#qi-hardware IRC log for Thursday, 2015-06-04

rjeffries$2M USD raised against $50K USD goal. C.H.I.P. will happen. (Uses lower end Allwinner ARM based SoC)16:10
ysionneausuper cool, yet another closed source cpu based hardware o/16:15
pcercueicomposite video output? Get real16:19
wpwrakhehe. great reactions ;-))16:24
wpwraki wonder if this simple adjustable regulator could actually work: https://neo900.org/stuff/werner/tmp/cmpreg.pdf16:27
wpwrakthe 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 again16:28
wpwrakso far the theory. the question is whether this would actually work or whether i'd get mad oscillations, etc.16:28
wpwrakovershoot 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 capacitor16:30
wpwrakso 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 mV16:35
wpwrakwhat 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 currents16:39
whitequarkwpwrak: oh well, I can say what would be wrong in my view17:56
whitequark(though it's pretty much EE 101 so I would be surprised if you haven't considered that still?)17:56
whitequarkyour "equivalent circuit" for the FET relies on an invalid assumption. that is, that the turn-on/turn-off time of the FET is insignificant17:57
whitequarkyou 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 MCU17:58
whitequarkwhich will limit the charge/discharge rate of the gate.17:58
whitequarkthus, you will operate the FET in the region where it's least efficient pretty much all the time17:59
whitequarkin the best case you will get, well, a linear regulator, not entirely unlike lm31718:00
whitequarkwpwrak: https://cds.linear.com/image/7413.png18:00
whitequarklooks familiar, isn't it?18:00
whitequarkwpwrak: oh, for that matter, tl431 is pretty much that too, except also with a BJT18:09
whitequarkI'm guessing a p-FET is too inefficient in this case, and an n-FET would require a dedicated gate drive18:09
whitequarkwpwrak: http://www.ti.com/lit/ds/symlink/lp2975.pdf does exactly what you describe18:12
whitequarkthey seem to have needed a more complicated control circuit for ripple rejection18:13
whitequarkah, no, nope, the second opamp is for overcurrent protection18:14
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 td18:24
whitequarkbut that's not about FET delays.18:25
wpwrak(resistor) FETs don't need that, only BJT ;-)18:25
whitequarkbut of course they do18:25
wpwraknot unless ~1 uA is a problem :)18:26
whitequarkI am not talking about steady state18:26
wpwrakah, i see. naw, even then it should be fine. you get a few MHz, tops. that should be pretty negligible.18:27
whitequarkthe first is the turn-on transient18:27
whitequarkwhich /can/ kill an MCU's output cascade18:27
whitequarklike, i have myself killed a few MCU pins before i figured that out18:27
whitequarkit's pretty reliable.18:27
whitequarkyou need at least an ohm there or something.18:27
whitequarkthe second is the average dissipated power at your "few MHz", which is actually a ton18:28
whitequarklet's calculate18:28
wpwrake.g., the DMG1013T has a "total gate charge" of 580 pC18:29
whitequarkand its threshold voltage?18:30
wpwrak[0.5, 1] V18:30
whitequark900m© at -2.5V18:30
whitequarkor well18:30
whitequarkreally, what your MCU runs on?18:30
whitequark3.3 probably18:30
wpwrakyes, 3.3 V18:31
wpwrak(LP2975) neat ! :)18:32
wpwraknegligible :)18:33
whitequarkseems you're right18:33
wpwrakmaybe you get this sort of trouble with larger FETs18:33
wpwrakthis one is pretty much as small as it gets18:33
wpwrak(for that kind of form factor)18:34
whitequarkyeah, that's where my guess was coming from18:34
whitequarkI still think you won't be able to get it to switch that fast and it will work as a linear reg18:34
whitequarkbut that's just another guess18:35
whitequarktry it18:35
whitequarkI wonder what the effective resistance of the MCU pin driver is?18:35
wpwrak200 Ohm18:36
whitequark8 MHz18:38
whitequarkI dunno.18:38
whitequarknow I'm interested.18:38
whitequarkbuild it already18:39
wpwraki guess something like 10 uF may even be enough there18:41
wpwraklet's be generous, 22 uF18:44
eintopfdid 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? :-/20:14
eintopfmakes no sense for me20:15
wpwrakcould be for identifying board variants, with different boards having different such connections20:15
wpwrakwhitequark: 1 V into 100 Ohm: http://downloads.qi-hardware.com/people/werner/cmpreg/static/23:03
wpwrak(fast mode, minimum hysteresis)23:04
wpwrakthe over/undershoot look more like measurement effects than anything else. lemme check if the probe compensation is still good ...23:06
wpwrakhmm. that was not a good idea. now it's much worse :-(23:22
wpwrakpro tip: the DMG1013T acts erratically after being shorted and emitting smoke23:33
--- Fri Jun 5 201500:00

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