Oscillator Frequency Considerations

For normal operation, leave the BOOST and OSC pins

of the MAX1044/ICL7660 open and use the nominal

oscillator frequency. Increasing the frequency reduces

audio interference, output resistance, voltage ripple, and

required capacitor sizes. Decreasing frequency reduces

quiescent current and improves efficiency.

Oscillator Frequency Specifications

The MAX1044/ICL7660 do not have a precise oscillator

frequency. Only minimum values of 1kHz and 5kHz for the

MAX1044 and a typical value of 10kHz for the ICL7660

are specified. If a specific oscillator frequency is required,

use an external oscillator to drive the OSC pin.

Increasing Oscillator Frequency

Using the BOOST Pin

For the MAX1044, connecting the BOOST pin to the V+

pin raises the oscillator frequency by a factor of about 6.

Figure 6 shows this connection. Higher frequency opera-

tion lowers output impedance, reduces output ripple,

allows the use of smaller capacitors, and shifts switch-

ing noise out of the audio band. When the oscillator is

driven externally, BOOST has no effect and should be left

open. The BOOST pin should also be left open for normal

operation.

Reducing the Oscillator Frequency Using COSC

An external capacitor can be connected to the OSC

pin to lower the oscillator frequency (Figure 6). Lower

frequency operation improves efficiency at low load

currents by reducing the IC’s quiescent supply current.

It also increases output ripple and output impedance. This

can be offset by using larger values for C1 and C2.

Connections to the OSC pin should be short to prevent

stray capacitance from reducing the oscillator frequency.

Overdriving the OSC Pin with an

External Oscillator

Driving OSC with an external oscillator is useful when

the frequency must be synchronized, or when higher

frequencies are required to reduce audio interference.

The MAX1044/ICL7660 can be driven up to 400kHz.

The pump and output ripple frequencies are one-half the

external clock frequency. Driving the MAX1044/ICL7660

at a higher frequency increases the ripple frequency and

allows the use of smaller capacitors. It also increases the

quiescent current.

The OSC input threshold is V+ - 2.5V when V+ ≥ 5V, and

is V+ / 2 for V+ < 5V. If the external clock does not swing

all the way to V+, use a 10kΩ pull-up resistor (Figure 7).

Output Voltage Considerations

The MAX1044/ICL7660 output voltage is not regulated.

The output voltages will vary under load according to the

output resistance. The output resistance is primarily a

function of oscillator frequency and the capacitor value.

Oscillator frequency, in turn, is influenced by temperature

and supply voltage. For example, with a 5V input voltage

and 10μF charge-pump capacitors, the output resistance

is typically 50Ω. Thus, the output voltage is about -5V

under light loads, and decreases to about -4.5V with a

10mA load current.

Minor supply voltage variations that are inconsequential to

digital circuits can affect some analog circuits. Therefore,

when using the MAX1044/ICL7660 for powering sensi-

tive analog circuits, the power-supply rejection ratio of

those circuits must be considered. The output ripple and

output drop increase under heavy loads. If necessary, the

MAX1044/ICL7660 output impedance can be reduced by

paralleling devices, increasing the capacitance of C1 and

C2, or connecting the MAX1044’s BOOST pin to V+ to

increase the oscillator frequency.

Figure 5. Basic Negative Voltage Converter

Figure 6. Negative Voltage Converter with COSC and BOOST

MAX1044

ICL7660

4

3

C1

10µF

*REQUIRED FOR V+ < 3.5V

VOUT = -(V+)

C2

10µF

V+

2

1

5

6

7

8

*

CBYPASS

+

MAX1044

+

4

3

10F

COSC

VOUT = -(V+)

10F

V+

2

1

5

6

7

8

CONNECTION

FROM V+

TO BOOST

MAX1044/ICL7660

Switched-Capacitor Voltage Converters

www.maximintegrated.com

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