I haven't made much noise about Anelok lately, but that doesn't mean that things aren't moving.
I had a good long look at the design for the next board revision - and promptly discovered two nasty little flaws in the battery / USB switching circuit, which would probably not have worked very well, if at all. For the following explanation, you may want to look at the old (buggy) schematics: http://downloads.qi-hardware.com/people/werner/anelok/tmp/anelok-20140603.pdf The idea is as follows: when USB is connected, we have 5 V on USB_VBUS, which then drives the p-FET Q1 that cuts off the battery. When USB is disconnected, the resistor R10 pulls the p-FET's gate to ground and battery current can flow. So far, so good. Now there are two problems: the first one is that FETs Q1 and Q2 can leak up to a few uA through the gate. With a pull-down of 1 MOhm, that would already raise the gate voltage by a volt or two. But that's the smaller of the two issues. The worse one is that current can flow out of the input of the MCU's voltage regulator (VREGIN). That's nothing uncommon since regulators are often designed to work mainly in one direction and not worry about the opposite direction. I have known that VREGIN behaves like that for a while, which is why there is diode D1. However, I didn't realize that this would also upset my little battery cut-off circuit. When on battery power, VREGIN would have a low resistance path to battery power, which would completely obliterate any effect of the weak pull-down R10 and try to close Q1. So the circuit would basically try to strangle itself. I'm not sure what exactly the end result would have been - it may still have worked, or perhaps only sometimes, etc., but this is certainly not what was meant to happen. Here's the corrected version: http://downloads.qi-hardware.com/people/werner/anelok/tmp/anelok-20140711.pdf I moved the tap to the other side of D1, so only a few uA can leak (*) from VREGIN to the FET. I also decreased R10 from 1 MOhm to 82 kOhm (same value as R12 so the BOM doesn't grow), to absorb any leakage of the FETs. This means that Anelok will burn 60 uA in R10 alone when on USB, but that's not much of an issue, at least not for now. (*) D1 is a Schottky diode and thus has a relatively high reverse current. I can replace it with a switching diode later, but I don't have any suitable part laying around. I also grew all the test points from a copper diameter of 60 mil (1.5 mm) to 80 mil (2.0 mm), making them just as large as those of the Y-Box. The 2 mm diameter worked will there, so there won't by any concerns about the test points being difficult to hit in Anelok. Here is the updated layout: http://downloads.qi-hardware.com/people/werner/anelok/tmp/anelok-20140712.png - Werner _______________________________________________ Qi Hardware Discussion List Mail to list (members only): [email protected] Subscribe or Unsubscribe: http://lists.en.qi-hardware.com/mailman/listinfo/discussion
