working > as an off switch. > > The easiest way to fix this is probably by adding an n-FET and a > pull-up resistor. Again, nothing major. > > It seems you cannot rely on power saving features on weird screen controllers. FETs are the hard way to teach them to stop eating precious energy :P > Then, power needs a closer look. The things to check there are: > - operation under USB power, > - operating current at 3.0 V (nominal battery voltage), > - operating current at 2.0 V (battery cut-off voltage), > - standby current at 3.0 V, > - standby current at 2.0 V. > > If you're only testing it with a workbench supply, don't forget to test it with a real coin battery. It would be a pity to produce next prototype and discover that it would need bigger capacitor ;)
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