MC34063A running at higher frequency than one set by timing capacitor - Page 1
Forum section: electronics / projects

author: AndrejaKo
author_rank: Frequent Contributor
timestamp: February 03, 2013, 07:00:03 pm
content: Hi guys!I'm making a boot converter using my junk-box components which will power a number of LEDs connected in series. For this I'm using MC34063A.I'm trying to keep this as simple as possible, so I'm relying on the current sense resistor to push the regulator into current limiting and avoid using series resistor on the LEDs (is this OK?).Here's the "schematic":My problem is that for some reason the switching frequency of the regulator is above its maximum value although I picked a suitable timing capacitor value.Here's a scope shot of the problem. Red channel one is the timing capacitor and yellow channel two is the output voltage.EDIT: Whoops! My ground clip made those huge peaks in the previous image. Here's a better one with a short ground lead.According to my calculations, using 440 pF capacitor (actually two 220 pF in parallel), I should get Ton of 11 ?s and 75 kHz frequency, but the whole cycle lasts for around 6.77 ?s. I've tried with several ceramic disk capacitors and several ceramic multilayer capacitors so the issue doesn't seem to be bad markings on capacitors or awful tolerances.This is on a breadboard, so I expect that its parasitic capacitances will make the equivalent capacitance of timing capacitors even larger.Any ideas why would this be happening?

author: Bored@Work
author_rank: Super Contributor
timestamp: February 03, 2013, 07:15:32 pm
content: From memory (read the datasheet) the MC34063 cuts a cycle short if the current limiting kicks in. This is done by faster charging (or was it discharging?) the timing cap. If your current limit always kicks in this might explain the higher frequency.

author: AndrejaKo
author_rank: Frequent Contributor
timestamp: February 03, 2013, 07:17:14 pm
content: Yes, I read about that too. Is that acceptable? I mean, can I expect damage to the chip from running at frequency higher than expected, even if the chip itself is increasing the frequency?

author: CarlG
author_rank: Regular Contributor
timestamp: February 03, 2013, 09:37:45 pm
content: A silly question perhaps, but are you looking in the device manufacturer's datasheet? Timing for TI and ONsemi devices are quite different. ST's diagram is hard to judge at 440 pF. Fairchild's datasheet doesn't give a clue about timing.I don't think the higher frequency will do any harm in itself, but if any max ratings are exceeded due to that, it might burn.I note that, assuming that R2 should read 370k, you've set the output voltage higher than max rating of the device. But I guess you expect the LEDs to limit the output voltage.In general, schottky diodes have high reverse leakage current. At 35V (as your schematic indicates, but beware that Vf of the LED:S are not that precise) reverse bias, the 1N5819 might get overheated even though it's rated at 40V.

author: AndrejaKo
author_rank: Frequent Contributor
timestamp: February 03, 2013, 09:50:03 pm
content: R2 is set to 370 ohms and output voltage is set at 35 V (as I indicated on the image), which is below the max output of 40 V, if I'm reading the datasheet correctly. I'm using the ON Semi part and ON Semi datasheet. I really couldn't find anything mentioned in it, the AN920 application note or the Excel worksheet about frequency changing other than due to use of timing capacitor.I'm aware of problems with Vf for LEDs. Unfortunately, the ones I have are datasheetless, but should have maximum current of 35 mA and Vf of 3.6 V. Right not, I get around 29 V out of the regulator and current of around 25 mA, which looks safe enough to me. I think I could mess with the current sense resistor to get a bit higher current, but what I have right now is good enough for my purpose, if it's going to be stable.Good point for 1N5819. I didn't even think about reverse leakage when I picked it. Right now it's not even warm, so I guess it should be OK. Still a good point to research.

author: CarlG
author_rank: Regular Contributor
timestamp: February 04, 2013, 09:01:47 am
content: Quote from: CarlG on February 03, 2013, 09:37:45 pmSorry about my late-night, quick&dirty answerI withdraw my statement regarding different specs. The TI and Onsemi equations are the same; I was only looking in the Ton/Toff diagrams. However, while the equations are the same, looking e.g at Ton for 1nF, TI diagram gives 20us while Onsemi gives 40us. I don't know what conclusion to draw from that. But maybe it's a non-issue, Bored@work's answer is obviously correct. The shape of CTvoltage shows that the current limit mode sets in as shown in AN920 fig 6.

author: hlavac
author_rank: Frequent Contributor
timestamp: February 10, 2013, 06:15:13 pm
content: Looks to me you need a bigger inductor so that the overcurrent fast charge occurs right before the end of the cycle, not half way thru.Or a smaller timing cap but I guess the frequency is already too high...

author: AndrejaKo
author_rank: Frequent Contributor
timestamp: February 10, 2013, 06:26:42 pm
content: Any ideas how big inductor should I get? SHould I look for say values near the current like 1.5 mH or 2.2 mH or go way up and get 10 mH or 22 mH? Also ideas how to get the exact numerical value would be helpful.I tried with 330 pF capacitor, which brings me to the datasheet maximum of 100 kHz, but it didn't help.Also I won't be able to provide meaningful feedback for a while. My only scope is being repaired at the moment and the manufacturer has a collective vacation until 17th Feb.

author: Mr Smiley
author_rank: Frequent Contributor
timestamp: February 11, 2013, 12:28:08 am
content: Hi,When i've used this chip to run strings of leds, correct me if i've been doing it wrong all these years, but R1 should be your current sence resistor, in this case you say you want 35mA which would make it 1.25/35mA = 35.7 ohms, R2 is a short. So you connect the top or your led string to the Cathode or your switching diode, the bottom or you led string goes to the top or your, lets say 36 ohm resistor, that point goes back into pin5 and the bottom of the 36 ohm goes to ground. And your cap goes from the top of your sting to ground.If you want to see if your inductor is saturating, remove C0 and look at pin 1 when you get your scope back.1mH does seem a tad large for 35mA, are you sure your calculations are correct. It's normally around 100uHAlso, the equations give values for discontinuous mode (it's not clear ), with your selected Vin min you will get continuous mode, and the frequence does change depending on the input voltage and output load. I've seen a data sheet somewhere that says if the scope values don't represent calculated values, increase the load or drop the input voltage to change the operating mode of the chip,Also, be very careful with Rsc, If it's just a midges to large, it will current limit.When using R1 and R2 you are not in current sense mode. As in Dave's video on this chip when doing load testing to find the efficiency, you can take out more or less of the calculated current. Those values determine a mid/optimized fixed output voltage point, NOT a fixed currentMr Smiley

author: AndrejaKo
author_rank: Frequent Contributor
timestamp: February 11, 2013, 01:26:24 am
content: I wasn't planning to be in the current sensing mode according to the way you describe it. My idea was to be in current limiting instead. To tell you the truth, it didn't even occur to me to only use R1 for current sensing. It looks like a god idea which I'll definitely try out as soon as I can. I do understand that my current choice of Rsc would not be optimal in that case, since Rsc should simply save me from any breakdowns at the R1 side of things.I got the inductor value from Excel worksheet which ON Semi provides. I did try to do some calculations and see how the worksheet number matches to datasheet formulas, but I'm getting results in couple of henries range. I'm probably too tired right now to do calculation properly.

author: Mr Smiley
author_rank: Frequent Contributor
timestamp: February 11, 2013, 03:22:28 am
content: Hi,There are plenty of on line calculators out there, just two as an examplehttp://www.nomad.ee/micros/mc34063a/index.shtmlhttp://dics.voicecontrol.ro/tutorials/mc34063/I hope people here agree, for long led strings go for constant current, the number of leds in the string is only limited by Vout max of the ic.I used two 34063's for a UV exposure box, one string of x10 350mA UV leds per deviceMeasure the voltage at the feedback pin, it it's lower than 1.25v then your current limiting with Rsc.If it's around 1.25v then your getting your full design current.Mr Smiley