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02/14/05 01:59
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 		 #87306 - To ULN or not to ULN
Responding to: ???'s previous message
Hi,

... ULN2803/4. If you keep in mind, that maximum current per output is only 200mA
But we have this sentence in the first page of the datasheet:
Output current(single output) 500mA

then only 200mA / 32 = 6.25mA is present per LED
In order to drive an LED mltiplexed in a 32 column arrangement, the needed current to have the same amount of light when driven constantly with say 20mA is NOT 32x20mA, it's √32x20mA=~5x20mA=100mA. Refer to this article from Allegro:
http://www.allegromicro.com/techpub2/...95031.pdf:
"Multiplexed displays must typically be operated at a current √n times the nominal operating current (where n is the number of digits)".
You definitely know the "inertia of vision" or "persistence of vision" phnomenon. The thing with this phenomenon s that human eye does not "calculate" a precise "integral" of the pulses, instead it "see"s a rather bigger amount of light than just an "integral" of pulses, that's why 5x20mA is what we need instead of 32x20mA.

Best.

List of 242 messages in thread
TopicAuthorDate
BIG I/O expansion            01/01/70 00:00      
   Like I said            01/01/70 00:00      
      Carry on            01/01/70 00:00      
   As i said before            01/01/70 00:00      
   Fanout            01/01/70 00:00      
      Great idea, but 2 things            01/01/70 00:00      
         Thanx Sasha            01/01/70 00:00      
   Why?            01/01/70 00:00      
      Yep, nice guess            01/01/70 00:00      
   outputs            01/01/70 00:00      
      I really need'em and can't multiplex'em            01/01/70 00:00      
         But why??            01/01/70 00:00      
         Can't you go serial?            01/01/70 00:00      
            Serial??            01/01/70 00:00      
            output of a CPLD            01/01/70 00:00      
               using CPLD outputs ..            01/01/70 00:00      
                  To buffer or not to buffer            01/01/70 00:00      
                     to buffer or not            01/01/70 00:00      
   timing problems            01/01/70 00:00      
   what on earth are you doing            01/01/70 00:00      
      Exactly!            01/01/70 00:00      
   complaints and more info.            01/01/70 00:00      
      Don't worry            01/01/70 00:00      
      Consensus - not conspiracy!            01/01/70 00:00      
      Head Bashing            01/01/70 00:00      
         Head banging            01/01/70 00:00      
      more questions            01/01/70 00:00      
      Summary of data and a suggestion            01/01/70 00:00      
         A typo correction.            01/01/70 00:00      
   well            01/01/70 00:00      
   More info. on the project            01/01/70 00:00      
      More info?            01/01/70 00:00      
      thevenin terminations on LE signals            01/01/70 00:00      
         Cable            01/01/70 00:00      
            Termination must exactly match!            01/01/70 00:00      
               Explanation            01/01/70 00:00      
   Stranger and stranger            01/01/70 00:00      
      Humor            01/01/70 00:00      
         Russel            01/01/70 00:00      
            tolerance goes both ways            01/01/70 00:00      
   Bingo !            01/01/70 00:00      
   Bingo !            01/01/70 00:00      
      repeating myself            01/01/70 00:00      
         bare CPLD            01/01/70 00:00      
            Not professional?            01/01/70 00:00      
   Forget about the second Bingo ! :)            01/01/70 00:00      
      Payam            01/01/70 00:00      
         The new decoder            01/01/70 00:00      
   Cos i am just too good to you :)            01/01/70 00:00      
      Wouldn't this be a bit expensive?            01/01/70 00:00      
   Trifle expensive            01/01/70 00:00      
      synchronous design            01/01/70 00:00      
         synchronicity            01/01/70 00:00      
   Not really            01/01/70 00:00      
      ...but there are clocks            01/01/70 00:00      
   a total mess            01/01/70 00:00      
      Dear Eric            01/01/70 00:00      
         Why didn't you just say....            01/01/70 00:00      
            Sub-micros and RS485            01/01/70 00:00      
         wrong approach            01/01/70 00:00      
            here are the links (I forgot)            01/01/70 00:00      
            Eric            01/01/70 00:00      
               serial/parallel            01/01/70 00:00      
                  Cables, Multiplexing            01/01/70 00:00      
                     more on MPX            01/01/70 00:00      
                        sunlight            01/01/70 00:00      
                           more on sunlight            01/01/70 00:00      
                              Could you please explain more?            01/01/70 00:00      
                                 Where is the current limiting from            01/01/70 00:00      
                                 more explaining            01/01/70 00:00      
                           LEDs are in danger!            01/01/70 00:00      
                              4538ing is nice            01/01/70 00:00      
                              What about this            01/01/70 00:00      
                              instead of 4538            01/01/70 00:00      
                                 all over the globe            01/01/70 00:00      
                                    RE: to do what            01/01/70 00:00      
                                       no need if non-mpx            01/01/70 00:00      
                  Question to Erik            01/01/70 00:00      
                     ST2221            01/01/70 00:00      
               EMI!            01/01/70 00:00      
                  not out, but in            01/01/70 00:00      
                  Transmitting parallel            01/01/70 00:00      
                     Series termination            01/01/70 00:00      
                        correct impedance of series termination            01/01/70 00:00      
                           termination            01/01/70 00:00      
                              Output current            01/01/70 00:00      
                                 Kai, is that right?            01/01/70 00:00      
                                    Series termination methode            01/01/70 00:00      
                                       serial termination, got it            01/01/70 00:00      
                                    series termination            01/01/70 00:00      
                                 cable of 100Ohm            01/01/70 00:00      
                                    Of course!            01/01/70 00:00      
                              used for parallel of thevenin terminatio            01/01/70 00:00      
                           Impedance of flat ribbon cable            01/01/70 00:00      
                              anyhow, termination may not be needed            01/01/70 00:00      
                                 Is needed for 2m long cables!            01/01/70 00:00      
                                    that long not needed            01/01/70 00:00      
                                       Agreed            01/01/70 00:00      
                                 re: termination may not be needed            01/01/70 00:00      
                              Thanx Kai            01/01/70 00:00      
                              Flat cable characteristic impedance            01/01/70 00:00      
                                 Look closely...            01/01/70 00:00      
                                    the the            01/01/70 00:00      
                                    Doh!            01/01/70 00:00      
                                       Initial Misconception            01/01/70 00:00      
                                          I can't even spell that...            01/01/70 00:00      
                        ferrite            01/01/70 00:00      
                           ground is part of signal path            01/01/70 00:00      
                              Vcc IS part of the signal path            01/01/70 00:00      
                                 Vcc PIN is part of signal path            01/01/70 00:00      
                           Depends on what you want to filter            01/01/70 00:00      
                              PC power supply            01/01/70 00:00      
                                 4700micro is too high            01/01/70 00:00      
                                    paralell            01/01/70 00:00      
                                       This cannot work!            01/01/70 00:00      
                                          we have a problem            01/01/70 00:00      
                                             Kai, Eric            01/01/70 00:00      
                                                you need to think            01/01/70 00:00      
                                                paralell resistor current limiter !            01/01/70 00:00      
                                                   Thanks Farshid, Reinforcing PC power sup            01/01/70 00:00      
                                                      Reinforcing PC power            01/01/70 00:00      
   Jan            01/01/70 00:00      
      We don't understand each other            01/01/70 00:00      
         What if only BIG I expansion            01/01/70 00:00      
            Your Own Thread            01/01/70 00:00      
      Metastability            01/01/70 00:00      
   how about            01/01/70 00:00      
   Maybe            01/01/70 00:00      
      Divide the problem            01/01/70 00:00      
      Own ideas            01/01/70 00:00      
      sidesteps            01/01/70 00:00      
   Thanks            01/01/70 00:00      
   I dunno Kai..            01/01/70 00:00      
      I dunnoJez            01/01/70 00:00      
      Agreed            01/01/70 00:00      
         CE radiation            01/01/70 00:00      
   Thanx for criticisims from Eric & Kai            01/01/70 00:00      
      the original poster            01/01/70 00:00      
         Eric, Kai            01/01/70 00:00      
            if you can't explain            01/01/70 00:00      
               paralell resistor current limiter !            01/01/70 00:00      
      Then, why don't you discuss it?            01/01/70 00:00      
         taking our warnings seriously and discus            01/01/70 00:00      
      world record            01/01/70 00:00      
         world record +1            01/01/70 00:00      
   Eric            01/01/70 00:00      
      Cost Is Final Issue            01/01/70 00:00      
         lowest cost            01/01/70 00:00      
            Yeah.......            01/01/70 00:00      
            now drop the multiplex            01/01/70 00:00      
      Let's compare parallel vs. serial            01/01/70 00:00      
         Time to confess            01/01/70 00:00      
            simple bitbang            01/01/70 00:00      
               something ain't mising here?            01/01/70 00:00      
                  more            01/01/70 00:00      
               not even that...            01/01/70 00:00      
                  you can, but            01/01/70 00:00      
                  Cool            01/01/70 00:00      
      urband legends            01/01/70 00:00      
      No influence            01/01/70 00:00      
   This heart of mine on fire            01/01/70 00:00      
      HC595's and ULN2803's            01/01/70 00:00      
         typpo            01/01/70 00:00      
         column xistors            01/01/70 00:00      
            Instantaneous!            01/01/70 00:00      
               saturation and off state not on and off            01/01/70 00:00      
                  What is a 'xistor'??            01/01/70 00:00      
                     Okay, transistor            01/01/70 00:00      
                     xistor            01/01/70 00:00      
                  Saturation and Gain            01/01/70 00:00      
                     FETs and gate capacitance            01/01/70 00:00      
                        Fets Vs Transistors            01/01/70 00:00      
                           Which MOSFET            01/01/70 00:00      
                           More Qs on P-MOSFETs            01/01/70 00:00      
                              Best text ...            01/01/70 00:00      
                              Mosfets            01/01/70 00:00      
                              Low 'Ugs' and low 'Ron' is crucial!            01/01/70 00:00      
                                 Bingo!            01/01/70 00:00      
                                 Soft-start circuit            01/01/70 00:00      
      answers and comments            01/01/70 00:00      
         ghosting            01/01/70 00:00      
            what            01/01/70 00:00      
               ghosting            01/01/70 00:00      
                  ok            01/01/70 00:00      
                     this is probably not the case with Payam            01/01/70 00:00      
                        no, worse            01/01/70 00:00      
            P-MOSFETs            01/01/70 00:00      
         typicals            01/01/70 00:00      
            These probably might go            01/01/70 00:00      
            yes, you are and so what            01/01/70 00:00      
               "them" explained            01/01/70 00:00      
   another solution            01/01/70 00:00      
   Serial is a must!            01/01/70 00:00      
      Serial is not a must!            01/01/70 00:00      
   Additional facts            01/01/70 00:00      
      Multiprocessor?            01/01/70 00:00      
      To ULN or not to ULN            01/01/70 00:00      
         ULN2803, human eye response            01/01/70 00:00      
            typical again            01/01/70 00:00      
         consider everything            01/01/70 00:00      
         Equivalent brightness with pulsing            01/01/70 00:00      
      Re: Additional facts            01/01/70 00:00      
         You put many of them...            01/01/70 00:00      
            Re: You put many of them            01/01/70 00:00      
               the matrix revisited            01/01/70 00:00      
   here are some more additional facts            01/01/70 00:00      
      NFI            01/01/70 00:00      
         Me, not subtle ???            01/01/70 00:00      
            Subtle as a brick!            01/01/70 00:00      
   one little detail            01/01/70 00:00      
      30° is it enough?            01/01/70 00:00      
   Calculating ... please wait!            01/01/70 00:00      
      answers/comments            01/01/70 00:00      
         six-layer boards?            01/01/70 00:00      
            no 2 layers possible            01/01/70 00:00      
               6 layer for 20$???            01/01/70 00:00      
                  here            01/01/70 00:00      
                     PCB discussion extracted            01/01/70 00:00      
               a trace about 1 inch            01/01/70 00:00      
                  20A on a 2mm track!            01/01/70 00:00      
                  holy smoke            01/01/70 00:00      
                     Okay.            01/01/70 00:00      
                        again            01/01/70 00:00      
                           colloquialism            01/01/70 00:00      
               Calculating a bit...            01/01/70 00:00      
                  'Three' layer design            01/01/70 00:00      
                     layers - LED - EMI            01/01/70 00:00      
                        LEDs don't understand ground bounce            01/01/70 00:00      
                           But the four 74HC595 and ULN2804 do!            01/01/70 00:00      
                              595s don't, ULN ... not important            01/01/70 00:00      
                                 not so            01/01/70 00:00      
                           Ground bounce            01/01/70 00:00      
                              Sure, shall take care of it.            01/01/70 00:00      
      Don't underestimate heat dissipation!            01/01/70 00:00      
   Kai            01/01/70 00:00      
      Payam            01/01/70 00:00      
         less complex/less expensive            01/01/70 00:00      
         Did you try the multiplex?            01/01/70 00:00      
            Yes            01/01/70 00:00      
            go ahead and try            01/01/70 00:00      
            Square root increase            01/01/70 00:00      
               Distributed power supply            01/01/70 00:00      

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