TTL supply voltage

Discussion in 'General Electronics Chat' started by count_volta, Dec 13, 2010.

  1. count_volta

    Thread Starter Active Member

    Feb 4, 2009
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    Hi, I have made a little project with TTL gates and flip flops. I am using a dc power supply as the power source. The dc adapter is rated as 5V, but actually outputs a little higher. I have connected it to my project, and it outputs 5.64V. I am wondering if the source voltage has to be exactly 5V or I could get away a little higher. I know TTL is sensitive to this.

    This is what the data sheet for the 7474 D flip flop says. The other chips have very similar characteristics.

    ABSOLUTE MAXIMUM RATINGS

    VCC Supply voltage –0.5 to +7.0 V

    RECOMMENDED OPERATING CONDITIONS

    MAX 5.5V

    From your experience, is 5.64V supply voltage safe for TTL chips, regardless of what the data sheet says?
     
  2. iONic

    AAC Fanatic!

    Nov 16, 2007
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    Your on the edge of reliability, but if you place a diode on the source you could hit 5.00V just perfectly.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    The Vcc range for 74xx series is 4.5v to 5.5v.

    If you operate the ICs outside of that Vcc range, don't expect them to perform to their guaranteed specifications.

    Many DC adapters aka "plug supplies" aka "wall warts" are not regulated; they have a specification like "5v @ 500mA" or similar; which means that when the load is 500mA, the nominal output voltage will be 5v.

    You can use a simple diode in series with the positive supply to reduce the adapters' voltage by 0.6 to 0.7v. A 1N400x series diode should work OK for up to a 1A load.

    Make certain to use a reasonably large capacitor after the diode to provide a reasonably constant 5v. Plug supplies typically have pretty small capacitors in them, if at all.

    You will also need a 0.1uF/100nF bypass cap across Vcc/GND on each and every 74xx series IC, otherwise you will have odd problems. See the sticky thread.
     
  4. count_volta

    Thread Starter Active Member

    Feb 4, 2009
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    I was going to use a diode anyway. Thanks. As far as the bypass capacitors on Vcc and gnd of each 74XX chip, I didn't do that. I'm using about 15 of these chips.

    My design works perfectly so far. I tested it many times. If I don't use the caps, will I have problems later? I soldered my design on a pcb board already. Everything is running perfectly.
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    I gave up on old high power TTL about 35 years ago when I discovered low power Cmos logic ICs. Ordinary CD4xxx Cmos ICs work from an unregulated supply from 3V to 18V.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    Then you need to add 15 0.1uF/100nF bypass capacitors, one per IC.

    Without the required bypass capacitors, the odds are not in your favor.

    If this is for an assignment, your instructor/professor should reduce your score for omitting the required bypass capacitors. Had you done such a thing for a project at a company, they might just show you the door.

    Bypass capacitors are not optional.
     
  7. count_volta

    Thread Starter Active Member

    Feb 4, 2009
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    About the bypass capacitors. This is my own personal project, not for a class. Also, hey I'm still learning right. We used many 74XX chips in class before and they never told us to add bypass capacitors. You guys know many things from experience that they don't teach us in school. That's why I ask you.
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    It seems that the teachers of electronics don't know anything about electronics.
     
  9. count_volta

    Thread Starter Active Member

    Feb 4, 2009
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    SgtWookie you mean like this?

    [​IMG]

    By the way, the clock of my logic is 1hz. I think the frequency is too small for this to make a real difference.
     
    Last edited: Dec 14, 2010
  10. beenthere

    Retired Moderator

    Apr 20, 2004
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    The bypass cap does not need to span the chip. It does need to be close to pin 14/16. On a PCB you make a connection to the ground plane.
     
  11. Audioguru

    New Member

    Dec 20, 2007
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    Your clock has a low frequency but the logic switches at a high frequency.

    The output of a logic IC switches very quickly with a fairly high current that must come from the bypass capacitor (isolated from the power supply by the inductance of the wiring). The switching frequency is 20MHz to 60MHz even when it switches only one time per second. Without a ceramic bypass capacitor then the power supply voltage to the IC bounces up and down at 20MHz to 60MHz each time the output switches that messes up the logic.
     
  12. count_volta

    Thread Starter Active Member

    Feb 4, 2009
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    I am not sure I understand your explanation of what switches at 20-60Mhz in the TTL. Then again all I know is that inside these chips are BJT's configured in weird ways that I never studied before. I am actually going to take a class where we study the inside of logic chips. Can't wait for it.

    Are you talking about the propagation delay between the rising edge of the clock and the time when the output reacts to the rising edge? Is this what is switching fast? The propagation delay is very small? I remember it was about 5ns.
     
    Last edited: Dec 14, 2010
  13. Audioguru

    New Member

    Dec 20, 2007
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    When a TTL output goes high or goes low it changes extremely fast which is a very high frequency transient. The output current is also fairly high which causes the supply voltage to jump up and down for a while at 20-60MHz if there is no high frequency bypass capacitor, which messes up the logic.
     
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