Pin Description

Pin

Name

Function

Voltage Inverter

Voltage Doubler

1

FC

Frequency control for internal oscillator:

Same as inverter.

FC = open, f

FC = V+, f

FC has no effect when OSC pin is driven externally.

2

CAP+

Connect this pin to the positive terminal of

charge-pump capacitor.

Same as inverter.

3

GND

Power supply ground input.

Power supply positive voltage input.

4

CAP−

Connect this pin to the negative terminal of

charge-pump capacitor.

Same as inverter.

5

OUT

Negative voltage output.

Power supply ground input.

6

LV

Low-voltage operation input. Tie LV to GND when

input voltage is less than 3.5V. Above 3.5V, LV can

be connected to GND or left open. When driving

OSC with an external clock, LV must be connected

to GND.

LV must be tied to OUT.

7

OSC

Oscillator control input. OSC is connected to an

internal 15 pF capacitor. An external capacitor can

be connected to slow the oscillator. Also, an

external clock can be used to drive OSC.

Same as inverter except that OSC cannot be driven

by an external clock.

8

V+

Power supply positive voltage input.

Positive voltage output.

Circuit Description

The MAX660 contains four large CMOS switches which are

switched in a sequence to invert the input supply voltage.

Energy transfer and storage are provided by external capaci-

tors.

Figure 2 illustrates the voltage conversion scheme.

When S

age V+. During this time interval switches S

open. In the second time interval, S

and S

cycles, the voltage across C

the anode of C

cathode of C

in the switches, and no ESR in the capacitors. In reality, the

charge transfer efficiency depends on the switching fre-

quency, the on-resistance of the switches, and the ESR of

the capacitors.

Application Information

SIMPLE NEGATIVE VOLTAGE CONVERTER

The main application of MAX660 is to generate a negative

supply voltage. The voltage inverter circuit uses only two ex-

ternal capacitors as shown in the Typical Application Circuits.

The range of the input supply voltage is 1.5V to 5.5V. For a

supply voltage less than 3.5V, the LV pin must be connected

to ground to bypass the internal regulator circuitry. This gives

the best performance in low voltage applications. If the sup-

ply voltage is greater than 3.5V, LV may be connected to

ground or left open. The choice of leaving LV open simplifies

the direct substitution of the MAX660 for the LMC7660

Switched Capacitor Voltage Converter.

The output characteristics of this circuit can be approximated

by an ideal voltage source in series with a resistor. The volt-

age source equals −(V+). The output resistance R

function of the ON resistance of the internal MOS switches,

the oscillator frequency, and the capacitance and ESR of C

1

and C

where R

MOS switches shown in

Figure 2.

High value, low ESR capacitors will reduce the output resis-

tance. Instead of increasing the capacitance, the oscillator

frequency can be increased to reduce the 2/(f

Once this term is trivial compared with R

ther increasing in oscillator frequency and capacitance will

become ineffective.

The peak-to-peak output voltage ripple is determined by the

oscillator frequency, and the capacitance and ESR of the

output capacitor C

DS100898-21

FIGURE 2. Voltage Inverting Principle

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