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| General Electronics Chat This forum is for general chat about electronics, eg: Dont know what a part does? Dont know how to read a circuit? Want to get an opinion? |
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I'm looking to use some of the 4000 series logic chips at around 5MHz (mainly the 4017, AND gates and inverters) for a dot-matrix LED controller. I was wondering if these chips will work efficiently at these speeds? If so, what are their limits?
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| | #2 | |
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Get the data sheet for the HEF4017 from: Datasheet Archive - Free Datasheet Search Engine :: PDF Datasheets :: Data Sheet :: Datasheet :: Application Note : 的参数 : 规*书 : 技术资料 : 데이터시트 : データ・シート : Fiche technique : Datenblatt : описание Look at the bottom of page #4.
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| | #4 |
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The max recommended clock for a Texas Instruments CD4017 is 2.5MHz with a 5V supply and 5MHz with a 10V supply.
__________________ Uncle $crooge | |
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| | #6 |
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Looking at the datasheets, it would happily run at 5.5MHz at 15V. I was just wondering if anyone has had any experience running these chips are high frequency. If, so, what to look out for?
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| | #7 |
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At 15V and at 5.5MHz then some of the ordinary Cmos logic chips might be hot. They can't slew that fast so their Mosfets spend a lot of time in the linear region charging and discharging internal capacitance and getting hot, instead of being saturated where they are designed to be cool. 74HCxxxx high-speed-Cmos switch much quicker and have a lower supply voltage so they would be cool. My two-cents, I haven't tried it.
__________________ Uncle $crooge | |
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| | #8 |
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Yes, I'll go with the 74HC series. Thanks for the advice.
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| | #9 |
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Sorry to bump again but I am not worried about the 74HC04. Will not be able to sink enough current for my application. I need it to sink about 40mA (on the output pins). I'm assuming I will need to use a transistor to do the job, could anyone suggest to me how I could connect it up? I've attached a schematic to show how it's connected at the moment. The second attachment is just my idea.
__________________ when you post that reply, im just kidding. Last edited by danielsmusic; 26th October 2008 at 06:10 PM. | |
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| | #10 |
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The max allowed output current from a 74HCxx is 25mA and the max for a package is 50mA. Two inverters can be paralleled to drive 40mA. The LEDs need series current-limiting resistors. A single inverter can drive a PNP emitter-follower transistor and the LEDs still need series current-limiting resistors.
__________________ Uncle $crooge | |
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| | #11 |
| AG's right here, but parallel more than just two together. Our standard practice was to always use four; it's a bit more wiring but easier on the gates that way. Max current still 50mA...
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| | #12 |
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A single 74HCxx inverter or gate will try to make an output current of 60mA. Two in parallel will drive 40mA into LEDs perfectly.
__________________ Uncle $crooge | |
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| | #13 |
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Unfortuantly, because of size and cost requirements I cannot use more than one inverter per set of LEDs. Can anyone suggest to me a good transistor that would switch at around 1MHz? And recommend a good way to wire it?
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| | #14 |
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One 74HC04 costs only $.47US at Digikey today. It has has six inverters inside. The inverter can switch quickly because its output is push-pull. A single transistor turns off slowly but a complimentary push pull pair can switch quickly.
__________________ Uncle $crooge | |
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| | #15 |
| Digi-Key - 296-4024-5-ND (Texas Instruments - SN74ABT540N) Can go quite fast and pull 128mA to gnd on EVERY pin | |
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| capabilities, chip, frequency, logic |
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