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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4 Responses to Maxim 785 Power Supply Controller

  1. Frank says:

    Another great post!
    The process and layer identification is very helpful.

    I see a handful of laser trimmed resistors including one with what appears to be course and fine adjustment (Just above the VH pad). Maybe to tune in the oscillator frequency.

    The others (near the center of the chip) appear to be trimmed to achieve matching.

    • Gary says:

      Thats a really good observation!! Laser trimming of resistors is fairly uncommon, I was not expecting that. Tommorow I will update the post with an image of the laser trimmed resistor network.

  2. Frank says:

    Mask editing makes a lot of sense. The trims here are much cleaner than the ones on a die like this : https://imgur.com/a/y06Ur

    (This is a AD543 die, similar to the AD633 which was brought to my attention by YouTuber Kerry Wong he was sold some counterfeits on eBay, video here: https://www.youtube.com/watch?v=W0YRIjx8fjk)

    I think the metal mask was revised as well – you can see a serpentine resistor (located North-East of the trim detail photo) that was shortened by moving its metal contact further along from the end of the defined Si/Chrome feature. Another possible edit is just above the noted trim resistor you show – there is a resistor that has been shorted by the metal layer.

    Does the mask labels in the lower left of the chip denote mask number followed by a mask revision letter?

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