Need help with a step-down regulator IC - Page 1
Forum section: electronics / beginners

author: bfrigon
author_rank: Contributor
timestamp: November 29, 2014, 04:17:10 am
content: Hi,I don't have much experience with step-down regulators. This is the first time I use one in a project.It is aADP2303ARDZ-3.3from Analog Device.Input : 6.5 VOutput : 3.3 VLoad : 280mAThere is noise on the output, about 350 mV peak to peak and it follows the switching frequency (700 kHz).I did put two 47uF (X5R type) caps on the output as suggested by the datasheet. I was not able to find the exact model of caps they recomended so i used this one :http://www.digikey.ca/product-detail/en/JMK316ABJ476MLHT/587-3399-1-ND/4157512I used a 4.7uH inductor (CLF10040T-4R7N) :http://www.digikey.ca/product-detail/en/CLF10040T-4R7N/445-9298-1-ND/3687735What could be the cause?

author: Skimask
author_rank: Super Contributor
timestamp: November 29, 2014, 04:48:54 am
content: Without a schematic, or even a picture of your completed board...meh...your fruit juice is expired?

author: bfrigon
author_rank: Contributor
timestamp: November 29, 2014, 05:16:44 am
content: I did post a schematic in the attachments (circuit.jpg).I build a test board with what you see on the schematic, nothing else. The power is supplied by my bench power supply and the load is just a bunch of leds on a breadboard.

author: Paul Price
author_rank: Super Contributor
timestamp: November 29, 2014, 08:09:23 am
content: You are missing an inductor 5uH to 330uH between the two output caps C25 C28, the bigger the uH in value, the cleaner the output, and using a large value inductor is better in most cases. if possible make the grounding point connections of all components having a ground very close to each other, and all grounds on one big fat thick PCB trace area. The output ground connection should lead to the output point from the right side of this central grounding point. Using a 22uH inductor should lower the output switching noise to <20mV P-P.

author: Zero999
author_rank: Super Contributor
timestamp: November 29, 2014, 12:03:44 pm
content: What sore of capacitors are you useing for C25 and C28? If the answer is ceramic, then their capacitance will be much lower than stated with over half the voltage rating applied.

author: bfrigon
author_rank: Contributor
timestamp: November 29, 2014, 01:07:52 pm
content: Quote from: Hero999 on November 29, 2014, 12:03:44 pmThey are ceramic.in the datasheet, they recommend using X7R or X5R. What should I use? i'm limitted on board space, it needs to be a 1206 package.Quote from: Paul Price on November 29, 2014, 08:09:23 amI replaced the 4.7uH inductor with a 10uH, there is already an improvement. The noise is now at 125mV p-p.I don't have the space on the board for a second inductor. Considering that, would a single 22uH still be sufficient to maintain the noise within 20mV ?

author: bfrigon
author_rank: Contributor
timestamp: November 29, 2014, 01:29:36 pm
content: Quote from: AcHmed99 on November 29, 2014, 12:59:49 pmThanks, I will try the tant caps.By the way, I tested it with a 280 mA load for now. But i will eventually need at least 1.5A with 1A reserved for future addons.

author: bfrigon
author_rank: Contributor
timestamp: November 29, 2014, 01:38:37 pm
content: Quote from: AcHmed99 on November 29, 2014, 01:23:56 pmWhere did you find this calculator ?

author: rob77
author_rank: Super Contributor
timestamp: November 29, 2014, 01:52:43 pm
content: http://schmidt-walter.eit.h-da.de/smps_e/smps_e.html(googled it using phrases from the images posted by AcHmed99)

author: Mad ID
author_rank: Regular Contributor
timestamp: November 29, 2014, 04:05:01 pm
content: It looks like you are not measuring correctly, i.e. this looks as magnetic coupling to your probe.Did you use a short ground lead like the one in the picture? Without it the noise will be 10-100x bigger than it is..

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 29, 2014, 05:19:12 pm
content: Quote from: Mad ID on November 29, 2014, 04:05:01 pmSeconded. Either your layout, or your probing technique, or both, is bad. This device and type of circuit requires large, stitched, ground pours for thermal management and carrying ground currents.Regarding damping (R+C across the diode): contrary to "conventional rule of thumb", you can actually intentionally increase trace inductance around the diode, which will shift the ringing to a lower frequency and higher impedance, where it is easier to snub or dampen.Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 30, 2014, 02:02:17 am
content: Quote from: AcHmed99 on November 29, 2014, 07:42:29 pmMy classic example:- I designed a 5kW high frequency inverter module. Multilayer board, "minimized inductance". It was built with ISOTOP / SOT-227 power modules, and I estimate it had around 15nH loop inductance. For something about as wide across as your hand, that's not bad at all.- It rang like a motherfucker, 80% overshoot. Now, when you're starting with 650V DC link... that's a bit of a problem. We had to buy 1200V transistors just to do initial testing.- Ok, so, it's multilayer, we've got connections to the opposite supply rail nearby. Let's try putting some clamp diodes in there.8A diodes: toasted.12A diodes: toasted.30A diodes: survived. But the overshoot is still over 10%, and even being generous with stray inductance on the diodes and connections, that's over 40V applied across the die itself. In just a few nanoseconds.By the way, the diodes failed mysteriously, barely warm at all. I suspect electromigration, but I really haven't heard anything concrete about high current failure modes. (Hot-spotting is unlikely because forward drop is resistive and PTC at those currents. Maybe there's something spooky like nonuniform forward recovery -- who knows, manufacturers are very quiet indeed about this phenomenon.)- Later rev, I explicitly added about 100nH of stray to the layout.Instead of ringing at 60MHz, peaking over 100A, and pounding the ever-loving shit out of the clamp diodes, this brought it down to a manageable 10MHz, the worst case overshoot to 10%, and the peak current down to 20A.What's more, a proper RCD clamp snubber handled the energy cleanly, dissipating it (of which there is slightly more at high loads, due to the E = 0.5 L I^2 stored in the supply inductance) dissipated safely in a moderately sized resistor (there was under 400W total dissipation under worst case operation -- always over 90% efficiency for a 5kW high frequency module). At full rated output, the snubber power was a small fraction of losses, so this is a perfectly reasonable sacrifice.- The moral of the story is, it's not even a sacrifice. Your inverter necessarily has junction capacitance and loop inductance. The only reason you're suggested to minimize inductance is because, if and only if you can get the resonant frequency higher than the switching speed, or the impedance sqrt(Lstray/Cj) low enough, then it won't be stimulated by the transients. In reality, especially with switching speeds so fast now, that this is neither possible nor reasonable, one must match the impedance to the circuit (roughly, switch Vpk / Ipk = Zo), and assign a reasonable frequency for the loop (usually, f_o > 20*f_sw is reasonable without incurring too much losses, while still being reasonable to deal with).Tim