Lpc2000

I believe you are referring to the MC34063A?  If so:

http://www.nomad.ee/micros/mc34063a/index.shtml
This page is nice as it calculates the switch peak current.

In reference to circuit on above page:
Depending on your output voltage, solve output of R1-R2 voltage divider
(input pin 5).  This is the max level you would drive via PWM.  The R1-R2
voltage divider would be omitted in lieu of PWM signal.  Use a voltage
divider on PWM signal to scale for max voltage.  This will prevent undesired
voltages on the MC34063A output.  As Tom mentioned you would want to add
filter to the PWM signal.  And it should be noted when controlling the
MC34063A via PWM, the input voltage must be regulated. 


The MC34063A looks like a nice part for dropping rectified 24VAC down to
3.3VDC.  Thanks for the tip Al.


Joel