STAA*TECH Journal
Getting Negative Voltages
From switching Regulators
By James Beck
Greater Southern Dist. Co.
Atlanta, GA
Hello again, didn't quite make it in last
nonth' s issue, but I'm back again. As
pranised, we will cover methods of getting
negative voltages from switching
regulators that only offer positive
outputs.
August 1986 (8)
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The main problem that is encountered with
attenpting to accooplish this, is the fact
that switchers use high-frequency pulses
to operate. Standard techniques, like we
covered in the previous nonths, won't work
all that well. M:>st rectifiers work OK at
60 Hz, but at 15 KHz and up, they tend to
·leak backwards across the junction.
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Fortunately in this age of the
semiconductors ;mere there is a problem,
someone has come up with a solution.
In steps the ICL7660. This wonderful
little chip takes a positive input and
converts it to an equal voltage of
opposite polarity. Fig. 1 shows the pin
out of the chip. All that is needed to
make it function are 2 capacitors and a
diode.
The typical efficiency of the device is
well above 95%. The value of the
capacitors is not critical; I have had
great results using lOuf caps. '!be output
current is low, but -can easily power
dynamic RAMs and OPanps. It may not work
with some of the newer games that use
hybrid custom chips for the RGB
generation. These hybrids suck up a lot of
current.
Fig. 2 shows the configuration for
converting voltages under 3.SV and Fig. 3
shows the configuration for converting
voltages g~eater than 3.SV. The whole unit
can be asseubled on a Perf Boa.rd in under
15 minutes and the chip can be purchased
for less than $4.
well, I haven't come up with a topic for
next DDnth as of yet, so if anyone has any
suggestions -give me a call at
. 404/352-3040.
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