DIY LED TFT Backlight

October 31, 2010November 3, 2014 | Mario

Today I finally got around to finishing my first design for a switching LED display backlight driver. The circuit is built around a LT3518 switch mode driver IC working in buck (step down) mode in this application.

Someone figure out why SMD hand-soldering always looks ugly on macro images…but trust me on this, the circuit works like a charm ;-) The flux residues don’t hurt performance, but you really do want to keep any solder-balls or whatever metalliccy is floating around your circuit away from the tight parts. If necessary, rinsing it with alcohol while carefully assisting with a brush does the job, followed by a soft coating of plastic spray to prolong the circuit’s life.

My design was done rather quick and dirty because it needed to get done. Just don’t expect THE most absolutely high frequency capable layout, though I guess that prominent airwire in the first picture crashes this illusion anyways. Consider this more of a first prototype to check out what this chip has up its sleeve in buck mode after already trying boost mode on Tobi’s LED driver, which will also be documented later on. So far, it seems to keep up with my demands quite well, it will be modified and optimized later on.

Measurements (all taken at maximum brightness with a tRMS multimeter):

	V in	I in	V out	I out	P in	P out	Efficiency
Mains	20 V	0.25 A	9.6 V	0.400 A	5.0 W	3.84 W	77 %
Battery	11 V	0.41 A	9.6 V	0.400 A	4.51 W	3.84 W	85 %

Higher efficiency on battery is just perfect, even though it is barely into the operating voltage range. The measurements made on the go, so the numbers might not be absolutely accurate. If there is time, the scope will clear that up. Also, keep in mind that battery voltage will drop some over time. According to standard Li-Ion specs, it ranges from 12.75 V charged to 9.3 V discharged (although I have never seen it go below 10.5 V so far).

The schematic and board layout will be available in the next few days in EAGLE format, together with more photos of the “implanted” driver. Need to clean up the parts naming and stuff first.

While I am on it, enjoy my wicked hand drawing skills :-D

A few things to add to the drawings: First, the schottky-diode is of SK34A type, secondly the ceramics are all of X7R-rating type (refer to datasheet of the LT on this one) and thirdly the connection between TGEN and CTRL/VREF ist also an airwire, which can clearly be seen dominating the circuit in the pictures and is unbelievably oversized.

This article will be followed by some pictures of the LED array and driver mounted inside the display. The former was already installed before I started this blog so I didn’t think of taking some shots at the time. It is nothing spectacular really, some chip-on-board (COB) slimline LED modules bought over at led-tech.de, soldered together and cut to correct length, then glued into place instead of the CCFL with thermal conductive glue.

NOTE: While testing the device to its limits, I am experiencing some problems at very low system voltage. When on battery only, the notebook’s internal supply lines drop to around 10 to 12 volts as opposed to rock-solid 20V when the mains is connected.

This driver was designed with up to 20V and only down to about 12V input in mind, which means a maximum of 80% switching duty cycle at low voltage. I figured that the internal supply lines would be thoroughly regulated but was mistaken as it seems. As soon as the voltage drops a quantum too far while powered only by batteries (eg. more power is drawn by any device), duty cycle rises over some magic number around 96% at 250kHz (see data sheet) and flickering occurs as the IC loses control over the current. This is why there certainly will be a v2 of this, and it will be buck-boost-mode to handle low voltage situations even better. I might also want to fix minor problems with shutdown mode still allowing a very small current through the ICs internal switch, but in comparison this one is totally unimportant.

Right now the v1 driver is successfully implanted in the screen bezel and works nicely. Screen brightness even beats the original CCFL backlight, though the lower screen border is not absolutely uniformly lit. I could care less, thanks to the windows taskbar using up that space. The notebook is a SX65S by the way.

Harddisk troubles

September 16, 2010July 7, 2011 | Mario

Somehow I managed to get a little work done last night (2AM), so here’s the story about the harddrive:

Some time ago, a friend dropped a dead 1TB harddisk on my workbench. It was installed in an brand new external disk enclosure and had signed off as soon as it was loaded with data. Eeek!

His first attempt was to write to the manufacturer of the drive and ask for a replacement PCB – some companies do it and in most cases the PCBs can be simply exchanged if the revision numbers are identical (they MUST be if you don’t want to risk even more data loss). In this case, they only offered him to exchange his drive for a new one. That would have been fine except for the data being lost.

So, I offered to take a look at the device. After plugging it in for the first time, I immediately noticed that the BIOS could not recognize the disk at all – it even hung during disk ID-ing, which is a strong sign of the drive controller not grasping what the heck is going on anymore. Meanwhile, the disk itself made beeping and whirring noises, followed by sharp clicks. As I could not hear the distinct noise of the spinning discs, I figured the spindle motor controller was a nice place to start off.

Current harddrive PCBs (WD10EACS in this case) consist of two main function groups which are fully integrated into generic ICs or ASICs, meaning application specific integrated circuits.

WD10EACS circuit board

The ICs in the red area are the buffer and the main controller. This chip contains the whole intelligence of the disk. SMART programs and everything regarding data organization or transfer runs in here. The buffer IC temporarily stores the data being written/read while it has not been processed by the head mechanism or sent to the pc. I also marked some other important stuff on the PCB along the way.

The green area is what makes your disk spin. The spindle/VCM driver IC (STM SMOOTH L7251 3.1) generates a three phase motor drive signal for the platter spindle and moves the arm according to the main controller’s wishes. You can find a datasheet on the web, but it is for the predecessor L7250, which is similar in function but not in its pinout.

Now, what most people don’t realize is, how complicated harddrives are. They have full onboard diagnostic programs (which are VERY poorly documented of course, and these well-protected secrets are what makes data rescue companies so special) and even though they look simple on both outside and inside, they have to be precisely calibrated for optimal performance. Hence the need for exactly matching PCBs. But, since this case has all indications of hardware failure, no diagnostic program will fix the damage.

Motor drive testing in this case is best done with the drive board unscrewed from the disk. This will eventually increase the “failed start” SMART counters in the disk’s long-term memory, but whatever – it’s busted anyway. The reason for unhooking the board is that the motor coils, if not defective, present unknown resistances and inductances between the drive outputs and make your measurement extremely difficult. After connecting the four probes of my digital scope to the spindle connector, I started sampling and plugged the board in…

Scope screenshot of motor phases

…and this is what happened. Excuse the poor contrast, I usually only use white backgrounds when printing, to save toner.

What you see is the main startup algorithm doing its job. The controller first tries to find out what position the rotor magnet in the spindle motor is in, so it can generate matching signals for rapid acceleration. It does that by applying voltage to the different coils in changing combinations, following a pre-programmed pattern. After measuring the current rise-time for each combination, it can calculate the magnetic influences in the motor coils and from that the actual motor position. To explain the different traces, yellow blue and green are the actual three phases and the magenta-colored trace shows the center tap which is not present right now, since the motor is not connected. The phases are switched against the rail voltages by MOSFET half-bridges integrated into the IC.

I marked the important part in the graph – the yellow phase is missing something on the top of its waveform – there should be short high-pulses like on the other two. The controller notices that and tried to restart the process at the red mark. After failing twice, it decides to try and spin the disk some. This is called coarse drive mode, the visible pulsing is not meant to spin the motor to a certain frequency but rather to just turn it in case the motor is “stuck” in a non-discernible state (even though that should never happen). This part of the signal is responsible for the whirring noise as the motor actually moves, but poorly so because of the missing high level on one phase.

The next step will be to either find an exchange for the driver chip since I can’t find a matching PCB, or find a way to replace the internal MOSFET bridge of the drive with an external one. If anyone knows where to get the actual L7251 3.1 datasheet, I’d be grateful for a hint.

EN_BLOG goes high

September 15, 2010February 20, 2012 | Mario

Hi there!

Since this is my first time blogging, i’ll just introduce myself for the moment.

I am a student of electrical engineering and information engineering (though I am not quite sure what it really translates to – well you get the idea!) at the Technical University of Darmstadt, Germany, currently 22 years old and I have been submerged in the world of electronics ever since primary school. I take a great interest in electrical/electronical and all kinds of technical craftsmanship, designing circuits and figuring stuff out – so I decided that now would be as good a time as ever to start publishing my own thoughts among the others that float around the net – possibly to help someone along the way on the same path.

Why I am writing this blog in english…I have loved the english language since I learned to read it and I read most of my books and materials in it (that is, if I can get it this way). The main purpose though is to enable a wider community to read what I write – and it is a wonderful new experience for me.

What you will get to see here – well, obviously mostly electronics stuff, but I don’t want to bind myself to it – other things might occasionally slip in.

Usually, I spend most of my time working with digital logic (hence the title) and microprocessors such as the famous ATMega series and it’s siblings, but right now I am digging into the world of switched power converters with special respect to LED highpower drivers and controllers. On other ends I do lots of audio stuff and am a great fan and thorough reader of boards such as www.diyaudio.com.

I guess this has to be it for now as I need to get back to studying for another few days. After that, there’s four week’s worth of holidays which will be well spent to equal parts on sleeping, getting out of this hole or whatever it has become during the last two months (the end-of-term-tests take QUITE a while here) and of course lots of soldery stuff :3

To give a few hints – on my workbench:

A switchmode LED driver small enough to fit inside a 2 cm flashlight head – complete with lithium ion battery management and mcu-controlled dimming and extra features.

A dead harddrive that I am currently reverse engineering a little while trying to find a way to repair the damage to its spindle control circuit.

The prototype for a pc-based surround-light-system to be installed on regular TFT displays

A driver circuit for my recently finished conversion of a CCFL TFT display to LED backlighting (based around the LT3518-IC produced by Linear Tech.) There’s also a friend in Norway waiting for the same thing for the same reason – Greetings to up there, by the way ;-)

A self-designed AVR-based LaserTag game system (though I do have to admit that it has parallels to the more present MilesTag, but hardware and software are different)

Some smaller reverse-engineery things (I really do like figuring out how things are made and how to get them to work in my own devices or make them do things they were never supposed to do in the first place) which have now become possible, thanks to a digital scope.

Loads of other interesting stuff, but got to keep myself focused for the moment and finish what has been left alone the most.

So enjoy your stay – I’ll be back for more in a few days.

~ice