How to choose the capacitors for the LDO.

Discussion in 'General Electronics Chat' started by Rolexme, Apr 10, 2008.

  1. Rolexme

    Thread Starter New Member

    Jan 8, 2008
    I have a question about the LDO chip TLE4266G. What we need is to transfer 12V to 5V , I read the datasheet of the chip , and find out the Figure3 on page7 .

    Here is the question : There are two capacitors before the input pin of , one is 470uF , the other is 100nF . I know that the larger capacitor is to filter the High Frequency Noise and the lower capacitor is to filter the Low Frequency Noise .

    1. What if I change the 470uF to 100uF ? What kind of effect it will be ?
    2. The 100nF should be ceramic kind . What about the 470uF ? Should it be aluminium electrolytical or tantalum electrolytical or other material? (Our space is very limited , especially the height of the capacitor , I know aluminium electrolytical capacitor is very large.)

    Thank you. Your reply is highly appreciated.
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    1. The effect depends on the source of the DC voltage you are regulating down. If the source is a rectifier diode, then making the ripple smoothing 470 uF capacitor smaller will increase the ripple component and possibly degrade the regulation.
    2. A 470 uF ceramic capacitor would be as large as a pig. If space is tight, then use tantalum. Be sure to get a voltage rating at least double that of the source - 25 volts in your case.

    Question - if your source is 12 volts and the regulator is for 5 volts, why use a LDO type? They are more for situations where the source is only 1.5 volts greater than the output. Also, those 7 unused volts are going to show up as heat. SMT regulators are hard to keep cool.
  3. Rolexme

    Thread Starter New Member

    Jan 8, 2008
    Thank you for your great help . My 12V is from the accumulator . It is said that the tantalum is two times price than the aluminium electrolytical , is that right ?

    About the question you asked : I am sorry that is not the original design of mine. I think it's not wise to use a LDO to transfer 12V to 5V too. I think it's better to use a DC/DC converter to do that , is that right ? But the price of the DC/DC is higher , and this is also a balance from PRICE to Function ,hehe .

    Am I right ? Your future advice is highly appreciated.
  4. Norfindel

    AAC Fanatic!

    Mar 6, 2008
    What about the LM317? It's a standard linear regulator, and it's better than the 7805. It comes in TO-200 and TO3 packages, and also surface mount.
    Is this something to use in a car? I think i readed somewhere that the voltage varies a lot in cars, maybe you need to take special precautions as to not damage the regulator.
  5. nanovate

    Distinguished Member

    May 7, 2007
    Another point on the tantalum type is that you need to derate its voltage by half... so for a 12V input you would want a 25V tantalum.

    The output cap selection is usually more critical for a LDO.
  6. Rolexme

    Thread Starter New Member

    Jan 8, 2008
    Hi, Norfindel, You are right , this product is used in a car , and the environment for the electronics is bad .

    Hi, Nanovate, thank you for your advice.

    I find the price from the manual of premierelectronics , and find out that : when the quantity is 100+,the normal aluminium electrolytical is 0.07$, but the tantalum is 0.7$!!! It's so expensive that exceed my imagination.

    Thank you a lot.
  7. Rolexme

    Thread Starter New Member

    Jan 8, 2008
    Do you know a software which can simulate the character of the parallel capacitors ?