> I'm looking for a switching regulator chip. Something inexpensive like
> the MC34063, but not limited to 100 KHz because I need to reduce the
> physical size of my layout, especially the inductor. Not having to use a
> current sense resistor would be nice too.

If you are willing to forgo any sophistication such as protection and
startup management then a 'roll your own' hysteretic buck converter
running at > 1 MHz with comparator, bipolar hi side switch (MOSFET
with extra driver components)  and either external reference or added
reference may be able to be done. Using a darlington may (or may not)
make drive easier. Getting this working is simple. Getting it working
really well is 'a bit of an ask' overall and it depends on what volume
you want whether the playing is worth it, but using the smallest
available packages the end result could be quite tiny. If the 8V in is
stable and accurate enough (eg another regulated output) it may be
able to be used to provide the reference.

As I know you know, driving a MOSFET at that frequency is going to
need a fast driver or super-optimised luck. You'd be OK at 100 kHz
with little more than an NPN/PNP pair but for 1 MHz plus you are going
to need an extra transistor and some extra componentry to get driver
speed. That's why a single bipolar or perhaps darlington (with its own
problems) may be easier.

A possible out exxists IFyou can find a comparator that is fast
enough, cheap enough and with enough drive power. If driving a single
bipolar high side switch an LM339 (or 1 per pkg variant) may do quite
well. It will slew high to low on a 5V supply in about 100 ns typical
and low to high in about double that. The 6 mA guaranteed, 16 mA
typical on Vdd=3D5V is not good enough to drive a FET gate capacitor
fast enough but may be OK for a bipolar.

It would take minimal bench (or SPICE) time to trial an LM339,
BC807-40 or your favourite high-Beta equivalent (500 mA+ testbed) , a
1N5817, a TL431 and not too much else. With 6mA guaranteed drive you
need a beta of about 160 at 1A which is achievable. (Zetex will always
oblige). This is looking like about 4 x SOT23 or smaller actives (1
pkg comp, PNP, 431 if needed, schottky) plus passives to taste.
Inductor is "small". Be careful not to inhale the 0201 resistors :-).



  Russell McMahon
--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.