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lm5007 buck switcher troubles

Discussion in 'Electronic Design' started by Jamie Morken, Dec 22, 2004.

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  1. Jamie Morken

    Jamie Morken Guest

    Hi all,

    I am using the LM5007 from national:
    "http://www.national.com/pf/LM/LM5007.html"

    I designed the circuit identical to the circuit shown on page2 of the
    datasheet, but it is not working. I believe it is something to do with
    the 200Kohm R1, as when I measure the voltage on pin6, it is under 0.7V
    which means the chip is in shutdown mode. I tested the chip with input
    voltages from 12V up to 75V and it is in shutdown mode no matter what.

    I disabled the shutdown mode by increasing the voltage on pin6 but it
    fried the chip too. Any ideas on this? I have ordered the evalboard
    from national to see if it works! :) I designed my board the same as
    the eval board.

    cheers,
    Jamie Morken
     
  2. The formula given for the on time dependent on the resistor connected
    to pin 6 says that the time is proportional to the value of the
    resistor and inversely to the input voltage. This implies that the
    full input voltage is applied across the resistor. So I suspect that
    pin 6 connects to an internal current mirror that has only a diode
    drop to the ground pin (which they neglect ot include in the
    formula). What ever current you pour into this pin, the mirror pulls
    down on the internal 'on time' timing capacitor. So you are probably
    seeing a reasonable voltage on pin 6.
     
  3. You need the resistor R1 and it has to be roughly 200K for it to work
    properly (depending on maximum input voltage). It just looks like a
    pullup in the first schematic, but it's actually doing more- it sets
    the output switch on-time.


    Best regards,
    Spehro Pefhany
     
  4. Jamie Morken

    Jamie Morken Guest

    Hi,

    Hmm.. any ideas why it isn't working then? :) pin6 is below 0.7V so it
    is in shutdown mode, but I am not sure why..

    cheers,
    Jamie
     
  5. No, sorry. I followed the data sheet and had absolutely no problems
    with R1=200K, my choice of inductor and capacitors that fit their
    recommendations (series 1-ohm resistor). I have the evaluation board
    too, somewhere.


    Best regards,
    Spehro Pefhany
     
  6. Jamie Morken

    Jamie Morken Guest

    Hi,

    Did you use 100K for R2 as well? I did. With the 200K R1, and 100K R2,
    and a 12V output voltage setting, will these chips be able to handle 14V
    to 75V with the 200K R1 and 100K R2 values, or is it best to set these
    values for R1 and R2 based on a static input voltage?

    I noticed an error on my design, I forgot to hook pin4 to ground, I did
    this now but I am still getting no output voltage, maybe I fried the
    chip by not having this pin grounded? :)

    cheers,
    Jamie
     
  7. That's approximately what I did in that particular product design-
    it's almost the same as page 2 on the data sheet.
    Best read the data sheet carefully. ;-) It was almost a year ago,
    don't remember the details (it gets into both continous and
    discontinous mode, depending on load). Power dissipation was the main
    concern.
    That's the only ground pin. It certainly won't work very well without
    it. ;-)


    Best regards,
    Spehro Pefhany
     
  8. Jamie Morken

    Jamie Morken Guest

    Hi,

    I had to replace 4 of 6 chips as not having this pin grounded fried
    them, but they seem all to be working fine now! :)

    cheers,
    Jamie
     
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