Maxim 785 Power Supply Controller

Another chip from the clamshell iBook from 2000.  Another Maxim power chip, this time it is from the main PCB.


Its a MAX785C which I am pretty sure is a dual (3.3V & 5V) PWM buck regulator for laptop computers.



I could not find a datasheet for the MAX785 but I did find one for the MAX786 which as you will see is almost certainly a minor change, or a slightly different version of the MAX785.

Here is the 4.6 mm x 2.8 mm dieMAX785 Die Photo (click on image for high resolution version)

For the MAX786 Maxim were once again kind enough to publish a chip topography 🙂 and you can see it matches the MAX785 die photo and pad layoutHere is my MAX785 die photo with the pads annotated, you can clearly see the symmetrical and  identical 3.3V & 5V PWM supply blocks with the 3.3V on the left.

Looking at the transistors zoomed in I can see a single Aluminum layer with 5 μm gate lengths and two polysilicon layers.

Referring to the Maxim Reliability Report I found whilst researching the earlier Maxim chip I can say with confidence the MAX785 was made on the Maxim SG5 process that looks something like this  (The transistors here are pretty drawn pretty ugly IMO)Its a CMOS process with a PNP transistor, zener diode and a Chrome/Si precision resistors. I have annotated the previous image with the layersThe layer 2 (PNP base drive) is/was unusual, they are making vertical PNP transistors.  In a standard CMOS process you can build lateral PNP transistors using the regular process.  However the gain of the lateral transistor is normally very low (As the base is defined lithographically you cannot make the base very narrow, unlike a diffused vertical base.)  Here is a large Bipolar transistor on this chip which is sandwiched between an two arrays of them to make  high current drive transistors.

Most of the large output transistors in the PWM blocks are I believe multi-finger MOSFET devices, here is a zoom image of one of them (It s hard to see the polysilicon gate as they have stitched metal lines along the gate, you can see the single gate contact at the very bottom right of the image.)


Update:  Laser trimmed resistors?

Frank commented that he could see some laser trimmed resistors, this was a very eagle eye observation! A bit of background, most silicon resistors are made with polysilicon or diffusions, and there are a number of variables that limit their accuracy such as thickness, width, dopant concentration, amount of dopant electrically activated during thermal processing.  It is typical that a resistor value is at best +/-10%.  For this reason most designs require only differential or ratio accuracy, and here the variables cancel out and you get very accurate matching.  Well over 90% (Perhaps 99%) of analog ic’s make do with these resistors. Occasionally a design needs an accurate absolute resistor, for these, a few processes (Like this Maxim SG5 process) offer a precision thin film resistor. These are made from thin (Typically 20nm-100nm) metal layer like Chrome used here.  I believe they can be made with +/-1-2 % accuracy. Sometimes a design needs even more absolute accuracy and for that they laser trim the thin film resistors, using a high power laser to ablate the metal track usually on a probe station where they measure the desired signal before and after trimming.

This is the section of the die where the thin film resistors have been cut.

It is a very complex serpentine that enables a wide variation in resistor value depending on where cuts are made.  After staring at this a bit and thinking some more, I don’t think they are laser trim cuts. The cut material looks too clean for laser ablation. What I think has happened is they designed the part with a un-cut serpentine resistor,  and then evaluated the initial prototypes making various laser cuts, and then used the results to change the resistor mask. In high volume, this would be much cheaper than trimming every die on a probe station.

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Texas Instruments TL494C PWM Control Circuit

The TL494C is a pulse width modulation control circuit, designed for power supply control, from the small power board on a clamshell iBook from 2000.

I have read the datasheet a few times and find it a quite complicated device.  I think is used to regulate the load when charging the battery, but I’m not very sure of that.

From the datasheet which was first published in January 1983 and revised March 2017, a 34 year old active document (And active part) that has to be some sort of record!

Onto the die photo which is really nice 🙂
(As always click on image for high resolution version)

The die size is 2.08 mm x 1.9 mm (3.95 mm2) another single metal Bipolar process.


From this diagram in the datasheet I was able to identify the pins.  The output transistors are obvious and I can see the error amplifiers and some of the other functional blocks



Here is my annotated die photo

There are some interesting devices on this chip, look at this 6 terminal beast in the error amplifier – unfortunately I have no idea what it is.

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Maxim MAX675 Voltage Reference

Do you remember the clamshell iBook from around 2000?  In my opinion it was one of Apples best ever products. Rugged and one of the first laptops with mass user appeal. I used one for several years, it was a bit heavy but I really liked it.


So I thought I take a look at some silicon inside one.




This is a small board that links the battery to the main PCB that is in the base nestled close to the hinge
You can see the battery connection in the bottom image (Top left). The board is clearly related to battery charging and the main power regulation to the board, but I couldn’t really figure out exactly what it does, and why it needs its own board. There are a couple chips that I thought I would take a look at, a Maxim MAX675 a 5V Precision Voltage Reference and the Texas Instruments TL494C Pulse Width Modulation Control Circuits.  Both had data sheets that I was able to locate.


The datasheet says the MAX675 takes a 15V input (From the battery) and outputs a precision 5V.




In the datasheet is this image showing the metal layout.  I have seen this before in datasheets, but only in very old ones.  I have no idea why they show it.And here is the die photo, which shows the metal layout is identical(click on image for high resolution version)

It is fabricated on a really old single metal Bipolar process, in fact I know that it is made on a 12 μm 24V Bipolar process. I know for certain as when I was looking for the datasheet I came across this document  (Maxim Product Reliability Report) published in Dec 1997.  It’s a strange document to be published on the internet, it shows Maxim’s reliability monitoring data on seven different processes as of 1996.  It also (again strangely) shows some details of the process including the layer thicknesses and a diagram of the process in cross section.  Here is the Bipolar process diagramIt is an old Bipolar process, made on <111> substrate. Very early on in 1960’s and 70’s Bipolar devices were made on <111> orientated silicon (I think because diffusions were faster).  MOS could only be made on <100> and eventually even Bipolar moved to <100> orientation. The epitaxial silicon (The layer of silicon that the active devices sit in that is grown after forming a patterned doped buried layer) is very thick 17 μm!   Here is the list of mask layers given

And if you look at the bottom of the die photo you can see the mask layer identification giving us a colour key for the die photo.The only difference is between 4 & 5 we have an extra layer 45.  I think 45 is the N+ emitter and layer 5  is a resistor option (Presumably the P base or N” emitter do not have the right sheet resistance (Ω/square) to make desired resistor values.  You can clearly see blue resistors that match the number 5 colour.

Whilst analyzing the die in the microscope, something looked odd, then it hit me, the N+ buried layer is massively mis-aligned by about 18μm! Look at the buried layer topography around this transistor.And again here

The N+ buried layer is a very low resistance path, necessary to lower the transistor collector resistance, which is a key transistor parameter allowing current flow from emitter to collector with a low ‘ON’ resistance. You can see the pattern in the die photo because an oxide mask is used when the dopant (Either Arsenic (As) or Antimony (Sb)) is diffused in to make the 4.5 μm deep buried layer region, this creates a step in the silicon that is propagated up through the epitaxial layer (And is necessary to align the subsequent layer to the buried layer). Clearly this was a working device, but it is hard to see how when for example in the transistor above the contact down through the epi to the buried layer is not even over-lapping the buried layer. I know of a phenomenon called epitaxial pattern shift that caused distortion,  but this is clearly only in horizontal plane, it appears perfectly aligned vertically. If it is epitaxial pattern shift it is shifting ~1μm for every microns silicon that is grown so ~45°.  Even if it is pattern shift how did they align to it?  It is quite the mystery to me.

I am going to publish the Texas Instruments TL494C as a separate post to keep the size manageable.


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My Silicon Decapsulation and Die Photo Process

Important Disclaimer:  Chip decapsulation involves using hot concentrated acids.  It should only be done by persons experienced at working with acids safely. This article is for entertainment/education please do not try this at home.

I have had readers ask to show the process I use for decapsulation, so here goes. There are only a few places on the web where amateurs are doing chip decapsulation and imaging chips. It is not tremendously difficult, but it does involve using hot acids. Something normally done in a laboratory setting within a fume hood/wet bench.  Doing this safely in a home environment takes some special care.

Equipment & Safety

Here is my set-upSafety is paramount. I do this for fun and don’t want my hobby to turn into a dangerous activity. So a few comments about my set-up regarding safety

  1. I always use gloves and eye protection. A bit of acid spilt on your skin will give you a burn, but you only have one pair of eyes!
  2. It is done under a range hood with a strong fan that vents outside.  A range hood that re-circulates the air is not suitable. This is critical as the hot acids will give off some nasty fumes.
  3. Only use small amounts of acid are used.  Most of my chips are small so I find using 10 ml beakers half full works fine for nearly all the chips I look at (Less than 8 ml of acid).
  4. A pipette is used to transfer acid to the beaker so I am not pouring large bottles of acid.
  5. There is a double containment. I built a glass tray (From an old oven door) and also have the hot plate in a metal tray.  Thus if I do spill a beaker of acid it is contained and easily cleaned up.

Decapsulation Acids

Most silicon chips in consumer products are bonded to a metal lead frame and then potted in a plastic compound.  To decap them you have to dissolve the plastic away using acids. There are three acids I know that can be used for decapsulation of silicon chips,  Sulphuric acid, Nitric Acid and Red Fuming Nitric Acid.  Fuming Nitric Acid is the most aggressive acid and can even decap most plastic packages without needing heat. However it is very expensive, very difficult to obtain.  And you really do not want to be using even small amounts of this outside of a proper lab web bench/fume hood containment.

Nitric vs Sulphuric

I have tried both Nitric and Sulphuric acids, both have their pluses and minuses

  • Nitric acid can decap at lower temperatures ~110°C
  • It is more aggressive etching plastics and can result in cleaner decaps.     – However
  • Nitric acid tends to attack metals very strongly so the bond pad area and metal leading off the bond pad gets etched away.
  • It is expensive and harder to obtain


  • Sulphuric acid does not attack the metals so aggressively
  • It is significantly cheaper.   – However
  • You have to heat it to higher temperatures ~260°C.
  • (Because it does not attack the bond pads as much) you are usually left with ultra-thin gold bond wires still attached that you have to pluck off the die.

I have taken to using Sulphuric in recent months. I bought the cheapest grade (Technical grade.) You can see the acid is pretty brown, it should be clear.  It was like that when I bought it, the discolouration is apparantly caused by a small amount ~1% of organic contamination (Perhaps from being in a plastic container too long).  For what I need it does not matter, the decapsulation process is very dirty.

Decapsulation Process

I use a cover glass over the beaker to contain fumes (though it is intentionally not air tight so gases can escape though the beaker lip).  The pyrex jug is for collecting the waste acid (A viscous black sludge) that I dispose of as hazardous waste. In almost a year and nearly 200 decaps I only have about 1L collected.

Here is my target chip. I usually cut the leads off.  A small chip like this goes straight in; a larger die I may cut, or sand off some of the plastic to reduce the amount of work the acid needs to do.




I heat the acid up for ~5 mins until it reaches ~250°C  (Not quite there yet in this shot).  When you lift the cover glass, there are fumes clearly coming off.




Into the acid it goes.



After about 30 secs the acid is starting to turn black as the plastic is dissolved.





A minute in and the acid is now black.




I leave the chip in the acid at ~255°C for 10 mins.  It looks like each chip needs a slightly different time for perfect results, but normally 10 mins is good.



Decant and Slide Preparation

After 10 mins, I take it off the heat and allow to cool for ~3 mins, then I drain off the liquid.  and rinse in acetone  (Note: adding acetone to acid is extremely dangerous, you have to ensure the beaker is cool and only a tiny amount of liquid sludge remains) and I then tip the contents out onto kitchen paper.

What you see here is small metal fragments (the lead frame and bond wire attachments) and a small die, can you see the die? If not you are not alone,this is a very small die (0.5 mm x 1.5 mm) it literally took me two to three minutes to find it amongst the lead frame and other small pieces of metal. I can barely see it without a magnifying glass.

I use a magnifying glass with light.  At this stage (If I used Sulphuric acid) I usually need to pluck bond wires off of the die, using tweezers. This is a difficult process and a stereo microscope would be better, but I don’t have one of those 🙁  I use teflon plastic tweezers to avoid/reduce damage to the die.This is a process that I still need to improve at, as I frequently scratch the die trying to pull off these really thin (Typically ~25 μm diameter) wires that are difficult to grab with the tweezers. (I can barely see them in the magnifier).After this I clean the die up in a beaker of acetone sitting in an ultrasonic bath of warm water. A cheap ultrasonic jewelry cleaner for a few minutes does the job.The die is then mounted onto a microscope slide.  It is important the die is perfectly clean on the underside and sits flat on the slide.  To hold it on the slide I  use a strip of double sided tape.

Here is the small die ready for the microscope.


It’s a metallurgical microscope (Meaning the light hits the specimen from above through the objective.) Manufactured by Olympus in the mid 1980’s. The rate of progress of optical microscopes has slowed down, with very little improvements in recent time. This 30 year old Olympus BH2 microscope likely still has better optics than any consumer level microscopes you can buy new today (Such as OMAX brand etc.).  I made one change to the microscope. The lamp housing had broken off so I removed it (with its 100W tungsten filament bulb) and retrofitted an LED light source. I designed and built the LED light and it has been working very well.

The microscope has a trinocular head with a Canon DSLR camera (EOS 1300D) to capture the images. I use Canon’s EOS Utility live view software to display the image on my PC.  The camera is set-up so that it is parfocal with the eyepiece objective. That means when it is in focus in the eyepieces it is in focus on the camera.  Which is key to making it easy to use.  There are four Olympus NeoDplan objectives 5x (NA=0.1), 10x (NA=0.25), 20x (NA=0.4) and 80x.  The 80x objective has an NA=0.9 but only a 0.18mm working distance. For imaging bare silicon die is ok as they are pretty flat surfaces. The NA (Numerical aperture) is important because it is the NA that defines what you can resolve not the magnification. You can zoom an image taken with 5x objective lens to the same magnification as one taken with 20x or 80x objective, but you will not resolve the same features because the NA is lower.

Die Photo

My die photos are typically imaged with the 10x or 20x objectives. They are a mosaic of images that are stitched together. I take between 10 and 120 images depending on the size of the die and objective used. Between images the stage is moved manually. I would like to acquire an automatic x-y stage that has servo motors moving the slides more efficiently.

To stitch the images together I use Hugin which does a good job but is not so easy to use. Ken Sherriff published a very useful tutorial for how to use Hugin to stitch die photos.  Once assembled I then use another good but complex software tool, Raw Therapee to adjust contrast and tone.  It can also do a Richardson-Lucy deconvolution which I have found to work quite well at sharpening my silicon chip microscope images taken with the lower NA objectives.

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