Nat Semi LM3485 Buck Controller

The datasheet calls this a Hysteretic PFET Buck Controller.  I had to look up “hysteretic” and even knowing it was related to hysteresis I still don’t fully grasp what it means?!  Whilst I am discussing technical english –  ‘buck’ controller means voltage step-down (vs. Boost = step-up)  where does buck come from?  I have always wondered why not just use step-down?

The National Semiconductor LM3485 was designed in 2001 and is a 1.7mm x 1.7mm die fabricated with a 2 metal analog process

This is the functional block diagram from the data sheet

The PMOS driver is the large structure at the top of the die (Either the one on the left or in the middle 🙂 )  There are a ton of different transistor structures, many of them look like Bipolar transistors, but to confirm one would need to remove the metals and stain the junctions or use scanning capacitance to resolve the junction types (Neither of which I can do.)  In this image I can count four different transistor types (I think circular emitter NPN and PNPs, and rectangular NPN, and PNP) – I do not know why they chose to use all these types.

And then there is this (Bipolar) transistor – a very odd arrangement.


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2 Responses to Nat Semi LM3485 Buck Controller

  1. Patrick M says:

    Hello. I love your website. The tear-down microscope pictures are absolutely stunning. Great Work. Please let me know if it is possible to use some of your die pictures on my website A stellar acknowledgment will be attached. The website construction is still in progress with an approximate 300 more entries to add.

    Best regards.

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