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Is there a practical difference between Open Drain and Open Collector outputs, or are the terms used interchangably? If they are indeed different, what are the contexts where each is advantageous? My hunch is that they are functionally equivalent, but Open Drain is implemented with FET technology while Open Collector is implemented with BJT technology.

vicatcu
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1 Answers1

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That's right, open collector is BJT, open drain (C)MOS. In logic circuits where currents are low the saturation voltage of the BJT may be a bit higher than the voltage drop due to \$R_{DS(ON)}\$ for the FET, but it still will be much lower than the maximum voltage for a logic low.

Exception: you can't use a TTL open collector output as a level shifter to interface with very low voltage CMOS. This inverter can operate on a Vcc as low as 0.8 V, and the datasheet gives a maximum low level of 0.25 Vcc, that's 0.2 V. An open collector will have a voltage drop higher than that. (This device even specifies a maximum low level input voltage of 0 V, which is simply impossible.)

note
Apart from wired-ORing (think I2C) open drain/collector is often used to control something operating at a different voltage than the logic, often a higher voltage. With some low-voltage logic families this no longer possible, since the FETs will not tolerate the high voltage and for that reason some families will have clamping diodes built-in to protect the output FETs.

stevenvh
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    They probably forgot to say 0V +-10% :-) – Olin Lathrop Aug 04 '12 at 11:59
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    @OlinLathrop that saying in datasheets kills me. Every time I see it I throw something. – Kortuk Aug 04 '12 at 13:28
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    @Kortuk-SMASH!! – vicatcu Aug 04 '12 at 13:58
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    Hey @vicatcu, may I remind you that you're in *my* living room! That goes for you too, Kortuk. – stevenvh Aug 04 '12 at 13:59
  • Hmmm. Has anyone tried using the link on the TI data sheet called "Submit Documentation Feedback" yet? – zebonaut Aug 12 '12 at 20:35
  • @zebonaut - I have now! I'll check the mailbox later this morning for the new version of the datasheet :-) – stevenvh Aug 13 '12 at 07:14
  • @stevenvh - Heh, just be careful: Writing to TI, Maxim, National (now TI) and whoever about strange stuff in their data sheets and applications notes may become a bit of an addictive hobby. Don't ask how I know. – zebonaut Aug 13 '12 at 07:42
  • @zebonaut - You think I'm doomed now? It was only my first! :-) – stevenvh Aug 13 '12 at 07:44
  • @stevenvh - There's still one way out: If you're lucky, they don't tell you that they will change the data sheet, but instead will ask you to use their own E&E community forum for advice. Like, Dames en Heren, yeah: If I need reliable data for a part in some industrial circuit, I don't care about Min/Max specifications, I don't even care about typical specs, what I prefer is some gossip related to me on a manufacturer's E&E community by happy users or sales representatives. – zebonaut Aug 13 '12 at 08:01
  • @stevenvh: no updated datasheet so far. – Federico Russo Jun 15 '13 at 10:54
  • Many data sheets require nearly-impossible tolerances for things. Consider, for example, what would be necessary to design a circuit to work 100% reliably if there were no guarantee that a part could tolerate an input being one microvolt above VDD, nor that an output wouldn't swing fully to full VDD. Since it's not uncommon for different parts on a board to have VDD differ by a microvolt, ensuring (using only data sheets) that a part whose VDD was 3.000001V couldn't overdrive a part whose VDD was 2.999999V would be annoyingly difficult. Of course, in practice such issues are ignored. – supercat Aug 21 '14 at 17:32