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 🙂
TL494C die photo(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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6 Responses to Texas Instruments TL494C PWM Control Circuit

  1. Frank says:

    The “beast” is a split collector PNP. Each of the collector segments around the emitter center give a ratio of the reference current (Where is the base?).

    Notice largest collector segment biases the input differential pair of the op-amp.

    See slide 20 in this presentation: http://www.diegm.uniud.it/driussi/teaching/dispense/CurrentMirrors.pdf

    Also mentioned in chapter 4 in “Bipolar and MOS Analog Integrated Circuit Design” by Alan B. Grebene

    Beautiful photos as always, keep up the good work!

    • Gary says:

      Thanks thats interesting! knowing that I looked carefully at both devices on the die photo and I think the base contact is the long contact in the centre and bottom of the image. The 5 salmon pink regions (p collectors which looks to be the same diffusion as the general p isolation) are surrounded by the light green n-epi, which in turn is surrounded by a deep blue diffusion which I think is an N++ sinker diffusion. The only contact to that is the long one at the bottom which I think is the base contact.
      The paper you linked to is a very good read on current mirrors. I have often wished I had chosen analog design as a career, but like reading that I find it gets complicated very quickly and I am not smart enough 🙂

      • Frank says:

        Good job spotting the base.
        In the last photo it appears the base connects goes through a diode (? the structure directly to left of the “beast”) to the collector which is then driven by an NPN from the voltage reference (makes sense to drive the op-amp bias circuit from the on-board voltage reference).

        However if you look at the same structure on the other op-amp (left side of the chip) you see base and collector directly connected (looking below the “beast” and below the elliptical emitter structure) in the traditional diode connected way of a current mirror. I wonder why the asymmetry.

        With regards to the op-amps, compare the layout with the TS321 (https://zeptobars.com/en/read/ST-TS321-SOT23-opamp-LM358A-LM324). A lot of similarities in the input stage. Notice the split collector PNP on the TS321 (bottom-middle) only has 3 collectors – the input darlingtons aren’t being biased like the ones on the TL494 error amps.

        You certainly know your fab processes!
        Another great resource for information on this older analog layout stuff is https://archive.org/details/bitsavers_signeticsdcsAnalogApplications_33415016

        • Gary says:

          That Signetics document is a great link. I know the fundamental analog circuit layouts in ic’s are still relevant today. I have downloaded the pdf and will spend some time with it. I swear I can recall seeing that book cover in the first lab I worked in (Back in 1981)!

        • Frank says:

          Correction: these input stages aren’t darlingtons – see https://www.eevblog.com/forum/projects/trivia-lm193/ for discussion.

          Also see https://www.onsemi.com/pub/Collateral/LM324-D.PDF for the full schematic of the TS321/LM324. This shows all transistors, allowing to identify some more features present in the TL494 error amps.

          For example Q2 & Q5 on the LM324 schematic are present in the TL494 error amps and are being biased by the “beast”.

          This is one of those situations where it’s difficult to identify structures because when going from schematic to layout they merged devices (Q2 merged with Q3 and Q4 merged with Q5 on the LM324 schematic).

          Merged devices are discussed in Chapter 13 of “The Art of Analog Layout” by Alan Hastings.

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