DC/DC buck converter TPS55386 - Page 1
Forum section: electronics / projects

author: iwasz
author_rank: Contributor
timestamp: May 23, 2015, 08:57:42 pm
content: Hi guys!I am facing a problem with TPS55386 from TI. I tried to copy the reference design as closely as I could, but obviously I did something wrong. Schematic :The output voltage (one chanel is set to 5V, and the second to 4.2) of both channels looks similar :So as you can see it is kind of turning on and off every few milliseconds. Outputs areunloaded, but it looks like some over-current protection has been turned on? What do you think?

author: Skimask
author_rank: Super Contributor
timestamp: May 23, 2015, 09:26:20 pm
content: RFDSPage 9Let's see the circuit board.

author: iwasz
author_rank: Contributor
timestamp: May 24, 2015, 09:49:04 am
content: Here it is. I power this thing from +12V not 24 as depicted on the schema. Both input and outputs are marked red. The waveform was taken on the marked ouput. Both input and output caps are tantalum instead of ceramic (should be the latter) but I don't think it matters so much. Besides I tried to solder some ceramics for output (but I haven't had 22µF but smaller) and it haven't changed the output waveform:And here I turned off the top layer for visibility:I also replaced the entire IC for another, hoping that the IC is faulty, but of course it it not the case, both behave the same. I also tried with other power inductors (replaced 12µH with 82µH to see what would happen) and again no luck!

author: Waff0l
author_rank: Newbie
timestamp: May 24, 2015, 07:15:32 pm
content: The ~10ms between startups indicates hiccup mode.One way to get there is by output overcurrent, but as they are not loaded this isn't an option.The datasheet states another option to get into hiccup mode when the output drops below 85%:QuoteYour waveform has more than 1Vpp of ripple on the output, probably caused by the output tantalum capacitor's ESR. This may cause the regulator to fall out of regulation.The ESR of an tantalum cap is in the order ofohms. A ceramic one has practically no ESR (<15mOhm), which could fix the issue.

author: Skimask
author_rank: Super Contributor
timestamp: May 24, 2015, 07:28:23 pm
content: You've got your answer in your first post...along with that reference to page 9 of the datasheet.

author: Niklas
author_rank: Frequent Contributor
timestamp: May 24, 2015, 09:41:14 pm
content: Quote from: Skimask on May 24, 2015, 07:28:23 pmI am not so sure that that is the reason here. Check Figure 26 on page 22 and compare it with the plot that the OP showed earlier. The signal is discontinuous and not a sawtooth.Can we have a look on both copper layer separately and perhaps also some data on the inductors (manufacturer, saturation current etc)?

author: Skimask
author_rank: Super Contributor
timestamp: May 24, 2015, 09:47:28 pm
content: Oh...hey...you're right, possibly.I just noticed that the wave in the 'scope picture 'apparently' goes from ground to 5v. I was thinking it was something like 4v-5v.Either way, I still think you gotta have a load on it to work properly.

author: rx8pilot
author_rank: Super Contributor
timestamp: May 24, 2015, 10:21:17 pm
content: First thing I see is that there is hardly any copper in your current path. When I lay out a converter, the current paths take priority and do not get thermals. All the other signals and components get worked in after. The PCB layout is just as critical and delicate as the schematic itself. A perfectly good schematic can easily fail with a poorly considered PCB. The switching noise can interfere with the signals that are mV sensitive and any parasitic inductance will cause problems that are hard to rack down like switch node ringing.See page 29 in the data sheet.Before changing anything, load the output to 1A or so and record the result. The tantalum caps will not likely work A few parallel ceramics are better. I have a 5v TPS based converter with 7x 10uf X7R caps on the output and it is quite clean and stable.It may be pulse skipping with no load and the big ESR of the output cap may be why the waveform is ramping the way it is. The 2ms rise time on a converter operating at 600khz is nuts (although that may be the soft start in action). Maximum capacitance and minimum ESR should be considered as well, since the device cannot start a big capacitor without seeing an over current event. The input cap being high ESR could starve the system on startup causing a false over-current sense which will shut it down and a reset will take place. Probe the PVDD pins right at the device to see what you are getting.

author: iwasz
author_rank: Contributor
timestamp: May 31, 2015, 11:29:08 am
content: Thanks for your insights guys. I didn't managed to get it running. Well I did, but partially. First i reeducated myself a little bit about ceramic capacitors (thanks to yours suggestion) and replaced my caps both on the input and output to quite big ceramic ones (both X7R, but tried X5R and Y5V as well). No luck. Then I discovered, that feedback of channel 1 has correct amplitude (0.8V), and feedback of channel 2 is to low (0.6V) and probably causes the IC to switch to under voltage fault. I replaced nearly every component in the channel 2, starting from the voltage divider - I simply rise its voltage to see what would happen and it worked, but the output voltage dropped to 3V. So I made another divider with proportional, but lower resistors so it could provide more current, and it worked... until it stopped to work. So probably I have some other bug, maybe on the PCB itself (but it was electrically tested by the fab), and I give up on this. I simply rise the channel 1 voltage a little bit and power the device from channel 1. The important thing about this dual output TPS is that when a fault is detected on one channel, both sops working. Maybe it's obvious, but it surprised me at some point of debuggingIn the next revision I will switch to some simpler buck converter (for example TI's simple switcher rated 3A), I'll power it from 12V (or 24) and it will give me desired 5V. Then I''ll pass this on to the big regulated LDO to have 4.2V (rated 2A or 3A) to power a energy hungry GSM modem (2A peak). The power loss on the LDO would be 2.4W (peak) or less If I increase the voltage. Maximum voltage for this modem is 4.8V. Then I'll go with another LDO for the usual 3V3. That's the plan.Once again thanks for the replies.