_______________Detailed Description

The MAX1680/MAX1681 switched-capacitor voltage

converters either invert or double the input voltage.

They have low output resistance (3.5

Ω) and can deliver

up to 125mA output current. These devices operate at

one of two selectable frequencies: 125kHz/250kHz

(MAX1680) and 500kHz/1MHz (MAX1681). This pro-

vides the flexibility to optimize capacitor size, operating

supply current, and overall circuit efficiency. Frequency

selection also allows for minimizing coupling into other

sensitive circuits. These devices contain no internal

divider; the oscillator frequency equals the switching

frequency. The devices can easily be cascaded to pro-

duce a higher output voltage, or paralleled to deliver

more current.

The MAX1680/MAX1681 feature a shutdown mode that

reduces supply current to <1µA (SHDN = high). OUT,

in the inverter configuration, pulls to ground in shut-

down mode. Shutdown is not available in the doubler

configuration; connect SHDN to OUT.

__________Applications Information

Voltage Inverter

A simple voltage inverter is the most common

MAX1680/MAX1681 application. It requires three exter-

nal capacitors (including the input bypass capacitor)

as shown in the

Typical Operating Circuits (inverter

configuration). Although the output is not regulated, low

output resistance produces a typical drop of only 0.44V

with a 125mA load. This low output resistance makes

the devices fairly insensitive to changes in load (see

the graphs for Output Source Resistance vs.

Temperature and Supply Voltage in the

Typical

Operating Characteristics section).

Voltage Doubler

The MAX1680/MAX1681 can be configured as a volt-

age doubler with two external capacitors as shown in

the

Typical Operating Circuits (doubler configuration).

When loaded, the output voltage drop is similar to that

of the voltage inverter. The minimum input supply range

is slightly higher than in the inverter configuration.

Calculate ripple voltage using the equation in the

Capacitor Selection section.

Frequency Control

A frequency-control pin, FSEL, provides design flexibili-

ty. Each device has two selectable frequencies:

125kHz/250kHz (MAX1680) and 500kHz/1MHz

(MAX1681). This allows optimization of capacitor size

and supply current for a given output load. Table 1

summarizes the frequency options .

Table 1. Nominal Switching Frequencies

125mA, Frequency-Selectable,

Switched-Capacitor Voltage Converters

6

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______________________________________________________________Pin Description

Selects operating frequency. MAX1680: 250kHz when FSEL is low, 125kHz when FSEL is high. MAX1681:

1MHz when FSEL is low, 500kHz when FSEL is high.

FSEL

1

Positive Charge-Pump Capacitor Connection

CAP+

2

Negative Charge-Pump Capacitor Connection

CAP-

4

Power-Supply Ground Input

GND

3

Logic Voltage Input. Connect LV to GND.

LV

6

Power-Supply Positive Voltage Input

IN

8

SHDN

7

Negative Voltage Output

OUT

5

Power-Supply Positive Voltage Input

Power-Supply Ground Connection

Connect LV to OUT.

Not available; connect to OUT.

Shutdown Input. Driving SHDN high disables the

charge pump, and the output goes to 0V. SHDN is a

CMOS input.

Positive Voltage Output

MAX1680

250

FSEL = LV

125

FSEL = IN

FSEL CONNECTION

MAX1681

1000

500

FREQUENCY (kHz)

INVERTER

DOUBLER

PIN

FUNCTION

NAME