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2015-06-04 16:10 <rjeffries> $2M USD raised against $50K USD goal. C.H.I.P. will happen. (Uses lower end Allwinner ARM based SoC)
2015-06-04 16:10 <rjeffries> https://www.kickstarter.com/projects/1598272670/chip-the-worlds-first-9-computer/
2015-06-04 16:15 <ysionneau> super cool, yet another closed source cpu based hardware o/
2015-06-04 16:19 <pcercuei> composite video output? Get real
2015-06-04 16:24 <wpwrak> hehe. great reactions ;-))
2015-06-04 16:27 <wpwrak> i wonder if this simple adjustable regulator could actually work: https://neo900.org/stuff/werner/tmp/cmpreg.pdf
2015-06-04 16:28 <wpwrak> the theory is that the comparator opens the p-FET until Vout ~ Vin (~ = approximately equal). then it closes. when the load discharges the silo capacitor, Vout drops and the comparator opens again
2015-06-04 16:28 <wpwrak> so far the theory. the question is whether this would actually work or whether i'd get mad oscillations, etc.
2015-06-04 16:30 <wpwrak> overshoot should be around td * Imax / C where td = propagation delay of the comparator, Imax = maximum current through the FET, and C = the capacitance of the capacitor
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2015-06-04 16:35 <wpwrak> so even if we assume an extreme worst case of Vout = 0, with Ron = 1 Ohm, C = 100 uF, and td = 1 us, that would be 1 us * 3.3 A / 100 uF = 33 mV
2015-06-04 16:39 <wpwrak> what puzzles me is that nobody seems to use this sort of regulator circuit. seems pretty ideal if you need a programmable reg, already have DAC and comparator (common in many MCUs), and don't need huge currents
2015-06-04 17:56 <whitequark> wpwrak: oh well, I can say what would be wrong in my view
2015-06-04 17:56 <whitequark> (though it's pretty much EE 101 so I would be surprised if you haven't considered that still?)
2015-06-04 17:57 <whitequark> your "equivalent circuit" for the FET relies on an invalid assumption. that is, that the turn-on/turn-off time of the FET is insignificant
2015-06-04 17:58 <whitequark> you miss a resistor right after the comparator, which will be required to bring the current through the pin in line to absolute maximimum ratings of the MCU
2015-06-04 17:58 <whitequark> which will limit the charge/discharge rate of the gate.
2015-06-04 17:59 <whitequark> thus, you will operate the FET in the region where it's least efficient pretty much all the time
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2015-06-04 18:00 <whitequark> in the best case you will get, well, a linear regulator, not entirely unlike lm317
2015-06-04 18:00 <whitequark> wpwrak: https://cds.linear.com/image/7413.png
2015-06-04 18:00 <whitequark> looks familiar, isn't it?
2015-06-04 18:09 <whitequark> wpwrak: oh, for that matter, tl431 is pretty much that too, except also with a BJT
2015-06-04 18:09 <whitequark> I'm guessing a p-FET is too inefficient in this case, and an n-FET would require a dedicated gate drive
2015-06-04 18:12 <whitequark> wpwrak: http://www.ti.com/lit/ds/symlink/lp2975.pdf does exactly what you describe
2015-06-04 18:13 <whitequark> they seem to have needed a more complicated control circuit for ripple rejection
2015-06-04 18:14 <whitequark> ah, no, nope, the second opamp is for overcurrent protection
2015-06-04 18:24 <wpwrak> (turn on/off time) naw, it should only not be excessively slow. the comparator also has delays in the tens if not hundreds of nanoseconds. so you can just add any FET delay to the comparator's td
2015-06-04 18:25 <whitequark> but that's not about FET delays.
2015-06-04 18:25 <wpwrak> (resistor) FETs don't need that, only BJT ;-)
2015-06-04 18:25 <whitequark> but of course they do
2015-06-04 18:26 <wpwrak> not unless ~1 uA is a problem :)
2015-06-04 18:26 <whitequark> I am not talking about steady state
2015-06-04 18:27 <wpwrak> ah, i see. naw, even then it should be fine. you get a few MHz, tops. that should be pretty negligible.
2015-06-04 18:27 <whitequark> the first is the turn-on transient
2015-06-04 18:27 <whitequark> which /can/ kill an MCU's output cascade
2015-06-04 18:27 <whitequark> like, i have myself killed a few MCU pins before i figured that out
2015-06-04 18:27 <whitequark> it's pretty reliable.
2015-06-04 18:27 <whitequark> you need at least an ohm there or something.
2015-06-04 18:28 <whitequark> the second is the average dissipated power at your "few MHz", which is actually a ton
2015-06-04 18:28 <whitequark> let's calculate
2015-06-04 18:29 <wpwrak> e.g., the DMG1013T has a "total gate charge" of 580 pC
2015-06-04 18:30 <whitequark> and its threshold voltage?
2015-06-04 18:30 <wpwrak> [0.5, 1] V
2015-06-04 18:30 <whitequark> 900mΩ at -2.5V
2015-06-04 18:30 <whitequark> or well
2015-06-04 18:30 <wpwrak> yes
2015-06-04 18:30 <whitequark> really, what your MCU runs on?
2015-06-04 18:30 <whitequark> 3.3 probably
2015-06-04 18:31 <wpwrak> yes, 3.3 V
2015-06-04 18:32 <wpwrak> (LP2975) neat ! :)
2015-06-04 18:32 <whitequark> hrm
2015-06-04 18:33 <whitequark> 0.58mA
2015-06-04 18:33 <wpwrak> negligible :)
2015-06-04 18:33 <whitequark> seems you're right
2015-06-04 18:33 <wpwrak> maybe you get this sort of trouble with larger FETs
2015-06-04 18:33 <wpwrak> this one is pretty much as small as it gets
2015-06-04 18:34 <wpwrak> (for that kind of form factor)
2015-06-04 18:34 <whitequark> yeah, that's where my guess was coming from
2015-06-04 18:34 <whitequark> I still think you won't be able to get it to switch that fast and it will work as a linear reg
2015-06-04 18:35 <whitequark> but that's just another guess
2015-06-04 18:35 <whitequark> try it
2015-06-04 18:35 <whitequark> I wonder what the effective resistance of the MCU pin driver is?
2015-06-04 18:36 <wpwrak> 200 Ohm
2015-06-04 18:38 <whitequark> 8 MHz
2015-06-04 18:38 <whitequark> I dunno.
2015-06-04 18:38 <whitequark> now I'm interested.
2015-06-04 18:39 <whitequark> build it already
2015-06-04 18:40 <wpwrak> ;-))
2015-06-04 18:41 <wpwrak> i guess something like 10 uF may even be enough there
2015-06-04 18:44 <wpwrak> let's be generous, 22 uF
2015-06-04 19:24 <eintopf> hi
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2015-06-04 20:14 <eintopf> did anybody ever see that two GPIO pins are connected together, in firmware one as output the other as input. And then they making some handshake algorithmn to confirm that the firmware running on the right board? :-/
2015-06-04 20:15 <eintopf> makes no sense for me
2015-06-04 20:15 <wpwrak> could be for identifying board variants, with different boards having different such connections
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2015-06-04 21:32 <eintopf> mhh
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2015-06-04 23:03 <wpwrak> whitequark: 1 V into 100 Ohm: http://downloads.qi-hardware.com/people/werner/cmpreg/static/
2015-06-04 23:04 <wpwrak> (fast mode, minimum hysteresis)
2015-06-04 23:06 <wpwrak> the over/undershoot look more like measurement effects than anything else. lemme check if the probe compensation is still good ...
2015-06-04 23:22 <wpwrak> hmm. that was not a good idea. now it's much worse :-(
2015-06-04 23:33 <wpwrak> pro tip: the DMG1013T acts erratically after being shorted and emitting smoke