Buck converters/electronics puzzles me

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#1

Anyone know anything about buck converters/switching regulators?

I am trying to set one up as a constant current supply - but am getting highly confused. What I’ve read on the internet seems to suggest it’s possible (I don’t need super high accuracy, trying to turn 5V supply into a 1-2V, 1.4A CC source, linear regulators heat up too much) but datasheets never include info and it goes over my head. I know it’s something to do with feedback - but what exactly, I am puzzled.

For example - ST1S31 - http://docs-europe.electrocomponents.com/webdocs/1282/0900766b8128247a.pdf

Cheers!
Calum

#2

Pin 3 controls how much electric is aloud through the regulator if it is conected to the output through a voltage divider the regular will try to mantain a constant output voltage and if pin 3 is conected to a current sense resistor the regular will try to mantain a constant output current

#3

Hear is a liner one , same principles but does not have the inpenitential pass element

http://www.ecircuitcenter.com/Circuits/curr_src1/curr_src1.htm

#4

Thank you so much - seems super obvious when you mention it but I’d been stumped for ages…

Plus I found a design tool by ST for the chip which let me input values and calculated neccessary components for it - so I’ve ordered a few parts and will see how it goes…

Cheers!

P.S don’t suppose you know anything about BQ2002C (Fast charge management IC’s)?

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#5

Never used one but the datasheet and application note look pretty straightforward.

#6

PS when you make it keep it small and tight and make sure you get a good solid ground.

If you leave certain wires to long smoke might come out, switching power supplys are funny like that

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#7

Great! Turns out my issues with the bq2002c were just because I misunderstood open drain outputs - working happily now! Cheers

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#8

Having more challenges implementing a battery charge chip and a switching regulator together.

I’ve tested the BQ2002C on it’s own (with LED/transistor as an output) and it works. I’ve also tested an ST1CC40DR (which I intend to use as a current source) and that works well too. I can use the EN pin to turn on/off the current and everything looked good…

However, when I connect the INH pin of the BQ chip and the EN pins of the ST1, it doesn’t work! (By connect - I connect 5V to the INH pin of the BQ through a 10k pullup resistor, then link the INH and EN pins directly with just a wire. Furthermore, I’m pretty sure it breaks the BQ chip - as afterwards they don’t work fully and just go on/off with no trickle option. Tried it with 3 different BQ chips and they all break and don’t work afterwards. Is there something obvious I’m missing, e.g back voltage from the ST1 inductor destroying stuff/need a diode somewhere/other magical effect?

Feeling deeply puzzled - if anyone has ideas I’m all ears. Can include a schematic but it is a little messy.

Cheers all!

#9

Sounds odd try contacting them with a 10 k resistance so you can see what is going on.

#10

Please post a drawing

#11

Aye it’s very puzzling. I’ve included a drawing - Bat+ and bat- go to battery to be charged (currently using an LED simulating a batt as more visible). Realised I meant CC to EN rather than INH - it’s clearer on the schematic.

Connecting them through a 10k resistor sounds like a plan - I’ve only got one working BQ left so keen to keep it happy - I was also thinking of powering both chips from their own 5V sources until I figure the problem. Thanks very much for the help.

SCHEMATIC buck and controller.pdf (29.7 KB)

#12

Cc is a output