SOLVED: My first TPS61175 Design... no gain... - Page 1
Forum section: electronics / projects

author: NivagSwerdna
author_rank: Super Contributor
timestamp: June 09, 2019, 02:26:02 pm
content: I've tried to build a .2A 35V supply using a TP61175 (see attached schematic) and not had any luck.I've had to make a number of bodges... Pull Pin4 HIGH EN, and more significantly Pin 11 LOW... NC (and I thought NC would be No Connection!)I connected the thermal pad to GND.For some reason it just isn't oscillating... Any idea what I could try?

author: Audioguru
author_rank: Super Contributor
timestamp: June 09, 2019, 02:40:12 pm
content: I guess you did not read the datasheet. It says pin 4 must be high and pin 11 must be low. It also shows a Schottky diode (your schematic shows an ordinary diode) and ceramic input and output capacitors.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: June 09, 2019, 02:53:39 pm
content: Quote from: Audioguru on June 09, 2019, 02:40:12 pmI have Pin 4 High (was switched but now high) and Pin 11 Low (was Not Connected, bodged, per earlier schematic, to GND), My D1 is a B550C-13-F, which is "DIODE SCHOTTKY 50V 5A SMC"; C31 is 10uF EEHZA1H100R, C32 4.7uF EEEFC1H4R7R. I did add extra Capacitance at the input but no change.So my output caps are both Polymer Electrolytic... could that make a difference? I'll go and read the datasheet againThanks for the interest

author: SiliconWizard
author_rank: Super Contributor
timestamp: June 09, 2019, 04:06:58 pm
content: I can't see anything clearly wrong with your setup. Could it be possible that powering the circuit while leaving the "NC" pin open could have fried the TPS61175 (before you figured it should be connected to GND?)The datasheet states "Reserved pin. Must connect this pin to ground." I suspect it's some kind of test pin and have no clue what it does... Have you tried swapping the TPS61175 with a new one?

author: NivagSwerdna
author_rank: Super Contributor
timestamp: June 09, 2019, 04:10:28 pm
content: Quote from: SiliconWizard on June 09, 2019, 04:06:58 pmI did swap with a TI Sample I had and same behaviour. It's very odd. I have ordered some 4.7uF ceramic as I don't have any of those to hand and will try again a few days.

author: SiliconWizard
author_rank: Super Contributor
timestamp: June 09, 2019, 04:20:52 pm
content: I suppose you have taken a look at the output with an oscilloscope? Is it at least trying to start and then stops or is nothing at all happening?

author: NivagSwerdna
author_rank: Super Contributor
timestamp: June 09, 2019, 04:36:10 pm
content: Quote from: SiliconWizard on June 09, 2019, 04:20:52 pmI'm not seeing any oscillation on my scope but haven't looked very closely at start-up.

author: SiliconWizard
author_rank: Super Contributor
timestamp: June 09, 2019, 04:38:10 pm
content: Might be interesting to make a single-shot capture upon powering it up.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: June 09, 2019, 08:55:46 pm
content: Quote from: SiliconWizard on June 09, 2019, 04:38:10 pmI have ordered some 4.7uF Ceramic... will report back once they arrive. Thanks all.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: June 10, 2019, 08:31:26 pm
content: Just for the ROFLs I took a few measurements whilst I wait...Firstly at the SS pin... increases to around 1.8V in a time of a few 10's of ms.[According to datasheet... Once the capacitor reaches 1.8 V, the soft start cycle is completed and the soft-start voltage no longer clamps the error amplifier output... sounds plausible!]CH1 Input Voltage 5V... CH2 SS

author: NivagSwerdna
author_rank: Super Contributor
timestamp: June 10, 2019, 08:33:45 pm
content: And it does seem to try for at least one attempt before giving up...CH1 Input voltage 5V.... CH2 output side of D1

author: NivagSwerdna
author_rank: Super Contributor
timestamp: June 10, 2019, 08:36:42 pm
content: Finally during the one blip I captured what was going on at SW...

author: SiliconWizard
author_rank: Super Contributor
timestamp: June 10, 2019, 09:09:01 pm
content: Just a thought after seeing your captures and reading the datasheet more carefully (see 7.4.1): have you tried your circuit without any output load? What you see may be due to the TPS61175 entering the "pulse skipping" mode (looks like it from the last capture) due to the extremely light load if you haven't added any additional load (basically just the feedback network + output capacitor). You could try adding a small resistive load to the output and see what happens.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: June 10, 2019, 09:24:46 pm
content: Quote from: SiliconWizard on June 10, 2019, 09:09:01 pmI have a 300 Ohm resistor across the output. It's meant to get to 35V... so it should really be trying harder. The Output is currently a 4.7uF electrolyic and a 10uF electrolytic and to be honest the PCB layout is not optimal for the output caps (although the other layout is IMHO not too shabby).I'm hoping adding a strategically placed 4.7uF ceramic will change the game... should arrive tomorrow.

author: thinkfat
author_rank: Unknown
timestamp: June 10, 2019, 09:43:29 pm
content: How did you build your prototype, on a perfboard, or a breadboard, or pcb? The layout guidelines are pretty detailed. For a reason, I would think.Gesendet von meinem Nokia 6.1 mit Tapatalk

author: SiliconWizard
author_rank: Super Contributor
timestamp: June 10, 2019, 11:44:08 pm
content: Quote from: NivagSwerdna on June 10, 2019, 09:24:46 pmAlright. Then conversely, you could try with a much lighter load (if you haven't already) and see if it wants to start. If so, the inductor could also be the culprit (getting into saturation)Quote from: NivagSwerdna on June 10, 2019, 09:24:46 pmI would expect excessive noise or ripple because of that, possibly reduced efficiency, but not starting at all? Of course we haven't seen your actual implementation so maybe it's a real mess... What kind of capacitors were you using? Anyway, keep us posted. But consider the inductor as well.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: June 11, 2019, 07:16:17 am
content: My Output Caps are currently... C31 is 10uF EEHZA1H100R, C32 4.7uF EEEFC1H4R7RMy Inductor is SRR1210A-270M which is interesting as I meant to order SRP6540-270M but I can't see why that would be a problem as my selection seems overrated if anything.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: June 11, 2019, 07:30:00 am
content: Quote from: thinkfat on June 10, 2019, 09:43:29 pmPCB. Most of the layout is OK IMHO... For the next revision I will improve some aspects. I think I am probably close enough to the guidelines that it should work. I did misinterpret NC as Not Connected but otherwise I was OK.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: June 11, 2019, 07:33:23 am
content: Quote from: SiliconWizard on June 10, 2019, 11:44:08 pmExactly my thoughts.Quote from: SiliconWizard on June 10, 2019, 11:44:08 pmHard to believeQuote from: SiliconWizard on June 10, 2019, 11:44:08 pmPanasonic Cylindrical Polymer Electrolytic (details above)Quote from: SiliconWizard on June 10, 2019, 11:44:08 pmI did desolder and measure the inductance and it came out OK but maybe I should try another at some point.

author: thinkfat
author_rank: Unknown
timestamp: June 11, 2019, 09:12:37 am
content: What about the inductor, what is the saturation current? Did you account for ripple current when choosing it?You went for a switching frequency of around 1MHz, I think. If I use the values from your schematic:Vin=9V, Vout=35V, fsw=1Mhz, L=27µH, VDassumed 0.3VUsing equation (6) from the datasheet I calculate a ripple current of around 0.24A.EDIT: I just saw that you posted the inductor type above, 4.2A is of course plenty.

author: thinkfat
author_rank: Unknown
timestamp: June 11, 2019, 10:19:52 am
content: BTW I ran your data through TIs power design workbench and while it comes up with pretty much the same values you have in your design, the output cap is suggested as combination of 1 10µF electrolytic and TWO 4.7µF ceramic caps, due to the heavy derating that applies to ceramic caps under DC. Also, Cin is suggested as THREE 10µF ceramic caps.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: June 11, 2019, 10:54:08 am
content: Quote from: thinkfat on June 11, 2019, 10:19:52 amThat's more than I have... and sounds rather expensive! The 4.7uF should arrive soon and I'll stack a couple and see if it makes a difference.

author: SiliconWizard
author_rank: Super Contributor
timestamp: June 11, 2019, 11:41:39 am
content: That's kind of weird... You could also try and play with the soft-start capacitor's value to see if there is a link with that, also check whether your input voltage source can provide enough current...I've seen a couple threads about the TPS61175 on TI's forums, but nothing really helpful...

author: NivagSwerdna
author_rank: Super Contributor
timestamp: June 11, 2019, 12:43:49 pm
content: Quote from: SiliconWizard on June 11, 2019, 11:41:39 amCurrently attached to bench supply with 500mA current limit. I think the scope pics suggest the supply is holding up. And I'm feeling the SS is also about right as the blip occurs after most things have stabilised.If I had to bet... I would say 4.7uF MLCC on the output will fix it.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: June 11, 2019, 09:03:30 pm
content: Firstly, thank you to those who have kept me company on this thread, particularly Silicon Wizard, it's a lonely business trying to solve something in your little electronics lab when nothing makes sense...Anyway so I added 4.7uF MLCC to the output side... no change.... added 4.7uF MLCC to input side no change!! Curious!I looked REALLY hard at the component selection but despite the Vf of my diode being 700mV.... and having doubts... I then decided I must be fundamentally on the right track and there must be a gremlin...R31 and R34 form the feedback divider... (Now this is the top layer... so no imaginative silk screen mirroring going on but...) (See PCB Layout below)Swapped the two components on the board and...[Spoiler: The R34 label is next to R31 and vice versa, must have got mixed up in layout ]OMG... two days (on and off )... of my life!

author: thinkfat
author_rank: Unknown
timestamp: June 13, 2019, 09:12:36 am
content: A good reminder to always check everything, and not assume something as being correct without proof. I guess measuring the feedback pin would have given the right clue instantly.Gesendet von meinem Nokia 6.1 mit Tapatalk

author: NivagSwerdna
author_rank: Super Contributor
timestamp: June 13, 2019, 10:57:53 am
content: Quote from: thinkfat on June 13, 2019, 09:12:36 am"Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth."