MC34063 Help - Page 1
Forum section: electronics / projects

author: tbj
author_rank: Contributor
timestamp: April 19, 2013, 08:34:41 am
content: Hi,I built a buck converter using the MC34063. I calculated the component values usingthis site, and here is the schematic and values I ended up with:The purpose of the circuit is to drive a high-power white LED with a 3.5v forward voltage - specifically,this LED- from a 9 volt alkaline battery.Here's the problem. When unloaded, the converter outputs a steady 3.5 volts, which is exactly what I expected. When I connect the LED, the voltage drops to about 2.7 volts, meaning the LED only draws about 70mA of its rated 350.What is causing the voltage to sag to this extent?Also, the efficiency seems to be poor - to get 70mA out I am measuring about the same on the input - surely it should be less? I know it won't be proportionally less due to efficiency losses, but still.Can anyone help?

author: BravoV
author_rank: Super Contributor
timestamp: April 19, 2013, 09:02:07 am
content: Suggestion, take a photo of your working circuit and post it here, it will definitely tell lots of stories and believe many members here will able to help you to sort it out.Also components selection that you put in there also matter, like the inductor type.Btw, that circuit is constant voltage, its not suitable for driving led which needs constant current.

author: c4757p
author_rank: Super Contributor
timestamp: April 19, 2013, 11:40:23 am
content: I am assuming you don't have a 0.429 Ohm resistor. What did you use for Rsc?

author: tbj
author_rank: Contributor
timestamp: April 19, 2013, 12:11:38 pm
content: Hi,I used an 0.47 Ohm for Rsc. I also have some 0.1 ohm resistors and tried substituting one in, but still voltage drop at the output.Inductor is a 390uH.Here is a pic of the circuit:I wanted a constant voltage source at 3.5 volts which I am then going to PWM to adjust LED brightness.

author: c4757p
author_rank: Super Contributor
timestamp: April 19, 2013, 12:42:37 pm
content: A few thoughts, looking at that picture:- Measure the resistance between pins 6 and 7 - $5 says it's a good bit more than 0.47. You may as well have more than that in contact resistance on that breadboard. That resistance is what sets the maximum output current. Inductance (long leads) can add to the effect as well. I have literally soldered SMD resistors directly onto the pins for breadboard prototyping, and it does make a difference.- Your layout needs to be a lot tighter. With the di/dt of a switcher, a couple inches between parts is absolutely huge. That Vcc capacitor needs to be right between the Vcc and ground pins of the IC. This will probably improve the efficiency as well.- As for efficiency - it tends to be bad at very low currents, so don't expect much.

author: kaindub
author_rank: Regular Contributor
timestamp: April 19, 2013, 01:11:51 pm
content: I would check whether your batery has the capacity to suply the amps, particularly the peak amps.9V batteries are really for low current applications - tens of milliampsRobert

author: c4757p
author_rank: Super Contributor
timestamp: April 19, 2013, 01:14:14 pm
content: Peak doesn't matter if you've got a big capacitor across it.

author: Unknown
author_rank: Unknown
timestamp: April 19, 2013, 09:44:35 pm
content: That LED forward current, wants 340mA and the LED will zener as in diode, before it's max rating... the PP3 battery is not going to provide enough current for long, especially if it's not Alkaline, even so your going to require 4000uf across the battery, otherwise use a pack of AA batteries for 6 volts, if no PSU is available !Place the battery across the IC V in to Gnd with the 4000uf, double-up your Gnd links from IC to breadboard too...

author: kxenos
author_rank: Frequent Contributor
timestamp: April 19, 2013, 10:17:53 pm
content: The first thing I would try is to set the output voltage a little higher, say 5V. If you use Rsc to limit the current then it will limit the current by dropping the output voltage so you will get your ~350mA. The other way I would try is to regulate the current instead of the voltage by means of pin 5: According to the datasheet Vout=1,25(1+R2/R1) and by rearranging you have Vout*(R1/(R1+R2) =1,25. That means that the converter regulates the output so that the voltage in pin 5 is 1,25V. That means that if you add a 0R437 from led to ground and take the node between led and resistor into pin 5 your're going to have about 1,25V in that pin when the current through the led will be ~350mA. In this situation you should set Rsc to something for bigger than 250mA output but smaller of the max. current that the IC can handle to protect it. Just my 2c.

author: Big_Al
author_rank: Contributor
timestamp: April 20, 2013, 11:31:14 pm
content: Make sure that wirewound resistor aint got too much inductance too.

author: free_electron
author_rank: Super Contributor
timestamp: April 21, 2013, 01:27:03 am
content: waay too much contct resistance in a breadboard, that inductor is probably not suited for switching apps and a 9 volts cell will collapes under a 130mA load ... ( 3.5 volt , 350 ma out is 9 volta 120 ma in + the loss in the convertor

author: nctnico
author_rank: Super Contributor
timestamp: April 21, 2013, 01:59:45 am
content: I agree. You can't breadboard switching power supplies. You need to make a PCB.

author: c4757p
author_rank: Super Contributor
timestamp: April 21, 2013, 02:02:30 am
content: Quote from: nctnico on April 21, 2013, 01:59:45 amWith something as simple as an MC34063 buck converter, you often can. I've done it many times. You just have to be very careful how you arrange the components.

author: Mr Smiley
author_rank: Frequent Contributor
timestamp: April 21, 2013, 02:26:15 am
content: Hi,Replace R2 with your led, I.e put your led between pin 5 and Vout.You calculate R1 so that at your required current of 350mA it drops 1.25v.So R1=1.25/.35 = 3.57 or approx 3.5ohmsIf you want to check if your inductor is saturating, remove Co and scope across R1 and look for a kink in the ramp.I'd up your output current to 400mA.If your not getting 1.25v at pin 5 check the drop across Rsc. If even slightly to high it can current limit and hide what's going wrong.Mr Smiley

author: Jeff1946
author_rank: Regular Contributor
timestamp: April 21, 2013, 02:37:37 am
content: You need a low resistance inductor. More importantly it is better to operate an LED with a constant current source. A new LED may draw more current at the voltage you are want. Maybe start off by changing the voltage to 3V which should be safe for the led and see how the converter works.

author: Rick Law
author_rank: Super Contributor
timestamp: April 21, 2013, 03:06:14 am
content: Hey TBJ,You are in luck - to get back into EE, I happen to be experimenting with this exact chip for the past few weeks. (I made a buck then a boost - success with both on second try last week) So I have all the parts some already connected and can tell you what to expect:Quote from: kaindub on April 19, 2013, 01:11:51 pmUsing 0.47ohm as you doOpen (no connection) @ 3.5V (your LED spec, 3.5V 350mA)Here are the numbers:@200mA output, output dropped to 3.45V (9V supply @115mA)@300mA output, output dropped to 3.42V (9V supply @170mA)@350mA output, output dropped to 3.38V (9V supply @191mA) *target 350mA@400mA output, output dropped to 3.34V (9V supply @211mA)@450mA output, output dropped to 2.81V (9V supply @208mA)From this point on, output voltage dropped like a ROCK, I pushed it to 450mA to show you, this guy drops like a rock once you draw too much.Robert is definitely right. Your 9V battery is not making it - it takes 191mA for your target. Your output will drop like a rock much sooner.Quote from: c4757p on April 19, 2013, 11:40:23 amNow, when c4757p talks, I listen. I trust his judgement since he helped me out before. So, I use a different Rsc here to show the point he made - I ran another .29ohm in parallel (along with connection clip resistance, figure about .35ohm in parallel with the 0.47ohm resulting about .21ohms.)@200mA output, output dropped to 3.33V (9V supply @122mA)@300mA output, output dropped to 3.24V (9V supply @175mA)@350mA output, output dropped to 3.27V (9V supply @200mA) *target@400mA output, output dropped to 3.23V (9V supply @222mA)@450mA output, output dropped to 3.25V (9V supply @253mA)@500mA output, output dropped to 3.21V (9V supply @274mA)@600mA output, output dropped to 3.14V (9V supply @315mA)@700mA output, output dropped to 3.08V (9V supply @370mA) *x2 LED@900mA output, output dropped to 3.02V (9V supply @477mA)Beyond this point, @920mA, output voltage drops like a rock.You can see that with a smaller Rsc here (more current), output voltage is more stable at a larger band. Since you used a larger Rsc instead of smaller, your voltage will collapse a lot sooner than the 0.47ohm (data group 1) and certainly a lot worst then 0.2xohm (data group 2). The voltage is not always a steady drop as you draw more current. See it perking up a bit @350mA. This matches with other experiments I have done. At certain point, the voltage perk up a bit then drops. I am not sure why (perhaps best matching).I set output to 3.5V (open). If you set output to 3.7V, 3.8V-ish and you will be around the 3.5V target for a large range of current draw.Let me add the following observation also:1. Your inductor is high - I use a 100uH on one, and 200uH on another. The appnotes suggest 170uH for the boost and 220uH for the buck. You are using over 300uH. My experiment shown mismatched inductor will cause voltage collapse sooner. I tried a whole bunch of unmarked inductors last week, and observed that the stability band is affected by inductance quite a bit. Unfortunately, the inductors I played with were unmarked and I don't have an LCR to check how bit it is.2. The aluminum electrolytic caps gave me the best result (I think). Until I found the misplaced caps, this is from recollection I cannot reconfirm.3. Yeah, very sensitive to wire length. No kidding here. With the second group results, the parallel resistor is connected via a 9-inch-wire clip and one end soldered. There is a lot of voltage jitter (60mV change). When I first connected the parallel resistor with both ends using clips, the voltage was dancing so much (200mV-300mV) it was practically useless.4. I found this chip to be very current sensitive. So I put in LOWER Rsc expecting higher current limit to give me a wider band of stability as load change. That works for me. However, the "precision reference" is only 2%. I do find a lot of fluctuation up to 5% mark because line-regulation is not so great either.5. Lastly, do you have some of those car charger for your old phone? You may find exactly the part you need in there. I disassembled a bunch of them last year (needed the car lighter adaptor plug only, but kept some of the parts). Of the 5 I disassembled and they all used this chip. That is why I started experimenting with this chip.Hope this helpsRick

author: c4757p
author_rank: Super Contributor
timestamp: April 21, 2013, 03:36:53 am
content: Quote from: Rick Law on April 21, 2013, 03:06:14 amI'm somewhere between flattered and baffled.There are people on here much more worthy of your trust

author: c4757p
author_rank: Super Contributor
timestamp: April 21, 2013, 03:40:59 am
content: Quote from: Rick Law on April 21, 2013, 03:06:14 amMC34063 should be able to cope with significantly high inductances. I just used a 2 mH inductor in a buck converter calling for 140uH (forgot how damn high the permeability of my cores was and didn't feel like removing turns) and it works fine! If your inductors were unmarked I'd be more suspicious of saturation and series resistance.

author: Rick Law
author_rank: Super Contributor
timestamp: April 21, 2013, 05:38:29 am
content: Quote from: c4757p on April 21, 2013, 03:36:53 am(not to tangent too far off and hijack the thread as I digress...)You helped me decode how a constant-current board works. I traced the PCB and couldn’t find any feedback mechanism to control current. I thought may be I misunderstood how something works. I suspected a zigzag on the PCB having something to do with something, I just was not sure initially. Your comment about the OpAmp layout being crazy pointed me to retrace that part. I found another trace that uses the zigzag as a current sensing resistor feeding back into the OpAmp.That was a failed board at the time. I did the trace to try to understand how things worked just to learn. Once I understood that last part, I knew the suicidal 78L05’s death is not critical. I used a couple of white LED’s as voltage regulator and actually got the CC circuit working for a (more) limited range of currents. The seller replaced that board free of charge.Good or bad, we all earn our reputation one way or another. You helped me learned something and for that I am appreciative. I have learned from your other comments in other posts as well. You have a good reputation in my book.

author: c4757p
author_rank: Super Contributor
timestamp: April 21, 2013, 05:47:40 am
content: Ah, I remember that! And I was just joking!Thank you.

author: Rick Law
author_rank: Super Contributor
timestamp: April 22, 2013, 03:01:28 am
content: Quote from: Rick Law on April 21, 2013, 03:06:14 amTBJ,(Editwhat I wrote a moment ago - the part I wrote as 9.8V "First, I forgot I say I was using 9.8V..." It was about something else. Pasted here by mistake. I was writing a note about trying different things, I want to simulate a near full battery and the text pasted here by mistake.) I removed the word "second" in the following paragraph since the "first" doesn't belong here.end-editI redid the second (changed Rsc) part with a capacitor change also. I replaced my 47uF (50V) with a 2200uF (16V). Along with the change in Rsc (to about .22ohm), I was able to get very stable 3.5V.I use a load of 20mA and set the voltage to 3.50V. At your target of 350mA, I was still at 3.49V. I increased the load all the way up from 20mA to 820mA and the voltage dropped only to 3.47V to 3.48V. This is excellent stablity. And, removing the load - no-load voltage is also exactly 3.50V.So, change the Rsc and the output cap; you would likely see much better results.Rick