buck converter vs buck controller

Discussion in 'General Electronics Chat' started by ak52, Jan 12, 2015.

  1. ak52

    Thread Starter Member

    Oct 15, 2014
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
    It looks like they call a controller an IC that controls an external FET while the converter has it internal.
  3. ak52

    Thread Starter Member

    Oct 15, 2014
    I see, operation of both are the same?, i mean both still need an LC filter at the output to smooth-en the signal?
  4. MrAl

    Distinguished Member

    Jun 17, 2014

    Some IC chips are completely self contained, except for the cap and inductor while others require an external transistor, and still others can be used either way depending on how high the output current has to be for your use.
    So a controller is more for use with an extra transistor, a complete converter has one in it already, and the combination type allows use without a transistor for lower current levels and with a transistor for higher current levels.
  5. ak52

    Thread Starter Member

    Oct 15, 2014
    I am facing a weird problem with my switch regulator!!
    Mods can move this to another thread as this does not come under the subject,if they wish.

    I got a few samples of LM2679-5.0 from TI,There is a tool on their website by which it gives us a full schematic on how to connect the regulator.
    I have connected the components as per their schematic.I have attached a screen shot.
    I have used an 1N4007 in place of D1,
    L1 = 10uH
    Cout = 194uF(100uF+47uF+47uF)
    I have not connected Rilim at the moment as i do not want any current limit.
    I have not connected Cin and Cinx also.

    Here are my observations:
    At no load my output varies from 5.0 to 5.3v
    Giving some load(Across Cout):
    1000E( load current = 5mA), output is stable.
    500E(load current = 10mA),output is stable.
    50E(load current = 100mA),output dropped by .3v,(4.7v)
    10E(load current = 500mA),resistor burned out.
    10E-5W(load current = 500mA),output dropped to 2.5v

    The switching regulator is supposed to handle up to 5Amps as per the data sheet.But at a load current of 0.5Amps the output changes completely.
  6. ronv

    AAC Fanatic!

    Nov 12, 2008
    Your substitutions and omissions aren't really good.
    What is the current rating and resistance of the inductor.
    The 1N4007 is only a 1 amp diode and pretty slow. Use one of the ones in the data sheet.
    I would add the current limit, even if it was 5K
    Put in Cin.
    These can be hard to get working on a breadboard so look close at the data sheet about the ground wiring.
    JWHassler likes this.
  7. Evil Lurker


    Aug 25, 2011
    Wow that IC screams compared to the LM2596's I'm used to tinkering with. Ok some pointers... first off you MUST use a Schlocky, preferably one rated higher than 5 amps and a reverse blocking voltage typically of around 40V like a SB540 (there are much better diodes out there, you just have to look for them). That 1N4007 will NOT be able to keep up with the speed at which that IC is switching at, it is rated about 20% of the current you are trying to run through it, and it has a horrible forward voltage drop.

    Also you MUST hook up the current limit resistor. Why? It doesn't control your output current, it controls the peak current going through your inductor. Basically the reason that pin is there is not to limit your output but to protect your inductor from saturating and your diode from letting out the magic smoke from excess current, especially in the case of a short on the output. Honestly, I'm surprised you didn't smoke the chip without that resistor installed.

    And you also MUST have a capacitor on the input, and it MUST be a low ESR type. I briefely skimmed over the datasheet, and I'm going to say that it is going to have to have something like a Nichicon HD or better or a Panasonic FM/FK whatever they are selling now. Now that I think about it, not having a cap on the input is probably what saved you from smoking your regulator.

    Anyhoos, these switch mode buck converters can be very finicky things and need high performance components for optimal performance. Ideally IC's like the LM2679 need to be installed on heavy double sided copper clad PCBs with a ground plane on the bottom along wiht "star" grounding to the ground plane using vias to minimize feedback loop interference.
  8. ak52

    Thread Starter Member

    Oct 15, 2014
    OK guys,i added the input capacitor, and current limiting resistor of 10K.So by the formula in the datasheet,current limit is (37125/10K),which is around 3.7Amps.

    Hmm i don't have a high current schottkey diode at the moment,i have replaced the 1N4007 with an 1N4148 at the moment,i have given a load of 10E(500mA),
    I see a considerable improvement in the voltage,but still voltage is dipping to 4.5-4.6 volts.
    Ammeter shows a draw of 430mA.
    I will try the same with a higher current schottkey and get back with the results.