Download ICL7660 Voltage level Converter

ICL7660 Voltage level Converter
Occasionally a circuit needs a few milli-Amps of a negative voltage for a specific circuit.
A typical example of this is an RS-232 Interface where the output must go to a positive as
well as a negative voltage. The ICL7660 was made exactly for such a purpose.
The ICL7660 is a fairly simple circuit. It has an oscillator with its own internal
components that generates a 10 KHz signal, four switching transistors and a Control
Circuit. The Control Circuit alternately charges and discharges the capacitors (connected
to pins 2, 4 and 5). The transistors steer the discharging of the capacitors so a Negative
Voltage is generated at the pin 5 output.
Output voltage is typically the same as the +V on pin 8, but of the opposite polarity. The
ICL7660 may operate on any voltage between 1.5 Volts and 10 Volts and supply about 3
milli-Amps of output current. The ICL7660A can operate at up to 12 Volts and supply up
to 10 mA. This is a suitable current level for an interface signal or provide a negative bias
voltage for older MOS logic devices. This era of technology just about coincides with the
beginning of Gaming devices. Early IGT Keno games were built around the Intel 8080
microprocessor. This is the application the ICL7660 was designed for. The 8080 required
+12 Volts, +5 Volts and -5 Volts.
More modern RS-232 Interface chips have a similar circuit built into them and generate
their own negative voltage.
(Referring to Figure 1)
Two capacitors are required for basic operation. 10 F electrolytic capacitors are
suggested by the manufacturer's data sheet. The capacitor connected across pins 2 (+) and
4 (-) is part of a Charge Pump circuit. The steering transistors build up a charge across
this capacitor then switch the polarity so that when it discharges it creates a negative
voltage at pin 5. The second capacitor at pin 5 (-) to ground (+) is called the reservoir
capacitor. The negative voltage is built up across this capacitor. As current is drawn from
pin 5 it comes from this capacitor discharging.
D1 protects pin 5 from going more positive than ground and is strictly a safety device you
may not see in most applications. A 1N4148 Silicon signal Diode would probably work.
The lower forward voltage of the 1N5817 Schottky diode is suggested. It is not required
for speed.
For operation below 3.5 Volts pin 6 should be connected to ground.
The oscillator section of the ICL7660 may also be used to make a Voltage Doubler. This
generates an output voltage twice that of the input voltage, minus the drop across the two
diodes. Again, 1N5817's were used for the lower voltage drop. 1N4148 types would work
with only a small drop in output voltage.
Figure 1 and Figure 2 may be combined to accomplish both a negative voltage generator
as well as a voltage doubler, but the total output current must still be below the level of a
few milli-Amps. The limiting factor is how much charge we can develop across the
capacitor connected to pin 2. See figure 3.
Operating Frequency
An internal R-C oscillator operates at around 10 KHz (when operating from 5 Volts). For
faster operation a CMOS / TTL clock pulse may be applied to pin 7 For slower operation
a capacitor may be connected between pin 7 and pin 8. A 100-pF capacitor will slow the
oscillator down to around 1 KHz. At this slower rate the Charge Pump and Reservoir
capacitors must be ten times as large (100 F).
For intermittent pulses of higher current the reservoir capacitor connected to pin 5 may be
made larger. A 100 F capacitor here will allow pulses of output current at up to 100 mA,
depending on the pulse width.
Devices may be put in parallel for higher currents or cascaded for higher voltages. In this
case the output (pin 5) of the first stage connects to pin 3 (ground) of the second stage.
Pin 8 (+V In) of the second stage is connected to ground, and the output is taken from pin
5 of the second stage. This simply puts the second stage referenced to ground and -V
instead of +V and ground. I see no reason this can't be continued for another (third) stage.
Speed of operation might start to be a problem. It would take some time after initial
power on before the output finally builds up to the desired voltage.
Regulating the output voltage
Since the output currents are so low we must regulate the output voltage by controlling
+V In on pin 8.
Similar products; MAX1044 (almost same pinout and function, pin 1 is different);
Si7661, ICL7662 (similar pinout, works up to 20 V); MAX1680 125 mA output
capability.