How to choose the capacitors for the LDO chip.

[rolexme]

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 low Frequency Noise and the lower capacitor is to filter the High 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.

 

[ecerfoglio]

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 .

If you have a 12 V supply and need 5 V you may use a "normal" regulator (7805) instead of a LDO one - LDO means Low drop, and you have a full 7 V to drop (12 V - 5 V)

The 7805 still needs capacitors on its input and oulput, but their values are less critical than with the LDO ones.

 

[Roff]

If your input voltage is rectified AC and you change the input side capacitor from 470uF to 100uF, your ripple voltage will go up by a factor of about 5, regardless of what type of regulator you use. Will your circuit still work under this condition?

 

[rolexme]

My 12V is from the accumulator .
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 . 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 .

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.

 

[vengat]

Passive Components

Can i have the Documents for Basic Capacitor and Inductor theory

 

[Roff]

My 12V is from the accumulator .

OK, but what charges the capacitor (accumulator)? If it's a rectifier of an AC source, then the ripple voltage will be inversely proportional to the value of the capacitor.

 

[ecerfoglio]

OK, but what charges the capacitor (accumulator)? If it's a rectifier of an AC source, then the ripple voltage will be inversely proportional to the value of the capacitor.

I think that accumulator means battery, not capacitor.

My 12V is from the accumulator .
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.

A 7805 should be cheaper than a LDO regulator.

Both are linear regulators, but the LDO is a special one that works well with a low voltage difference between input and output. With a 7 V difference you don't need to use an LDO.

If there are no long wires to pick up noise, a 7805 fed from a battery should work OK with small capacitors at its input and output - no need to use tantalum ones. Deppending on the load, you may use only a pair of 100 nf (= .1:mu:F) ceramic capacitors, one at the 7805's input and another at its output.

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 .

A DC/DC converter is usefull if you want to conserve battery power because it is much more efficient. A linear regulator (either a 7805 or an LDO) will dissipate a lot of power (the current times the voltage difference), but for low current applications the lower cost makes them the "best" option.

 

[Roff]

I think that accumulator means battery, not capacitor.

I just looked briefly at the Wikipedia entry on Google. It said capacitor, but if you read the next line in the article, it says battery. Makes a lot more sense, considering it's 12V. I may be slow, but I'm stupid.

 

[rolexme]

Thank you a lot ,my fellows .

The accumulator means accumulator battery which used in the car.

And question about 7805 which ecerfoglio mentioned : 7805 and LDO are both linear regulator , if they have the same input voltage and the same output voltage , what difference should it be between them ? It's just that the LDO is more suitable for the Low-Dropout situation?

 

[electronic spark]

Difference is 7805 is cheaper, I'd say.

 

[ecerfoglio]

And question about 7805 which ecerfoglio mentioned : 7805 and LDO are both linear regulator , if they have the same input voltage and the same output voltage , what difference should it be between them ? It's just that the LDO is more suitable for the Low-Dropout situation?

Difference is 7805 is cheaper, I'd say.

If you have enough voltage to drop (as in your case), both regulators should work but the LDO costs more and the value of the capacitors it needs is more critical. So te "normal" 7805 is the better option.

But if you have only a little difference between input and output (about 3 V or less) the 7805 stops working and the only option is a LDO regulator (or else a switching supply, like a DC/DC converter).

 

[Speakerguy]

Like said above, some LDO regulators have stability problems with certain values of output capacitors, and they are usually more expensive than cheap 7805 type regulators. If you already have the LDO you can use them, but if not I would buy the 7805 type.

 

[rolexme]

Thank you all a lot !