Debugging my PCB with a STM32 and TPS65185 PMIC - Page 1
Forum section: electronics / beginners

author: hatrix
author_rank: Newbie
timestamp: May 28, 2023, 05:08:03 pm
content: Hey!tl;dr: made a board with a power management IC, the PMIC doesn't work, I think there is a short somewhere, I don't know how to proceed to debug.I hope the post will be clear while not being too long, don't hesitate don't point it out if I forgot something, I'm still learning and I've been working for so long on that project I might be a bit short sighted.I'm kind of a noob regarding PCB building, as this is my second project. The first one had only 8 THT components.This second one is more complex and I even think a bit too much for meI've been building lately a board that's intended to drive a ED060SC4 E-Ink Paper Display (EPD) using a STM32F103C8 and a TPS65185 as the Power Management IC to generate the power rails.The idea is to be able to send simple commands via SPI to write on the screen.The EPD requires several voltages that the PMIC is expected to generate: -20V, -15V, +15V, +22V and a variable voltage of about -1.5V.It achieves that via two internal switching converters (±16V), two charge pumps (-20V, +22V) and two LDOs (±15V):It also has a 3.3V enable, is programmed via I2C and powers up via the pins "PWRUP" and "WAKEUP".The PCB I made is a 4-layer board with a standard stackup ofsignal / 3.3V / GND / signal. This is the first time I'm making a 4-layer board.I followed precisely the "typical application" of the datasheet of the TPS68185 to create my schematic.On the left is everything related to the STM32 and on the right to the TPS65185.I'm not sure if I can add spoiler tags here, so I'm posting links to the schematics of the project:And for the PCB layout:TPS65185 datasheet:https://www.ti.com/lit/ds/symlink/tps65185.pdfNow to the issue itself!I soldered all the components either via hot air or soldering iron, while making sure there's no bridging on the TPS65185 which has a QFN-48 package.Can't be 100% sure there's no bridging under the pad but I haven't used a lot of solder so I'm hoping it's fine there.When powering the board, the TPS gets hot fairly quickly without having been activated via its two pins.There is also a very notable voltage drop from my power supply (a simple 3.3V LDO connected to USB) that drops to 2.65V from 3.3V.To me, that indicates there is a short-circuit somewhereThe TPS does not generate any rail and only gets hot.On the other hand, the STM32 part of the board works fine. I can flash it and its GPIO work as expected.My question is fairly vague: how can I debug that board?I do not own an oscilloscope but only a multimeter.I checked for any shorted capacitor with my multimeter and could not find any.I did check the diodes orientation and for shorts, they should be fine.I also made sure the inductors are not shorted by desoldering them, applying the minimum amount of solder and resoldering.What Ididfind is that by removing L1 (the inductor at the top, 2.2µH, for +16V), the short-circuit disappears.Removing the capacitors C8 or C6 does not help.This is the extent of my testing, I don't have much idea on how to proceed now…I very hope this post wasn't too dense and I would greatly appreciate any insight you could have!Thanks!

author: Manul
author_rank: Super Contributor
timestamp: May 28, 2023, 06:19:54 pm
content: On unpowered board I would measure resistance between ground and VB_SW. If you see low resistance, most likely the switching mosfet of DCDC1 is cooked. If no, then I would follow the lead of VB pin. Datasheet describes VB as:QuoteThe input voltage essentialy passes through D3 into VB even if DCDC1 is non operational, it could be that there is something wrong with VPOS LDO and VDDH charge pump. Check pins related to those.

author: hatrix
author_rank: Newbie
timestamp: May 28, 2023, 07:45:10 pm
content: Quote from: Manul on May 28, 2023, 06:19:54 pmI'm seeing a resistance of 59 Ω between GND and VB_SW, I guess that qualifies as low resistance?

author: Manul
author_rank: Super Contributor
timestamp: May 28, 2023, 07:59:25 pm
content: Borderline. I'm thinking, could it be something else on the 3.3V rail, because it's used everywhere. Maybe repeat the test with L1 removed to isolate VB_SW from 3.3 rail. Also remove D3.And do same check with VB pin.

author: hatrix
author_rank: Newbie
timestamp: May 28, 2023, 08:11:02 pm
content: Quote from: Manul on May 28, 2023, 07:59:25 pmJust desoldered D3 and L1.For GND - VB_SW: 60 Ω as beforeFor GND - VB: 7 MΩ, decreasing to 0.8 MΩ, probably because of capacitors?

author: Manul
author_rank: Super Contributor
timestamp: May 28, 2023, 08:35:08 pm
content: 60 Ohms is not enough low to make the IC really hot (it's less than 200mW), but it might be that the resistance is dropping when significant voltage is applied. In any case it does not look good. It is likely that IC is damaged and you migh need to replace it. Biggest question is how it got damaged in the first place. If you can't answer this, you might end up cooking more ICs. Maybe ringing on the input? Sorry, I'm out of good ideas.

author: hatrix
author_rank: Newbie
timestamp: May 28, 2023, 08:49:00 pm
content: Quote from: Manul on May 28, 2023, 08:35:08 pmThe board I was checking only had the minimum components assembled for the TPS.I just checked the values for a board with everything (which also doesn't work) and I got about 250 Ω for GND - VB_SW.Probably my hot air soldering technique isn't the best and I fried both ICs?

author: Manul
author_rank: Super Contributor
timestamp: May 28, 2023, 09:06:14 pm
content: Quote from: hatrix on May 28, 2023, 08:49:00 pmHonestly, I doubt that. I think you would visibly burn the PCB earlier than destroy the IC. But I was not standing there and looking, so I don't know really.Quote from: hatrix on May 28, 2023, 08:49:00 pmSo the board was powered at least once? It would be interesting to check the board which was never powered. Or after fresh replacement of IC.

author: hatrix
author_rank: Newbie
timestamp: May 28, 2023, 09:08:20 pm
content: Quote from: Manul on May 28, 2023, 09:06:14 pmBoth boards have been powered up already. I made the second one because I thought I burnt the TPS at first, but the issue was still there.I'm going to replace the IC on one of the boards re-measure the resistance.

author: hatrix
author_rank: Newbie
timestamp: May 28, 2023, 09:27:55 pm
content: Quote from: hatrix on May 28, 2023, 09:08:20 pmJust soldered a new IC, I got:- 3.3 MΩ between GND - VB_SW- 32 kΩ, between GND - VB.All that still without L1 and D3.What does that mean?

author: Manul
author_rank: Super Contributor
timestamp: May 28, 2023, 09:36:54 pm
content: Quote from: hatrix on May 28, 2023, 09:27:55 pmIt means, that you can try to solder missing parts and power the board if you don't mind loosing 4 Eur. I suspect that the first two ICs got damage not from soldering. So it means, that you should expect this one to be dead too after powering.

author: hatrix
author_rank: Newbie
timestamp: May 28, 2023, 10:01:18 pm
content: Quote from: Manul on May 28, 2023, 09:36:54 pmI just soldered L1 and D3, this is what I have:GND - VB_SW: 103 kΩI just powered the board, no short. I connected PWRUP and WAKEUP to +3.3V and I have all my rails?!I got absolutely no idea why and how but it worksI'm powering the board with dupont cables off a breadboard, do you think some kind of ringing due to repeated bad connections could be the issue?

author: Manul
author_rank: Super Contributor
timestamp: May 28, 2023, 10:13:33 pm
content: Congratulations. I really don't know what happened to previous two ICs. The classic problem is inductance mixed with ceramic capacitors causing voltage overshoots. Read Linear Thechnology Application Note 88. Can it be the cause in this case I'm not sure. Just try to simulate bad power connection and see what happens.

author: hatrix
author_rank: Newbie
timestamp: May 28, 2023, 10:21:42 pm
content: Quote from: Manul on May 28, 2023, 10:13:33 pmIt's my last IC, I will try not to damage it until I can get my hands on more.Thanks a lot for your time and help, I really appreciate it!

author: Manul
author_rank: Super Contributor
timestamp: May 28, 2023, 10:37:08 pm
content: Or maybe you were very unlucky to get solder bridge to thermal pad. I've just noticed, that thermal pad is not ground. Other VB_SW adjacent pins are either N/C or ground, so that kind of bridging would either not cause problem or give a 0 Ohms dead short.

author: hatrix
author_rank: Newbie
timestamp: May 29, 2023, 08:06:08 am
content: Quote from: Manul on May 28, 2023, 10:37:08 pmThat could very well be, as I'm not very familiar with QFN soldering.I probably applied too much paste the first time and then tried to make a burnt chip work for weeks without a clue…For the next revision I will extend the pads of the QFN footprint, so the solder has a place to go if there's too much.

author: Manul
author_rank: Super Contributor
timestamp: May 29, 2023, 12:20:41 pm
content: Quote from: hatrix on May 29, 2023, 08:06:08 amThat's easy to test. You have Thermal Pad connected to various pins. So you can easily test for short between VB_SW and Thermal Pad.