#avr Logs

Sep 12 2022

08:57 AM twnqx: i had analog design
09:06 AM twnqx: hate*
09:06 AM twnqx: the datsheets are all crap
09:06 AM twnqx: "oh hey, here's a reference schematic for X" "ok, none of the values match the formulas we're giving, but you don't care, right?"
09:06 AM twnqx: "oh, and you don't need extra parameters, right? like inductor saturation? because we won't give that to you"
09:11 AM twnqx: chip's 37 years old (and still made, by at least 4 companies), yet none of them have a datasheet that seems to have correct data
09:12 AM LeoNerd: Maybe nobody actually knows how to do it; the real experts have all long-since left
09:12 AM LeoNerd: So all that's left is a bunch of companies still churning out silicon based on decades-old litho drawings nobody quite understands any more
09:12 AM twnqx: all i want is to build a +12V to -12V DC-DC with through-hole components :X
09:13 AM twnqx: (and +12V to -5V as a second switcher)
09:13 AM LeoNerd: Mmm.. What's the overall principle? Flying cap? Some sort of inductive thing?
09:14 AM twnqx: MC34063 :P
09:14 AM LeoNerd: Ahyes that one
09:15 AM twnqx: and so many numbers i find make... little sense
09:16 AM twnqx: the inductor needs to be larger the less current i draw... now if i don't know the current in advance, do i need the largest value? i am already in the mH then
09:17 AM twnqx: hm i wonder if that chip is simple enough to have a full spice model
09:18 AM LeoNerd: Inductor size has to be at least enough to matter.. no real problems if it's too big
09:18 AM LeoNerd: (other than cost/size)
09:18 AM twnqx: yes but what exactly is big enough
09:18 AM twnqx: their reference circuit has 88µH
09:18 AM twnqx: my calculations based to that parameters are at least 120µH
09:19 AM twnqx: if i go down in load i end up at 1mH
09:19 AM twnqx: huh interesting, rev D datasheet has a PCB layout that doesn't look freehand any more :D
09:36 AM exp: twnqx: can you link the datasheet you're reading?
09:36 AM exp: i'm interested to see
09:39 AM twnqx: ti.com/lit/ds/symlink/mc34063a.pdf?ts=1662959357217
09:39 AM twnqx: https://www.onsemi.com/download/data-sheet/pdf/mc34063a-d.pdf
09:39 AM twnqx: https://cdn-reichelt.de/documents/datenblatt/A200/MC34063A%23utc.pdf
09:39 AM twnqx: (unisonic)
09:40 AM twnqx: https://cdn-reichelt.de/documents/datenblatt/A200/MC34063_MC33063%23MOT.pdf 2002 edition onsemi
09:40 AM twnqx: right, haven't checked and grabbed the st micro yet
09:42 AM twnqx: https://www.st.com/resource/en/datasheet/mc34063ac.pdf redrawn schematics & PCBs, same values
09:42 AM twnqx: (90µH instead of 88)
09:44 AM twnqx: hm, they mention ton(min) for the inductor
09:45 AM twnqx: that might be the reason... because at low load, obviously the on time goes down
09:45 AM exp: you also have to pick an operating frequency
09:46 AM twnqx: in the reference circuit the capacitor is 1.5nF, which comes out as ~20kHz
09:46 AM exp: that will limit the maximum inductance you can use
09:47 AM twnqx: or rather, 5V+-10% input, -12V output, 0.1A current at 20kHz happen to yield 1.5nF
09:47 AM exp: it's an interesting chip because of its age, i would like to sit down and understand it fully
09:49 AM twnqx: i am still not convinced...
09:49 AM twnqx: i mean, if i draw less load, the on-time will go down, but at the same time, the inductor will "discharge" slower
09:50 AM twnqx: so i am not sure what to choose if i want "0 to 500mA, idk" output current - calculate for peak point, even if that yields less inductance (and more output capacitance)?
10:38 AM qu1j0t3: AoE has a lot of info about this, i think it may even have some discussion of this chip
10:39 AM * qu1j0t3 has been in that section a lot lately, am doing a buck step-down with NR111D
11:52 AM twnqx: what's AoE?
11:52 AM twnqx: modern buck stepdowns are pretty easy, i thin
11:52 AM twnqx: k
11:52 AM twnqx: never had issues with any design so far
11:54 AM twnqx: weeeird a TI order that got pushed 8 month early...
12:00 PM qu1j0t3: twnqx: The Art of Electronics
12:00 PM qu1j0t3: twnqx: Sure, but you still have to select all components
12:01 PM twnqx: yeah, normally a) a reference circuit can be used to verify my calculation spreadsheet, and b) the datasheet contains all information :P
12:01 PM twnqx: selecting components once you have the parameters ain't the hard part
12:03 PM twnqx: but a parameter off "this is the inductance you need", but no information about the peak/average current makes inductor selection a tad hard
12:03 PM qu1j0t3: their buck case study is MC34063
12:04 PM qu1j0t3: twnqx: not all datasheets are complete on that. e.g. the chip i am using has almost nothing on selecting the main inductor :)
12:04 PM qu1j0t3: anyway, AoE has lots of stuff on it
12:04 PM qu1j0t3: ch 9.6
12:04 PM twnqx: but i am building an inverting regulator :P
12:08 PM twnqx: i mean, pretty easy concept in general, use output voltage as ground, regulate so you get (positive) voltage between gnd and your output
12:09 PM twnqx: obviously need to keep in mind that vcc now is input voltage + abs(output voltage)
12:10 PM twnqx: i wonder if you could modernize the circuit with an external mosfet instead of pnp or npn
12:11 PM twnqx: i guess i'll have to buy (and blow up) a few.
01:50 PM twnqx: hmpf. the ton/toff resulting from the MC34063 datasheet differs from the generic inverting buck-boost one...