LM2621 SEPIC 3.3V Regulator fail - Page 1
Forum section: electronics / beginners

author: tmk854
author_rank: Newbie
timestamp: May 13, 2016, 12:37:17 pm
content: I am going over my head in terms of knowledge and experience and I am trying to build an LM2621 voltage regulator. It’s not working, so I am interested in feedback as I am probably making some trivial errors.LM2621 is a DC boost converter with 1.2V-14V input and adjustable outputDatasheet:http://www.ti.com/lit/ds/symlink/lm2621.pdfThe circuit I am trying to build is the 3.3V SEPIC regulator as shown in the datasheet:My design:Eagle schematic:https://dl.dropboxusercontent.com/u/20650993/LM2621%20Sepic/lm2621v4.schBoard:https://dl.dropboxusercontent.com/u/20650993/LM2621%20Sepic/lm2621v4.brd* UPDATE 2016-05-14: fixed image to include bottom ground copperBOMLM2621LM2621MMMSOP-8pdfC-S1uF 25V X7R ceramicCL21B105KAFNNNEC133pF 50V C0G ±5% ceramicCL21C330JBANNNDC233uF 10V tantalumNRD336M10R12C368uf 10V tantalumTAJC686K010RNJC40.1uF 35V ±20% tantalumTAJA104M035RNJD11A 40V SchottkySS14pdfL16.8uH 1.6A 0.06ohmDLG-0504-6R8pdfL26.8uH 1.6A 0.06ohmDLG-0504-6R8pdfR-F1150k ±5% 0,125WRC0805JR-07150KLR-F291k? ±5% 0,125W0805S8J0913T5ER-FQ160k ±5% 0,125W0805S8J0164T5ER6510? ±5% 0.125W0805S8J0511T5EI hook up the finish board to a lab supply and check 1.8V, 3.3V, 5.0V, 10V input at 50-100mA and the output goes up to 5-15V instead of the expected 3.3V. In general the bigger current limit I set on the supply the higher the output voltage gets.I have built the circuit 4 times and using two different board designs. First design was done at a fab house, second version etched at home. Both of them have the same output voltage behavior. I solder the components with hot air trying to maintain a reflow profile (1 minut preheat to 160C, 20s at 250C). Then I wash the completed PCB in an ultrasonic cleaner with isopropyl alcohol. Here are pictures of the board and components.*discregard solder paste balls, pic taken after "disassembly inspection"Help me internets, because after a few days of redesigning, rebuilding and inspection I have no clue what to check next. TIA.Tom

author: tmk854
author_rank: Newbie
timestamp: May 13, 2016, 03:05:05 pm
content: I need to update the invalid board layout image. There is a ground copper pour on the bottom layer, its just missing here.I can check the inductor current rating but I was under the assumption that 1.6A inductors can handle a 100mA output (since the regulator already fails at that level).Tom

author: tmk854
author_rank: Newbie
timestamp: May 14, 2016, 10:50:04 am
content: I have updated the board layout image and the ground plane is now visible.Quote from: AcHmed99 on May 13, 2016, 06:01:18 pmReturn path for which current? If you were talking about the ground return then there are several vias around the board (near C2, C3, L2, GND pin pad). Is that not sufficient?Quote from: AcHmed99 on May 13, 2016, 06:01:18 pmVoltage meter VDC setting. I have an oscilloscope but I wouldn't know what to measure and how to interpret the readings (yet).Quote from: AcHmed99 on May 13, 2016, 02:02:05 pmI think I incorporated that into my design and there is a single trace on the blue ground plane that I guess is not pulsing since it's the FEEDBACK voltage divider.If someone wrote a few obvious things to measure with a multimeter / oscilloscope then that would probably be really helpful in going forward.

author: tmk854
author_rank: Newbie
timestamp: May 15, 2016, 07:44:43 pm
content: I measured the output with a scope and it doesn't contain any abnormal noise.One thing that I've been meaning to ask is that while I was inputing voltage/current from my lab supply I didn't have any load connected, just the voltage meter. I reckon the regulator doesnt consume 100mA by itself, so that means it's shorting out somewhere? The reason I'm confused about this is that it's repeated in all 4 circuits I've built so I was assuming it was because of wrong component selection. Or is there a chance I've consistently "melted" the IC during hot air work? I guess I will try using a soldering iron for the 5th build to rule this out.My chips were bought from China, but are knockoffs for this kind of chips that frequent to take it into consideration?Thanks AcHmed for all the suggestions and help. I will definitely go in the direction of designing a layout strongly based on the pdf you linked or another one I found for the LM2621, if all else fails.

author: daddylonglegs
author_rank: Contributor
timestamp: May 16, 2016, 02:29:42 pm
content: Tom,I can't see any obvious reason for this not to work and, like AcHmed99, I wonder if you got a genuine part. If you can get TI to ship you samples or buy some from a distributor such as Farnell that might be better.It's also worth testing it with a load of a 100mA just to establish that it is not a question of minimum load (though, as AcHmed99 says, the hysteritic control should be fine at low loads).After being exposed to +15V (or worse), the output 10V capacitor is suspect . This is not the root cause but it may be a secondary problem.Conduct the remaining tests with at least a small load on the output (say 1kOhm, this will only draw a few mA).Try the following observations with a x10 oscilloscope probe and a short ground lead to one of the output ground pins. Make a DC coupled observation and then an AC coupled observation with a higher sensitivity if nothing shows up on the DC observation:The ground plane around pin 5 (SGND). This will give you an idea of how much things (including your observations) are bouncing around with switching noise.Output voltage (you will see something on the AC coupled reading).Input voltage, should be steady with a bit of ripple but check anyway.Pin 7, BOOT. Ditto.Pin 6, VDD. Ditto. Record the DC voltage.Pin 8, the switching node. This should switch between ground and Vi+Vout (I think). Record max, min and frequency; get a screenshot if possible.The anode of D1. Min, max, screenshot.Pin 1, PGND. Compare this with pin 5. My guess is that this will look horrible (more on that below). Min, max, screenshot.Pin 4, FB. Sticking a probe onto the feedback node of a working converter would be liable to screw the operation up, in this case that's less of a worry. This should be about a third of the output voltage.If you see something funny, record the amount of funny .Two things stand out in the layout. The first is the connection of pin 1 to ground. Pin 1 is the power ground that (according to the block diagram data sheet) handles the switching current. This should get a thick short trace to the ground plane/ bypass capacitors and be kept away from the feedback network. The example layout in the datasheet (figure 19) also shows a long thin trace that is shared with Rf2 of the feedback network! This is madness!There is a picture of the boost circuit evaluation board -http://www.ti.com/tool/lm2621eval- this is the board that TI actually built. It clearly shows pin 1 getting a short fat connection to the input and output capacitors. The feedback network is grounded at pin 5, signal ground.I suggest moving R-f2. I think the easiest place for it is to bridge it between C1 and the adjacent ground strip.Give pin 1 a connection directly to the adjacent ground via.The second is that the output capacitor C3's ground current goes directly into the signal ground, pin 5. I suggest removing it and fitting a new capacitor between the output and the ground connection for L2; i.e. place it physically alongside D1 and make connections with copper foil, braid or wire to 3V3 and to the ground via for L2.The aim is to connect things to PGND and SGND as appropriate and to reduce the area of the switching current loop.However, this doesn't explain the behaviour described so I would start with a known good IC. Good luck and let us know how you get on. Tantalum capacitors are suspect when sitting brand new in their packaging, in my jaundiced opinion, but they're particularly suspect when they've been over-volted or reverse biased. I think this comes from Bob Pease?

author: tmk854
author_rank: Newbie
timestamp: May 18, 2016, 11:51:31 am
content: I tested the circuit with a load in the form of a 1k resistor. Here are my measurements for how the circuit operated:Vin I-in  Vout9.00 4 03.308.00 4 03.297.00 4 03.286.00 5 03.285.00 5 03.284.00 5 03.283.00 8 03.282.40 10 03.76 *current jumping around 6-14V rapidly2.30 200 3.842.20 235 14.50<2.20 >200 ~141.30 300 7.411.20 250 4.841.10 104 2.931.00 0 turns offSo the regulator is working alright except for input voltage below 2.40V.Now I understand what was happening during my measurements with no load. When hitting ~2.3 input voltage, the capacitors charged up to the 14V and were discharging slowly while I changed the input voltage up/down, hence clouding an obvious pattern. Adding the load helped to discharge the caps quickly, so the measurements turned out repeatable this time.Here is a useful link about running SMPS without a load, worth reading for newbies like me:http://electronics.stackexchange.com/questions/80547/operating-a-switched-mode-power-supply-without-a-loadI am assuming that the problem with generating stable output voltage at Vin = 2.2V is due to bad layout and/or caps after violent treatment, so I'm not sure it is worth further debugging until I prepare a revised board layout. I will try to make the modifications suggested bydaddylonglegssoon to see if any particular one will fix the Vin 2.20 problem.Here are some oscilloscope screenshots:LM2621 SW (pin 8 ) @ Vin = 2.60V, Vout 3.3VLM2621 SW (pin 8 ) @ Vin = 2.30V, Vout > 14VLM2621 PGND (pin 1) @ Vin = 2.60VLM2621 Diode anode @ Vin = 2.40VLM2621 Diode anode @ Vin = 2.30VLM2621 Diode anode @ Vin = 2.25VLM2621 Diode anode @ Vin = 2.20VAfter that the diode just jumps to > 14V "flat".Basically around Vin=2.2 voltage everything just transitions from the status of "signal" to "noise". I wonder why it happens at this level precisely.Tom

author: daddylonglegs
author_rank: Contributor
timestamp: May 18, 2016, 01:01:30 pm
content: Power! Congratulations.Quote from: tmk854 on May 18, 2016, 11:51:31 amVDD (pin 6) needs to be between 2.5V and 5V, see sections 6.2 and 8.2.1.2.2 of the datasheet.In a boost configuration you can power VDD from the output as shown in the typical application circuit on page 1 of the datasheet. Before the circuit starts up, Vout will equal Vin minus one diode drop.In a SEPIC configuration things will be trickier, since there is a capacitor blocking DC conduction between Vin and Vout. You could try diode OR'ing Vin and Vout to VDD (with a series 500 Ohm resistor).It is not clear to me what the start up conditions for the IC are and whether there is an under-voltage lockout. :-( I'm not hugely impressed with the datasheet.

author: tmk854
author_rank: Newbie
timestamp: May 18, 2016, 07:38:57 pm
content: Good catch with VDD pin 6, I remember reading about that, but I guess there is too much new information at once.I found an "LM2621 Design Document" (attached) that describes a SEPIC design. In section "3.0 Schematic" there is also D1, a double diode package going from Vin & L2+ to the BOOT (pin 7).I will try both of these and hopefully Bob's  my uncle.

author: tmk854
author_rank: Newbie
timestamp: May 27, 2016, 04:25:29 pm
content: Success!!! I managed to get the module working after adding BOOT and VDD bootstrapping. Start up voltage is 1.12V. Once in regulation it can operate down to 0.38V for a 15mA load, so it will be a great mixed battery/USB regulator.BOOT is done as per "LM2621 Design Document" and VDD is a 2.4V zener controlling a PNP transistor. By default, Vin is connected to VDD through R6. Until Vin can break through the zener (Vin=0...2.4V), VDD is kept connected to Vout (+3V3). When Vin > 2.40V the PNP starts disconnecting Vout from VDD.I should probably disconnect Vin from Vout so that only one of them is connected to VDD at a time. Is adding another zener2.4V/NPN module the best way to do that?Bootstrapping fragments outlined in blue