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7.8mA

  • ebpDoug Wednesday, February 5, 2014 10:01 AM Reply
    An SN74HC595N is what is being sold here. SN is an ancient TI prefix dating back to their earliest days of TTL. These specific parts may not have the SN prefix because they may be from a different manufacturer. There used to be a lot of 74HC/HCT manufacturers, but some have quit as the parts are dropping into obsolescence. All are essentially equivalent, though there may be very slight differences. NXP (formerly Philips, with much of their older logic lines having come from their acquisition of Signetics) still make the HC595 (no pro would omit the HC) . None would drive more than the parts being sold here.
    I did a little searching. There are derivative parts made to be logically functional but with high-current driver outputs, such as the STPIC6D595 from ST Microelectronics - drop all of the prefix characters at your peril. This is obviously the sort of part hobbyists are confusing with standard HC logic. DX's supplier Jtron is probably doing the same thing, thinking they are offering special instead of a jellybean part.
    [edit] From TI data sheet for SN74HC595 "• ±6-mA Output Drive at 5 V". I think TI actually quit making the HC/HCT family, but are now doing it again since they acquired National Semiconductor. I note the data sheet is dated 1982, revised 2009

    ~~~
    TxCharlie,
    Oh lordy, Poly Paks. I used to buy a few things from them from time to time back in the good old days when shipping from the US to Canada was reasonable - back in the days when DigiKey was a little hole in the wall outfit with a 2 page "catalog" (did you know that the ND at the end of DigiKey part numbers stands for No Discount?) , nobody had heard of Future Electronics and Hamilton-Avnet and Arrow were kings. I remember the days when the HC and HCT families were being introduced and starting to swipe a little market share from LS. These days I do mostly analog design, so I haven't kept up with all of the new CMOS logic families. I still, on rare occasion, use 4000 series CMOS (or COSMOS, as it was originally designated by the good people at RCA).
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    post edited by ebpDoug on 2/5/2014 at 10:09 AM
  • firethorn Wednesday, February 5, 2014 10:21 AM Reply
    You have totally missed the point. This part according to the description has a max current of 7.8mA.

    According to the datasheet of the SN75HC595N, it's about 35mA.

    And ±6-mA is not the same as 6mA. That's a rookie mistake.

    So... I'm not sure what exactly you are trying to say since you have mistaken the main point. It's mostly a lot of words totally missing the main issue.

    Unless you're saying that these are standard parts equal to standard SN74HC595Ns with underrated current capacity. Is this what you are trying to say?
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    post edited by firethorn on 2/5/2014 at 10:25 AM
  • ebpDoug Wednesday, February 5, 2014 12:22 PM Reply
    ±6mA means the part can source (conventional current flowing from the positive supply out of the output) or sink (conventional current flowing into the output pin) 6mA while operating with specified voltage margins for the specified logic levels. ± means the drive is symmetrical - equal drive for logic high and low, unlike TTL which could sink much more current than it could source.
    The ±35mA figure (again, sourcing or sinking) is the absolute maximum current allowable. The part is not intended to be operated at that level. The output voltage, either high or low, at 35mA simply isn't specified. If it isn't specified, you cannot expect it to work in a repeatable fashion. The voltage at the pins at 35mA will vary from unit to unit and with temperature as the transconductance of the FETs varies. At that magnitude of current, much of the supply voltage will appear across the channel resistance of the FET that is on. You might get away with it, but it is beyond specifications and will lead to high and potentially destructive power dissipation. To use it that way is bad design, unless you don't care about reliability and consistency.
    A further limitation is the supply and ground pin currents. From the TI data sheet for the SN74HC595:
    "Continuous current through VCC or GND ±70 mA"
    That's the total current through those pins. Source 25mA from three outputs at once and you have exceeded the supply pin current. Sink 35mA at two outputs and you reach the absolute maximum for the ground pin current.

    From the NXP data sheet:
    Qn bus driver outputs (for output High state)
    IO = 6 mA; VCC = 4.5 V @ -40°C to +85°C 3.84 minimum 4.32 typical, maximum not spec'd @ -40 to 125°C 3.7 minimum, typical and maximum not spec'd

    IO = 7.8 mA; VCC = 6.0 V 5.34 5.81 min-typ -40 to 85, 5.2 min -40-+125

    There is your 7.8mA specification. The bus outputs can source 7.8mA while guaranteeing an output voltage of at least 5.34 volts and typically 5.81 volts when the part is operated from a 6 volt supply at between -40°C and +85°C. 6 volts is the highest recommended operating supply voltage (7V abs max). You will find the 7.8mA figure in the TI data sheet, too.

    All of the above specifications will apply to the parts being sold here, unless they are "seconds", which seems unlikely. DX and Jtron often get things wrong. In this case they didn't. The used a specification for the part as it is designed to be used.
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  • firethorn Wednesday, February 5, 2014 1:41 PM Reply
    pointless.
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  • sheepish Top 10 Forum Poster Wednesday, February 5, 2014 2:06 PM Reply
    There were several points in that post. Did you miss them?
    Remembering 30 years.
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  • firethorn Wednesday, February 5, 2014 2:23 PM Reply
    No. Did you?

    You realise he's agreed with what I was pointing out don't you?
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  • sheepish Top 10 Forum Poster Wednesday, February 5, 2014 2:30 PM Reply
    No. Did you?

    There were several points in that post.

    Duh.

    You realise he's agreed with what I was pointing out don't you?

    You mean the 7.8mA figure? Sure. It's what they're rated for. You mean the "but it's not right/fair/how I misuse mine"? Well, you live and learn. You might want to do something about it.
    Remembering 30 years.
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  • firethorn Wednesday, February 5, 2014 2:35 PM Reply
    @sheepish:
    I'm not certain you fully understand what is happening.
    1) Read the product description on dx.
    2) Read his last post. The first sentence with text in bold.
    3) Read the product description on dx again.

    So much back and forth, before he finally read the datasheet. You might want to think about doing the same.
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  • sheepish Top 10 Forum Poster Wednesday, February 5, 2014 2:41 PM Reply
    I am certain you do not understand DX. You might want to try reading what people have been saying.
    Remembering 30 years.
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  • firethorn Wednesday, February 5, 2014 2:46 PM Reply
    I cannot understand what you are saying. Honestly, contents of this thread aside, I really don't.

    If you're trying to say that dx has inaccurate information, then it looks like you may have missed out his last paragraph in his last post.
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