Roman Black wrote...

>If you found 2kHz with the other circuit,
>it should be possible to decrease the inductor
>down to 0.1mH or maybe much less than that,
>with the tiny RF inductors the wire size is
>larger with the lesser values, and increasing
>the speed should help with saturation issues.

With this thing arranged for operation off a 9V battery, expect about
20 KHz with a 2.2 mH inductor and a 20 mA load.  Note that in a buck
regulator like this, the relationship between inductance and frequency
isn't simple: the frequency depends on the amount of hysteresis
designed into the switch, the load current, and the output capacitance
as well.

If you'd like to try this circuit with 9V input, try making the
following adjustments to RPModified.gif that I posted on 8/27:

R2 =3D 1.0K
R9 =3D 100K
R5 =3D 3.3K
R10 =3D 6.8K

Experiment as suits you, and see what results you get.

>A lot of PIC projects only need a few mA, and
>a nice alternative to a 7805 regulator chip
>or resistor/zener would be great for 9v battery
>operated PIC gizmos.

If you want a good alternative to a 7805 for operating a PIC off a 9V
battery, use an LDO regulator chip like National's LP2951.  At 20 mA
it will work down to an input voltage of only 5.25 volts, at which
point your 9V battery is pretty much sucked dry.  The LP2951 only
takes a hundred microamps or so for its own use, compared to a
milliamp or two (IIRC) for a 7805.

The LP2951 also provides a very solid low-voltage inhibit output that
can be used as a PIC reset signal.

If you want the maximum possible battery life from a 9V unit at low
currents, I'd suggest using an IC switching regulator like Maxim's
MAX639.  It will be a lot more efficient than any of these circuits
we're playing around with: about 90% efficiency at 1 mA output, AFAIR.

Dave

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