So you want to boost the 5V to something like 36V and then use
LM317 to regulate the voltage to 0-30V @1A. It can of course be
done. However this defeats one of the major purpose of SMPS:
higher efficiency.

Suppose you want to get 3V @1A and suppose the boost regulator
is ideal (100% efficiency). We then have the following calculation:

Vin = 5V, Vout =36V, Iout =1A (because of the linear regulation)
Iin = 36V x 1A / 5V = 7.2 A. That is not a small current. The
total efficiency is now (3V x 1A) / (5V x 7.2A) = 8.3%
If the efficiency of the boost converter is 80%, we will have 
current of 9A at the 5V input!

The better method is to use a buck-boost/SEPIC/Zeta converter
which can do step-up and step-down. A SEPIC may be better here
because of the readily available PWM control ICs.

If the noise is a problem, a post linear regulator like 7805
or an LDO can be used.

Regards,
Xiaofan

-----Original Message-----
From: William "Chops" Westfield [mailto:westfw@mac.com]
Sent: Monday, September 05, 2005 6:41 AM
To: Microcontroller discussion list - Public.
Subject: Re: [EE] Lab-style power supplies...


Speaking of power supplies, has anyone seen a circuit for a 317-style
power supply with BOOST-MODE switching pre-regulator?  I want to get
0-30V @ 1A from ubiquitous high-current 5V...

BillW
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