Help! Finding out if an IC is working.

[cetchmoh]

Hi there!

A few month ago I spilled some liquid on my laptop and since then I try to repair it as good as I can.
I don't have any background in electronics and circuits except from my 1st semester of computer science.
I'm a desperate learner and people say I have excellent soldering skills when it comes to smd parts.
So I spent the last few weeks educating myself, learning about all this, and gradually repaired my laptop
so that it's working again. Just the backlight is left which doesnt work.

After a lot of googling and digging through russian and chinese forums I found the corresponding schematics and some tools to locate the parts on the board.
Now I'm left with one very tiny chip and it's surroundings.

If you like take a look:


The circuit builds a boost converter where on the left goes 12.5V in, and it should output 18 - 27V at the end of the diode if I understand everything correctly.
The maximum I measure is always 12.5V and I'm running out of ideas where to dig for the error.
I replaced the inductor, the schottky diode and the caps at the end to ensure that they are not the source for the error, which only leaves me with the IC in the middle which does the switching.
I don't really have an idea how I could debug this.
The chip is a TI LP8550 and they don't provide a full datasheet for it, but I found a datasheet for an LP8545 which looks very similar (feature wise), but is housed in a different package.
http://www.ti.com/product/lp8545

But I assume they work more or less identically?
I took my multimeter and measured the pins I could locate for voltage.


(edit: of course I did my measurements when the chip was soldered to the board!)


I get the following readings:
FB: 12.5V
FSET: 0V
PWM: 0.82V
ISET: 0V
EN: 3.1V
FAULT: 0V
SW: between 0.3 and 1.6V
FILTER: 0V
VLDO/VIN: 5V
OUT1: 0V
OUT2: 0V
OUT3: 0V
OUT4: 0V
OUT5: 0V
OUT6: 0V
VSYNC: 0V
SDA: 3.3V
SCLK: 3.3V
VDDIO: 3.3V

So the chip gets 3V on it's enable pin which should be sufficient. 5V on VLDO and VIN looks good to me according to the datasheet. I don't know if it makes some difference that VDDIO is 3.3V.
On FB I would expect more than the 12V (like I said, between 18 and 27V to drive the LEDS). PWM seems to be some signal where the duty cycle will result in 0.82V.
Would it be good to look at this with an oscilloscope?
The oscillating voltage I measure on the SW pin would indicate to me that the chip is doing something.
I don't really know for what I need ISET and FSET and I have no idea how to deal with that floating FAULT pin. The datasheet says its an open drain pin. Do I need a pull up resistor to get something from that?

But as you can see, I'm running out of ideas. I'm gladful for any hint how I should proceed from here.

[amyk]

Now I'm left with one very tiny chip and it's surroundings.

And we're left with a very tiny schematic... I can't see a single thing on that.

PWM seems to be some signal where the duty cycle will result in 0.82V. Would it be good to look at this with an oscilloscope?

Yes.

The oscillating voltage I measure on the SW pin would indicate to me that the chip is doing something.

Is that with a scope or a DMM? The latter does not work well for this purpose.

I don't really know for what I need ISET and FSET and I have no idea how to deal with that floating FAULT pin. The datasheet says its an open drain pin. Do I need a pull up resistor to get something from that?

ISET sets the current. FSET sets the frequency. You may want to look at the LP8552 and a few others in the LP855x family, since they're much closer to the 8550 than the 8545.

As for the FAULT pin, if the laptop uses it then there will be a pull-up resistor somewhere. It will be pulled low under a fault condition.

[cetchmoh]

Hey! Thank you for your reply!

Now I'm left with one very tiny chip and it's surroundings.

And we're left with a very tiny schematic... I can't see a single thing on that.

Errr... did you try to click on that schematic? It should show you the huuuuuuge schematic.

PWM seems to be some signal where the duty cycle will result in 0.82V. Would it be good to look at this with an oscilloscope?

Yes.

The oscillating voltage I measure on the SW pin would indicate to me that the chip is doing something.

Is that with a scope or a DMM? The latter does not work well for this purpose.

I don't really know for what I need ISET and FSET and I have no idea how to deal with that floating FAULT pin. The datasheet says its an open drain pin. Do I need a pull up resistor to get something from that?

ISET sets the current. FSET sets the frequency. You may want to look at the LP8552 and a few others in the LP855x family, since they're much closer to the 8550 than the 8545.

As for the FAULT pin, if the laptop uses it then there will be a pull-up resistor somewhere. It will be pulled low under a fault condition.

Alright. ISET and FSET makes sense and I measure the correct resistance to ground for both of them, but no voltage - I assume that this is correct behaviour.
The "oscillating" voltage I measured on SW did not oscillate today anymore - it's just somewhere at 0.2V. I don't have a scope here at the moment and would need to wait for a friend to help me with that.

The FAULT pin is left floating on the board - so I won't get something from it I guess.

Thanks for the hint with the datasheet to the LP8552. In the first moment it gave me a heart attack when I saw the connection diagram. But for the placement: when I look on the chip, the diamond on it should be located in the lower left corner and under it should be pin (ball?) A1. Right next to it should be B1 and so on.

What could an oscilloscope tell me about the chips behaviour? I guess I should get some nice square wave measuring on the SW pin?

[cetchmoh]

Another idea I have would be to communicate via i2c with the chip.
I ordered a bus pirate which should arrive next week. Perhaps I could interrupt the connection from SDA and SCLK and solder 2 wires to those points and see what bus pirate tells me when I try to read the EEPROM or the FAULT register... does this sound viable?

[amyk]

I see now; I have to click twice.

Something odd is going on if you get full voltage past the diode, at the FB pin, but only a very low voltage at SW (broken/intermittent trace?) If you had no voltage past the diode that would be more consistent. Either way, look at it with a scope. The '8552 datasheet has waveforms you can compare against.

Yes, you could try communicating with it via I2C directly (remove R9753 and 9757 to prevent conflicts with the host I2C bus). Make sure your ground references and applied voltages are within spec - the '8550 is probably 5V tolerant but other parts might not be. As I guess the '8550 supports control via I2C too, there is a bit in one of the registers that you can use to turn it on programmatically (and make sure the brightness, set in another register, is also nonzero...) You can inspect the source of Linux LP855x driver to verify the correct register address and bits (the '8550 looks like the '8552 from a quick glance, but you should double-check this.) Good luck.

[ChiefPFF]

Hi mate,

Going to be honest and say I've only scanned the posts above - tldr

BUT, that looks like a macbook LED driver is it? Anyway, I do a fair amount of these that come in water damaged and the IC dies in most cases. The most common passives to go are on the EN divider circuit. If you can get EN and VIN then just go ahead and replace the driver and you will probably be up and running.

[Telequipment]

http://www.ebay.co.uk/itm/170964892729?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

[cetchmoh]

Hi mate,

Going to be honest and say I've only scanned the posts above - tldr

BUT, that looks like a macbook LED driver is it? Anyway, I do a fair amount of these that come in water damaged and the IC dies in most cases. The most common passives to go are on the EN divider circuit. If you can get EN and VIN then just go ahead and replace the driver and you will probably be up and running.

You are spot on!
The thing is I tried 2 times to replace it, and I'm sure I failed looking at my results.
At the moment I'm trying to find some company which would replace it for me in a proper way. I guess fine pitch BGA is beyond my skills
I just tried to rule out if there is something else damaged, which most probably is after shorting the circuit.

[ChiefPFF]

Yeah repairing these things is a real PITA if you don't have the correct gear to hand - I prefer to use IR for these ICs. You really need to be sure of a sound install.

What is the P/N of the board you have there? the 820-?-? one..

[cetchmoh]

Yeah repairing these things is a real PITA if you don't have the correct gear to hand - I prefer to use IR for these ICs. You really need to be sure of a sound install.

What is the P/N of the board you have there? the 820-?-? one..

820-2936-B.
I already got the schematics and the board view
I wrote a lot during the painful way of debugging this issue on the macrumors thread at http://forums.macrumors.com/showthread.php?t=972699

[ChiefPFF]

Ah OK cool. I've got a couple of revA boards in the workshop - but unfortunately don't have anything else related to them so can't power one up at the moment.

I had quick re-read of this thread though. SW probably looks fishy - have you checked R to GND there?

[cetchmoh]

Ah OK cool. I've got a couple of revA boards in the workshop - but unfortunately don't have anything else related to them so can't power one up at the moment.

I had quick re-read of this thread though. SW probably looks fishy - have you checked R to GND there?

Sorry I took so long.
R to GND I measured an open, while on a healthy board from my friend I get a reading of ~400k.
I guess either the chip is fried internally or the balls don't make contact with the pads.
I removed the chip again, and perhaps I will try tomorrow to solder it again - hopefully with a bit more temperature control and this time with tinned pads.

Wish me luck

[amyk]

SW internally connects to a MOSFET which should normally be open with no power applied... of course if you have some residual voltage on the gate it might look like a high resistance instead.