LT8610AB step-down problem - Page 1
Forum section: electronics / projects

author: Mino
author_rank: Newbie
timestamp: November 03, 2016, 02:58:22 pm
content: Hi.I’m building a bench power supply and I have problems with LT8610AB step-down regulator. I have managed to kill two of them and can’t figure out what exactly is the problem. I attached schematics of two versions I’ve tested.First time I powered up the circuit (original version) it seemed to work OK. I could control output voltage using I2C potentiometer. I also loaded the output without issues. However, when I turned it off and on again, it wasn’t working anymore. There was no load at the output when I turned it off, so I thought that output capacitors discharged through LT8610 and this reverse current killed it.So, I added schottky diode on input to prevent reverse current (see version 2 schematic). I also connected SYNC pin to GND so that it will work in burst mode (to lower output ripple). Then I turned it on an off several times and it seemed to work OK. So I moved on to do some other testing, but today when I turned it off the LT8610 suddenly blew up and all the magic smoke escaped.Is there some obvious issue I’m overlooking?Thanks for any advice.

author: perieanuo
author_rank: Frequent Contributor
timestamp: November 03, 2016, 05:18:34 pm
content: Read carreful how to implement gnd,that's the thing.99% they oscillate you can't even catch that on oscillo only if you start with tension rise and current limit to protect the chip.so revise your gnd pcb implémentation.best regards ovidiuEnvoyé de mon HUAWEI NXT-L29 en utilisant Tapatalk

author: kxenos
author_rank: Frequent Contributor
timestamp: November 03, 2016, 05:34:54 pm
content: According to the datasheet, the en pin should be connected to the anode of the diode. (http://cds.linear.com/docs/en/datasheet/8610abfa.pdfpage 19). But this will cause the en pin to be one diode drop higher than vin. So I think the best way is to connect en pin to a voltage divider as shown in the same image. I think you have too much output capacitance and too little load.

author: Kalvin
author_rank: Super Contributor
timestamp: November 03, 2016, 05:38:28 pm
content: Have you tried a smaller inductor? The example circuits show ten times lower inductor (4.7 uH) compared your circuit (47uH).

author: tatus1969
author_rank: Super Contributor
timestamp: November 03, 2016, 06:49:04 pm
content: The datasheet says that you need to decouple intvcc with 1uf ceramic. Could cause instability not doing that.Have you checked output voltage response after line and load transients? Also sw node ringing could be high when the layout is not perfect.

author: wraper
author_rank: Unknown
timestamp: November 03, 2016, 07:05:46 pm
content: Better post your PCB layout, hope you don't do it on some kind of proto board.

author: BravoV
author_rank: Super Contributor
timestamp: November 03, 2016, 07:32:59 pm
content: Quote from: wraper on November 03, 2016, 07:05:46 pm+1 , post a photo of your work, this usually will help experienced members here to spot problem fast, like the layout, components used and etc.Although I'm not an expert, just learned from my own experience, a very basic question, do you have a scope ?According your freq setting, you're running at 700KHz switching and its synchronous rectification, imo, troubleshooting these without a scope is unthinkable, at least for me, cmiiw.

author: kony
author_rank: Regular Contributor
timestamp: November 03, 2016, 07:39:34 pm
content: Digital pot in DC/DC converter feedback, especially from Microchip is all but reliable. Been there, got burned on that.Also - why PGOOD pull-up is using output voltage as reference instead of digital logic voltage rail?

author: prasimix
author_rank: Unknown
timestamp: November 03, 2016, 09:50:08 pm
content: Why power ground pin 7 and 8 are missing in your schematic? Cvcc indeed should be connected and try everything without digital pot first to be sure that basic circuit is working as expected and then try to play with adding thing to the feedback loop that could indeed blow up IC instantly.

author: mij59
author_rank: Frequent Contributor
timestamp: November 04, 2016, 07:05:42 am
content: Not sure if its the case but according to the datasheet EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB.

author: tatus1969
author_rank: Super Contributor
timestamp: November 04, 2016, 07:16:45 am
content: Quote from: prasimix on November 03, 2016, 09:50:08 pmDon't think that pot can be the problem. He had limited the output range from small (2v?) to roughly 24v with additional resistors, the worst thing that can happen IMHO is unstable output but not destruction. +1 for the missing pins/pad!

author: Mino
author_rank: Newbie
timestamp: November 05, 2016, 11:19:14 am
content: Thanks for your answers.Quote from: kxenos on November 03, 2016, 05:34:54 pmHere is a similar design from LT:http://cds.linear.com/docs/en/lt-journal/LTJournal-V24N2-02-df-BenchSupply-Szolusha.pdfThey use LT8612 (higher current rating) and quite a lot of output capacitance. Voltage divider is there only to set input voltage range, right? According to datasheet it’s OK to connect EN to Vin.Quote from: Kalvin on November 03, 2016, 05:38:28 pmI calculated inductor value using formula 6 in datasheet and only tried this one (47 uH). Could high inductance or output capacitance cause it to blow up when you disconnect input voltage?Quote from: tatus1969 on November 03, 2016, 06:49:04 pmI missed that one. That could indeed be a problem. I didn’t get to test transient response or anything before it blew up.Quote from: mij59 on November 04, 2016, 07:05:42 amMultiple pins for Vin, GND and SW are correctly connected in footprint. I just didn’t include them in schematic symbol.I attached photo of relevant part of original layout and also whole layout in diptrace.

author: kxenos
author_rank: Frequent Contributor
timestamp: November 05, 2016, 03:29:56 pm
content: I copied your first schematic to ltspice and sure enough the simulation couldn't run. On the second attempt the f@@@king program crashed and I lost the schematic.The new version ltspice XVII is quite unstable.

author: prasimix
author_rank: Unknown
timestamp: November 05, 2016, 04:12:41 pm
content: If you are going to make another PCB (since intVcc capacitor is missing), maybe you can rotate C1 and make it as close as possible to the controller (U1). Don't know if D19 make any good. It should works without it.

author: prasimix
author_rank: Unknown
timestamp: November 05, 2016, 04:23:14 pm
content: It seems to me that you didn't take care about connecting signal (GND) and power ground (PGND) below the U1 (see Fig. 4 in datasheet). That is another possible candidate for a new PCB revision.

author: CraigHB
author_rank: Regular Contributor
timestamp: November 05, 2016, 06:46:29 pm
content: Absolute maximum voltage on the Bias pin is 30V. If input voltage is on the higher end any output transients could exceed tolerance on that pin and fry the chip. I actually fried an LT converter controller once due to voltage transients so it's not rare for that to happen. Other makers like TI tend to use more liberal tolerances where LT does not. You have to be more careful about transients with LT controllers.47uH is a high inductance for a switching frequency of 700kHz and an output of several Amps. Normally something under 10uH would be used depending on maximal output current and acceptable inductor current ripple. Sometimes those formula data sheets provide for inductor values can come up with less than ideal numbers. With a similar switching frequency I've done a boost converter for high current outputs that ran an inductor as low as 680nH. With that high inductance and more liberal capacitance you're more likely to see big start-up and load dump transients.That controller uses internal compensation. With the component values you're using you may be running right up against the limit of what it can handle. That can result in longer settling times, slower response, and of course larger transients.As mentioned make sure the thermal pad is properly soldered to the ground plane. I've had issues with thermal pads on SOP style packages failing to put the pad close enough to the PCB.You might want to put a series resistor on the En pin. Though it's acceptable to tie that pin directly to Vin, I've had issues in the past where internal shorts have resulted through logic pins tied directly to supply voltage. Putting a resistor inline provides a bit of insurance.Generally I prefer to use controllers other than Linear Technology because they seem to be less robust than other makes. They also tend to be considerably more expensive. Though I have successfully used their controllers in the past, it's a bitch to deal with problems like that. You really can't go through a pile of expensive chips to trial and error a resolution to your problem. When that happened to me I resolved it by going with a different make controller using a somewhat different design.

author: rfbroadband
author_rank: Unknown
timestamp: November 09, 2016, 01:53:55 am
content: not familiar with this specific chip, but:- during debug bleed some load current from the output to GND, thus in a no load condition you have at least some min. load current. I have seen many switching regulators become instable under no load condition.- carefully select the output capacitorthe data sheet warns about stability issues for lower value caps"A lower value of outputcapacitor can be used to save space and cost but transient performance will suffer and may cause loop instability."- I always do bode plot measurements of the feedback loop of a regulator to tune the stability of the loop. This is easier said than done (equipment). If you have the equipment, measure the bode plot of the feedback loop by injecting a small test signal into the feedback pin. Be careful with the amplitude of that test signal.- follow the guidelines of the datasheet for layout. A bad layout can ruin the entire design.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: November 09, 2016, 02:35:29 am
content: kxenos,You need a 1uF cap from INTVcc to ground. This is really important. See the datasheet for details.Regards,Jay_Diddy_B