School me on capacitors please! - Page 1
Forum section: electronics / projects

author: KTP
author_rank: Frequent Contributor
timestamp: June 06, 2012, 08:46:21 pm
content: I am looking at driving a bunch of RGB leds and to get things going quickly, I decided to just use a LMZ12003 adjustable switching regulator package to supply the ~4V to the driver chip (I want to run the system on 12V). This neat little 3A capable puppy is very small and low profile, includes the inductor, and is about $6 in quantity 100 (I was going to use about $5 in parts just to roll my own with one of those SOIC8 controllers + external inductor).Anyway, in looking at the datasheet for the LMZ12003, it recommends a multilayer ceramic low ESR cap and mentions that the eval board uses a 100uF 6.3V X5R model. I found these on Digikey for around $0.34 each in quantity 1000.I had no idea ceramic capacitors had gotten that small for that high of a value! (it is a 1210 package). What are the advantages of using SMT electrolytic caps now over these new multilayer ceramics? Is it just price?

author: mukymuk
author_rank: Contributor
timestamp: June 06, 2012, 09:01:39 pm
content: Cost would be the only reason I can think of.

author: Flávio V
author_rank: Regular Contributor
timestamp: June 06, 2012, 09:09:02 pm
content: Those multilayer ceramic caps are ceramic super capacitors..About the caps,you should be sure if the current than goes to those ceramic super caps reverses or not...if it does better use the ceramic super caps...if does't a normal cap can do it...i think....(a normal cap is a electrolytic in my words)

author: hlavac
author_rank: Frequent Contributor
timestamp: June 06, 2012, 11:37:34 pm
content: QuoteI'm sure you meant polarity of cap's charge voltage, not currentCurrent will necessarily reverse or you would not be able to discharge the cap. Even the voltage across the cap could go negative until it is discharged thru the ESR and nothing bad would happen. Only when the cap is charged in reverse bad stuff starts happening...

author: free_electron
author_rank: Super Contributor
timestamp: June 07, 2012, 12:01:02 am
content: Quote from: KTP on June 06, 2012, 08:46:21 pmHow about a 10uf in 0402 package ... dust in the wind ...Ceramic caps have several advantages over electrolytics- do not dry out- withstand more abuse ( temperature )- lower esr and eslroughly same price ( dpeending on package )drawbacks- larger values more expensive- you need to know what you select. there is many grades. class - i ii and iii. class i has no tempco , no microphony , ii has tempco.the x5r you picked could be good.- ceramics caps derate under temperature , voltage ,frequency. ( that is what X5R Y5V and other letter combinations mean... they tell you temperature boundaries and temperature drift .. z5u's can drift 80% .... )- voltage derating is heavily dependent on package.a 10uf 10volt X7R in 0805 will be around 8uf at 10 voltsa 10uf 10volt X7R in 0402 will be around 4 uf at 10 volts...capacitance is always specified at 0volts... you need to check the exact datasheet of the exact familiy and manufacturer. do not make the mistake of selecting a murata and placing a tdk or vice versa ! every manufacturer has their own ceramic material with its own unique properties... there is a post from me somewhere on the forum where i give the links to the software (free) that can show you the exact curves.the serious capacitor makers all have this available...

author: KTP
author_rank: Frequent Contributor
timestamp: June 08, 2012, 12:15:25 pm
content: Thanks for the replies...I think I will design a pad that can take either the 1210 package or the smallest of the surface mount electrolytic caps and this way I can have the option of going with either!