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From: Paul Burke
Subject: Re: LM317 or 7808 is better performance for 9V output ?
Date: Wed, 04 Dec 2002 08:56:31 +0000
Organization: Scazon Systems
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Tong Narak wrote:
> Hi all,
> A. LM317 require 5V head room.
> B. 78XX require 2V head room ( Is these correct ? ).
> I would like to set the output to 9V. Load ~300mA.
> (1) Which one is better proformance and efficiency ?
> (2) 7805 and 7808 are same efficiency when adjustable to 9V ?
A. Not in my data sheet, it only wants about 1.75V at 25deg, and just
over 2V over the range (typical values). The 780x are also quoted as 2V
Efficiency will be identical- all volts that aren't dropped in the load
are turned to heat in the regulator.
Don't forget to allow for ripple if you are taking the input from a
smoothed rectified AC supply- the dropeout voltage must be calculated
from the LOWEST point of the ripple at ful load.
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