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Help with SMT diode selection

  1. Jul 30, 2008 #1
    Greetings:

    I recently made my first prototype surface mount board. In designing it, all the SMT components were foriegn to me. I'm a ME, not and EE and my projects are fairly straightforward microprocessor based designs. I chose components similar to old standbys I've used with through hole designs.

    I needed to have a uP "wake on change" from one of 2 switch presses. I had 3 available inputs and only one of those had the wake on change feature.

    So, I used a pair of diodes to "share" the single wake on change pin withthe switches, and tied each switch to the other 2 pins. When a switch is pressed, it signals the wakeup pin (via one of the diodes), and it's own pin. The diode prevents the current from passing to the other switch's pin. The uP wakes, then polls the switch pins to see which of the 2 woke it up.

    I need an inexpensive diode for this job. There won't be any significant current through this, only the leakage current on the uP pin I think. The switch's ground a pulled-up uP line. It's a 3V (2 AAA batteries) powered, the pullups are 100k. When a switch is depressed it changes the shared pin from pulled up 3V, to grounded.

    It all works fine but I initially chose a S1A diode from Fairchild (SMA package). When it arrived I found the SMA is large and bulky compared to my other components, overkill at 1A 50V.

    Can someone recommend a good, inexpensive diode for this job. I would prefer it be:
    a) smaller
    b) cheap or as cheap
    c) low V drop so a low battery can still show a "high" on the shared pin after the V drop through the diode.

    Thanks!
    Greg
     
    Last edited: Jul 30, 2008
  2. jcsd
  3. Jul 31, 2008 #2
    BAV70 is a dual diode in sot23.

    It's common cathode, but common anode duals are also available.
     
  4. Jul 31, 2008 #3
    Perfect. Thanks very much!
     
  5. Jul 31, 2008 #4
    The sot23 has the advantage over the SMA in that it won't roll off the board...
     
  6. Jul 31, 2008 #5
    Agreed :) I am thrilled to get rid of the 2 SMA's and replace them with a single SOT23. I already have 2 other SOT23 (mosfets) on my board so I'm familiar with them. I only have to make a minor change on my schematic and PCB layout.

    I do need a common anode though. Now that I have a clue what to look for I found a BAT54A. It's listed a as Schottky, which I think is good for my app. Except for common anode, and a 30V rating instead of a 70V rating, it's specs are identical to the BAV70 you suggested. And it's price is the same. The BAT54A only has a Vf of 0.24V at low current (0.1mA) which I think I fall into (1 uA)

    The 30V is well within my 3.2V from my (2) AAA's so I think it will be OK for the app.

    I saw some other common anode's listed (e.g. MMBD1205, BAS35), but they appear to be oriented to higher current apps and have apparently higher Vf's.

    Think the BAT54A will work OK or should I find one rated higher?

    Thanks agian very much for the help.

    Best,
    Scott
     
  7. Aug 1, 2008 #6
    Sounds ok to me.

    The only problem with a schottky is likely to be the reverse leakage, which is considerably higher than that for an ordinary Si diode.

    Probably won't make much difference in your application, but if it does, then reduce the value of pullup/pulldown resistors.
     
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