pull down resistor help

Discussion in 'General Electronics Chat' started by nandax, Mar 26, 2009.

  1. nandax

    Thread Starter Member

    Nov 20, 2008
    12
    0
    I am using step down converter
    TPS62056 from texas instrument
    (http://focus.ti.com/lit/ds/symlink/tps62056.pdf)
    I'd like to use one switch connect to Vin and EN pin with pull down resistor to ground.
    can someone help me to determine the value of pull down resistor
    because the typ input currentof En pin is 2 uA (page 3), is that means i have to use big resistor?
    also the example in datasheet (page 12) use big resistor if I use 2 switches, do i have to use same value if I only use 1 switch?
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    Actually you need a resistor from EN to Vin with the switch from EN to ground. The 2uA requirement is advising you that you need to choose the R value so that at least 2uA can flow when your proposed disable switch is off and EN is pulled high.

    EN requires at least 1.3V as a logic high.

    If Vin is 9V say, then you could use at most (9-1.3)/2μA =< 3.9MΩ between EN and Vin. Let's call it 3.3MΩ max.

    But ...
    Use something more reliable - say 100k to 1MΩ, since Vin is likely to be less than 9V after your battery starts to go flat.

    Why the new thread?
     
  3. nandax

    Thread Starter Member

    Nov 20, 2008
    12
    0
    but I'd like to use it from low to high statement that is why i need pull down resistor, will it be the same value for pull down resistor?
     
  4. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    OK - I understand.

    The other method I suggested is normally preferred. Depending on the logic condition required, you make your switch condition either normally open or normally closed.

    But if you insist - then you can probably safely use the 10k to 1M range. I suggest the 10k low end as it's not zero (i.e. a short when you close your switch!) and not so low that it ruins your overall efficiency.

    The data sheet doesn't (??) give any clue as to what leakage current might flow out of EN to ground. Could be femto-amps to nano-amps?? Who knows. You just have to get EN below the low logic level - was it 0.3V? Don't have data sheet at hand at the moment.

    The only thing that should concern you with making your pull-down resistance too high is that the EN terminal might drift up into an unstable value when not switched to Vin. Pretty unlikely, even at very high resistances ~10MΩ.

    From your (the designer's) perspective it depends how efficient the overall design has to be when the EN terminal is pulled high and some of your precious supply current flows via your pull-down resistor to ground.

    Happy designing!:)
     
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