PTH08000 (3.3v for N64) capacitors question

[Life of Brian]

Yes, I am playing around with modding an N64. Yippee! I have a question regarding getting the 3.3v I need.

According to TI's spec sheet for the PTH08000:

"The minimum required input capacitor is 100 μF of capacitance. When VO > 3.4 V, the 100-μF electrolytic capacitance must be rated for 650-mArms ripple current . For VO < 3.4 V, the ripple current rating must be at least 500 mArms. The ripple current rating of electrolytic capacitors is a major consideration when they are used at the input.

When specifying regular tantalum capacitors for use at the input, a minimum voltage rating of 2 x (maximum dc voltage + ac ripple) is highly recommended. This is standard practice to ensure reliability. Polymer-tantalum capacitors are not affected by this requirement. For improved ripple reduction on the input bus, ceramic capacitors can also be added to complement the required electrolytic capacitance."


Okay, I know I need 100uF capacitors. Does this mean I would be okay with caps rated at 10v, or would I need something bigger? 2 x 3.3= 6.6v, so assuming "ac ripple" is negligible then would I be okay with using a 100uF 10v cap?

Long-winded question for a yes or no answer, isn't it? ಠ_ಠ

[Rekarp]

Your fine Brian

[Skyone]

10v is fine, man.

[Life of Brian]

Thanks for the quick replies, guys. I went ahead and made the circuit. I made two of them - do you know why?

I was feeling lazy, so I used a 75 watt soldering gun to make this because it was on the table already and my 15 watt iron was in my toolbox. Soldering tiny things with a huge iron is amusing sometimes.

I stuck my thumb in this picture for size reference - these things are tiny! TI sent them to me in a box that could have held about 500 of these, stuffed with all sorts of protective wrapping paper and whatnot. Kinda silly...


Underside shot. I couldn't order samples of the same thing twice, so I got one that had legs and one that didn't.


I'm pretty proud of making that tiny connection between the two resistors with that huge iron's tip.


Anyway, I guess more pictures will follow eventually in an N64 work log. I don't have a set time frame, however, so we'll see how long it takes.

[Mario]

What is the second capacitor for? I know one is needed and the other is optional, so what does it do?

[Rekarp]

What is the second capacitor for? I know one is needed and the other is optional, so what does it do?

You are talking about the input cap? It just smooths out the input voltage and helps prevent the regulator from getting a voltage "surge".

When you are drawing current from a battery the voltage slightly drops. Well the regulator "pulses" like a light switch being turned off and on really fast. This can cause the input voltage (ie battery) to go from lets say 7.4V to 7.35V really fast. Its mainly for protection. You don't need it but its a good idea.

[Life of Brian]

I had at least four on hand so I thought, "Why not?"

[timmeh87]

The input cap is on the 7.4v side and therefore 10v is not fine. Its fine for the output cap.

Putting a 1uf and 10uf ceramic on the input provides a faster response to smooth out small high-frequency ripples you get from a switching regulator while the larger electrolytic one deals with transients like suddenly increasing the load. Heck, this could possibly be the 'right' solution to the screen-lines--are-caused-by-these-converters-sometimes problem