Spec sheet interpretation

Comments

• DaveG Posts: 84

  2005-10-14 23:32 edited 2005-10-14 23:32

  Howdy,
  
  I believe a load of 15ma is way too much for the outputs of the 74HCT573.
  The spec sheet says the output drive current of each output is +-6ma, which means it can only source or sink 6ma.
  It also says each output can drive 15 LSTTL loads. 400ua(each LS load) x 15 = 6ma.
  You could possibly use a 74ACT573 which is rated at 24ma. IC buffers are also an option.
  
  However, I always use discrete devices like 2N3904s or 2N7000s to drive relays. Relays are inherently noisy and the more
  isolation the better. And be sure to put a diode across the relay coil.
  
  Dave G
• DaveG Posts: 84

  2005-10-15 04:24 edited 2005-10-15 04:24

  Howdy again,
  
  I finally looked at the Crydom relay spec sheet. In my previous post I suggested putting a diode across the coil.
  However, since the SCR relay doesn't have a coil, the diode isn't needed.
  
  I still would use an open-collector buffer chip or discrete drivers instead of driving directly from a 74ACT573, because
  the high output voltage of a 74ACT573 could be as low as 3.8v would may cause turn-on/turn-off problems.
  
  Dave G
• LoopyByteloose Posts: 12,537

  2005-10-15 11:37 edited 2005-10-15 11:37

  You also have to look at how much power the chip will handle as a total.
  
  If you assume that all 8 channels will be on at the same time....
  
  15ma X 8 = 400ma. Maybe it can, but maybe it can't - That 'Continuous current thru Vcc or Ground' does not look good.
  You might stand back with a fire extinguisher handy while you try.
  
  [noparse][[/noparse]I think it is telling you 70ma for the whole package, but you are reading optimistically].
  
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• walkbones Posts: 30

  2005-10-17 18:09 edited 2005-10-17 18:09

  Thanks for the advice.
  
  Dave, you said the 74ACT573 might be an option because it is rated +- 24ma. In one case we have, there is a 74HCT573 that is controlling reed relays that are 5v and 700 ohm (about 7ma each). It looks like the 74HCT573 can be directly replaced by the 74ACT573 without the need to remake the pcb because the pinouts are the same. Do you agree with that?·
  
  As for the other example that I asked about, where the relays are a greater load, thanks for your suggestion·to use transistors. What do you think about the ULN2003AN http://rocky.digikey.com/WebLib/Texas%20Instruments/Web%20data/ULN2001A-4A,ULQ2003A-4A.pdf
  
  
  
  Dan