> I'm searching for a low noise switching supply IC. The goal is to 
> get
> +5V/200mA and -5V/200mA from a +3.3V with less than 5mV ripple. 
> Ideal
> one IC.

Good comment from a competent friend:

_________________________________

For lowest noise a sine-wave converter is the ultimate.

Simple designs based on the Royer configuration can be built with one 
IC (or
even with just two transistors if you can tolerate a slightly more 
complex
wound component).  The Royer is zero-voltage switched so efficiency is 
very
high and noise is very low  - and even lower due to all power 
waveforms
(current and voltage) being sinusoidal.  It is also very scalable  -
virtually all CCFL backlights for LCD's are driven with Royer 
converters  -
so are most induction cooktops.

The Royer configuration was often used in valve communications gear 
powered
from low-voltage batteries  - with an electromagnetic vibrator 
(younger
listeners will have no idea what I am talking about) doing the 
switching.

I have built Royers rated between 1W and 500W to heat ball-bearings up 
150mm
diameter, weld plastic, transmit power across a rotating shaft, to 
convert
24V to 12V for noise-sensitive communications equipment on offshore 
yachts,
and to drive compact fluorescent tubes from 12V and 24V DC sources 
(with
dimming capabilities) for boats and mobile homes.

Note that the resonating capacitor is a critical component in any 
Royer and
especially so as the power level goes up.  A good quality component 
with low
series resistance, low dissipation factor, and adequate RMS current 
rating
is absolutely essential.  Care should also be taken with the wound
component(s) to ensure low DC resistance.  Litz wire is essential for 
good
efficiency at higher frequencies and power levels.  I have also 
successfully
used planar cores with PCB windings.

Most of my higher power designs have been based on the Unitrode (now 
TI)
UC3871 device but there are now plenty of other alternatives. 
Low-power
designs (up to 10's of watts) can be readily built with CMOS glue 
logic.


Regards,

Ken Mardle
Applied Digital Research Ltd.

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