DC/DC Converter question

[Jdo300]

Hello All,

I have a situation where there is an IC that takes 5V at 7mA on an isolated area of my PCB. I have two DC supply sources to choose from to power this chip; either a 5V supply (which has to be isolated from this chip) or an 18V supply which doesn't need to be isolated from it.

I currently have a simple linear regulator to drop the voltage from the 18V side down to 5V, but since my board runs on batteries (two 9Vs to be specific) I don't want to waste any extra power if I can help it. Would any of you happen to know of any companies who make super low-power step-down converters? I was looking on Mouser/Digikey for the small monolithic ones with all the guts in them but the lowest I could find were 1W units. and if I only draw like 7mA from it, the efficiency would be so bad that it wouldn't be worth using.

I also am thinking that some kind of charge pump may work since the power is so low but I have yet to find any step-down charge pumps.

Thanks,
Jason O

 

[berkeman]

That is a tough design point. Have you looked at the National Semi LM5009 buck step-down converter?

http://www.national.com/pf/LM/LM5009.html [Broken]

It looks to be their lowest-power buck. The efficiency plot on this web page implies that it may still be pretty efficient down at 7mA, but the plot doesn't go below 50mA (the datasheet should have a better plot, and a listing of the Iq for the part, which will limit the low Iout efficiency.

Beyond that, the linear approach seems your best bet. Or you could home-brew your own step-down buck circuit, optimized for very low Iq. What are your regulation and ripple requirements for this 5V IC?

 

[mheslep]

Linear Technology IMO is the best company in the industry for converters. A switching converter will get you 85% efficient against your 5V*7ma load, for a Pin of 41mw. If you use a linear you must waste the drop 13V*7ma, for an Pin of 126mw or ~3x worse.
http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1003,C1042,C1032,C1064,P49784

When you have a moment read Williams' '..Converters for Poets' app note. Its a master piece on the subject, along w/ most of his app. notes.
http://www.linear.com/designtools/app_notes.jsp