[sdiy] SMD part availability, manufacture

John Luciani jluciani at gmail.com
Thu Nov 13 19:20:43 CET 2008


On Thu, Nov 13, 2008 at 12:33 PM, Seb Francis <seb at burnit.co.uk> wrote:
> Ingo Debus wrote:
>>
>> Am 13.11.2008 um 01:31 schrieb Seb Francis:
>>
>>> For the power supply regulators, I've been looking at low dropout ICs and
>>> these seem pretty cool.  Am I really understanding it right that they can
>>> work with a output-input of down to like 0.2V.  This would mean only a tiny
>>> amount of heat dissipation so no need for any heatsinking.  Are there any
>>> disadvantages to these compared to standard linear regulators which need
>>> typically 2.5V output-input differential?
>>>
>>
>> Many LDOs need a low-ESR cap at the output.
>>
>
> Like a ceramic chip capacitor in uF range I guess.
>
> Just been reading this LT datasheet which goes into some detail about
> suitable caps..
> http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1040,C1055,P1778,D3903
> (page 15)
>
> They are saying that X5R or better still X7R ceramic caps are fine.

Unless you are doing a large number of boards or need very large caps I would
go with X7R SMD caps. You could be better off buying one reel of X7R caps
rather than two partial reels.

>
> They also say elsewhere that "Small ceramic capacitors can be used without
> the series resistance required by other regulators" which leads me to assume
> that some LDO regulators actually need a higher ESR to be stable.  So I
> guess it pays to read the datasheet!

A number of LDOs (and switching converters) require a series resistance for
stability. For example the datasheet for the MC33269 3.3V LDO recommends
a series resistance less than 10Ohms but greater than 0.2Ohms.

(* jcl *)

-- 
http://www.luciani.org



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