ICL7660S, ICL7660A

FN3179 Rev 7.01

Page 9 of 14

Feb 10, 2020

Typical Applications

Simple Negative Voltage Converter

The majority of applications will undoubtedly utilize the

ICL7660S and ICL7660A for generation of negative supply

voltages. Figure 15 shows typical connections to provide a

negative supply where a positive supply of +1.5V to +12V is

available. Keep in mind that pin 6 (LV) is tied to the supply

negative (GND) for supply voltage below 3.5V.

The output characteristics of the circuit in Figure 15 can be

approximated by an ideal voltage source in series with a

resistance as shown in Figure 15B. The voltage source has a

value of -(V+). The output impedance (RO) is a function of the

ON resistance of the internal MOS switches (shown in Figure

14), the switching frequency, the value of C1 and C2, and the

ESR (equivalent series resistance) of C1 and C2. A good first

order approximation for RO is shown in Equation 2:

Combining the four RSWX terms as RSW, we see in Equation 3

that:

RSW, the total switch resistance, is a function of supply voltage

and temperature (see the output source resistance graphs,

Figures 2, 3, and 11), typically 23

 at +25°C and 5V. Careful

selection of C1 and C2 will reduce the remaining terms,

minimizing the output impedance. High value capacitors will

reduce the 1/(fPUMP x C1) component, and low ESR capacitors

will lower the ESR term. Increasing the oscillator frequency will

reduce the 1/(fPUMP x C1) term, but may have the side effect of

a net increase in output impedance when C1 > 10µF and is not

long enough to fully charge the capacitors every cycle.

Equation 4 shows a typical application where fOSC = 10kHz and

C = C1 = C2 = 10µF:

Since the ESRs of the capacitors are reflected in the output

impedance multiplied by a factor of 5, a high value could

potentially swamp out a low 1/fPUMP x C1 term, rendering an

increase in switching frequency or filter capacitance ineffective.

Typical electrolytic capacitors may have ESRs as high as 10

.

Output Ripple

ESR also affects the ripple voltage seen at the output. The

peak-to-peak output ripple voltage is given by Equation 5:

A low ESR capacitor will result in a higher performance output.

Paralleling Devices

Any number of ICL7660S and ICL7660A voltage converters

may be paralleled to reduce output resistance. The reservoir

capacitor, C2, serves all devices, while each device requires its

own pump capacitor, C1. The resultant output resistance is

approximated in Equation 6:

Cascading Devices

The ICL7660S and ICL7660A may be cascaded as shown to

produce larger negative multiplication of the initial supply

voltage. However, due to the finite efficiency of each device,

the practical limit is 10 devices for light loads. The output

voltage is defined as shown in Equation 7:

where n is an integer representing the number of devices

cascaded. The resulting output resistance would be

approximately the weighted sum of the individual ICL7660S

and ICL7660A ROUT values.

Changing the ICL7660S and ICL7660A Oscillator

Frequency

It may be desirable in some applications, due to noise or other

considerations, to alter the oscillator frequency. This can be

achieved simply by one of several methods.

By connecting the Boost Pin (Pin 1) to V+, the oscillator charge

and discharge current is increased and, hence, the oscillator

frequency is increased by approximately 3.5 times. The result

is a decrease in the output impedance and ripple. This is of

major importance for surface mount applications where

capacitor size and cost are critical. Smaller capacitors, such as

1

2

3

4

8

7

6

5

+

-

10µF

10µF

ICL7660S

VOUT = -V+

V+

+

-

RO VOUT

V+

+

-

15A.

15B.

FIGURE 15. SIMPLE NEGATIVE CONVERTER AND ITS

OUTPUT EQUIVALENT

ICL7660A

R0 2RSW1 RSW3 ESRC1

++

 2R

SW2

RSW4 ESRC1

++



+





(EQ. 2)

1

fPUMP C1



-------------------------------- ESRC2

+

fPUMP

fOSC

2

--------------

=

RSWX MOSFET Switch Resistance

=



R0 2xRSW

1

fPUMP C1



-------------------------------- 4xESRC1 ESRC2

++

+



(EQ. 3)

R0 2x23

1

510

3



10



10

6

–



--------------------------------------------------- 4xESRC1 ESRC2

++

+



(EQ. 4)

R0 46 20 5

++

ESRC





VRIPPLE

1

2fPUMP



C2



----------------------------------------- 2ESRC2 IOUT



+







(EQ. 5)

ROUT

ROUT of ICL7660S



n number of devices



---------------------------------------------------------

=

(EQ. 6)

VOUT

nVIN



–

=

(EQ. 7)