Tonight I salvaged a Maxim ICL 7662 Voltage Converter from a circuit board that came out of a piece of dental equipment called a Cavitron. As I glanced over the data sheet on this little IC I was immediately captured by its simplicity of design and application. In my book the closer one can get to simplicity and still get the job done the closer one is to electronic beauty and elegance. This little circuit is beautiful and elegant. I looked the part up on Newark. Here is the listing:
Here is a link to the Data Sheet also in case you want to look it over for yourself. Also please note that the two illustrations posted later in this blog have been lifted from the Data Sheet.
I began my exploration of this circuit by hooking it up as illustrated in the data sheet's typical operation:
As you can see from the diagram and the bread board we have the ICL 7662, 2 10 uF caps, and a small 100 nF decoupler that I threw in across the power inputs to the chip. The circuit is so simple it is almost trivial but it works. The +15 volts comes in and a -15 volts appears magically on pin 5 of the IC relative to ground. Oops, I don't believe in magic so here is the explanation thankfully simplified a bit by the writers of the Data Sheet.
The picture above is an idealized operational diagram that I moved from the Data Sheet. The ICL 7662 has an internal clock of approximately 10 kHz. This clock can be over ridden by injecting a higher frequency into pin 7 of the chip. For my exploration the internal clock was convenient so it was used. From the diagram we can see that the banks of switches ( S1, S3) and (S2, S4) are alternately closed by the clock. When (S1, S3) are closed external capacitor (10 uF in this case) is charged with the Vin. In the second half of the clock cycle (S2, S4) are closed and the charge in C1 is transferred to external capacitor C2 (also 10 uF) which is wired such that it's positive terminal is tied to the common ground and it's negative terminal is the negative voltage output of the chip. The picture above shows the operation with the input voltage displayed on the small breadboard meter and the larger multimeter displaying the negative voltage. There is no load on the chip at this point.
For the fun of it I decided to carry this exploration one step further and hook the +/- 15 volt supplied by the ICL 7662 to an LM741 Op Amp. I decided to use a couple of 10 K resistors to center the inverting input of the 741 and a 50K potentiometer to provide a variable voltage to the non-inverting input of the 741. For an output I connected a bidirectional red / green LED to the 741 output pin. This should allow me to turn the LED from red to green by walking the voltage on the non-inverting input back and forth past the fixed voltage on the inverting input. Here is a picture of the finished circuit:
Since the picture doesn't show much here is a short video of the circuit in operation.
The Maxim ICL7662 IC can be very useful in situations where one wants to power an Op Amp, Comparator, or other circuit with a +/- voltage supply and not go through all the trouble of constructing a dedicated split power supply. The output current is practically limited to about 20 mA unless your circuit can tolerate voltage drops in excess of 10%. Check the graphs in the Data sheet if you have an application to verify that these limits will work for you.