BobG wrote...

>New, Related Question:

>Do you have to be more concerned with EMI when you use a switching DC-DC
>convert / regulator? =20

Yes, you do; possible interference to sensitive circuits in your own
device is always an issue, as are possible radiated and/or conducted
emissions that could interfere with other devices.

The LT1931 device you linked to below operates around a megahertz,
which could cause a lot of interference with nearby radios.

>I am considering this linear device
>http://www.linear.com/prod/datasheet?datasheet=3D569&product_family=3Dpo=
wer
>In a nutshell its 5V in, -12V @150ma out.  Its overkill for what I need =
(2ma
>at -12V for an LCD), but the chip is only a couple bucks, and I am =
afraid of
>discrete components:)

It's overkill, and at the very low currents your application requires,
it won't be very efficient either.  Take a look at the efficiency
curve for the "+5V to -12V Inverting Converter" circuit (I assume this
is the one you were thinking of); see how the efficiency drops off
sharply at low output currents?

=46rom what I see on a quick read of the datasheet, it looks like this
device was optimized to give high output power in a VERY small circuit
footprint; hence, the high operating frequency to make the inductors
small.  It doesn't seem to be targeted at low current applications.

>The data sheet actual has a sample board layout, warning, "If you don't =
lay
>out your PCB carefully, its not going to work".

And they REALLY mean it, too: if you have a layout that doesn't have
the right ground topology (i.e., the right arrangement of the current
paths in the circuit) the chip won't work.

>Is this normal? =20

Unfortunately, yes.

>Are switching regulators trickier to work with? =20

Sometimes yes, sometimes not so tricky.  At best, they take
considerably more care to design with than the trusty ol' 7805 linear
regulator.  At worst, they are unbelievably nightmarish to design.

=46or applications requiring a couple of watts of power, there are many
IC switching regulators that are fairly easy to design with.  National
Semiconductor's "Simple Switcher" chips are the ones I've used most
often, and they have the design procedure down to a cookbook approach
where you just plug some numbers into a few formulas, look up a few
component values in a table, and you're done.  Component selection,
particularly filter capacitors, is critical, and so is circuit layout;
but other than that it's straightforward.

=46or applications requiring more power, design becomes progressively
more and more difficult as the power increases.  Above a few dozen
watts, things get tricky; above a few hundred, it becomes "don't try
this trick at home".  Above a few thousand watts, it gets real scary.

=46or lower-power applications, the situation gets even trickier;
designing a switching regulator to convert 10 milliwatts at high
efficiency can be a real challenge because every piddling little power
loss within the circuit has to be minimized.

In any case, yes; switching regulators are generally more difficult to
design with than linear regulators.

>I gather
>that a DC-DC convert is similar to a switching regulator; the difference
>being that the output of the DC-DC convert will vary with changes in =
Vin. Is
>that correct?

Yes and no.  In some cases (such as the LT1931 you cited), the terms
are used interchangeably.

Here's a suggestion, for whatever it's worth:

The easiest way to generate a negative voltage for your LCD bias would
be with one of the RS232 interface chips (like a MAX232A) that
generate their own + and - RS232 levels using capacitive charge pumps.
Just ignore the RS232 transceiver circuits (ground the unused inputs
or connect them to Vcc, whichever results in the least current drain)
and use only the power section.

The efficiency will be a lot better than the LT1931, and so will the
price tag.

The only drawback is that you won't get a full -12 volts out of it,
only around -9V to -10V.  Would that be enough?

Hope this helps...

Dave

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