Detailed Description

The MAX868 inverting charge pump uses pulse-

frequency-modulation (PFM) control to generate a reg-

operation is obtained by enabling the internal 450kHz

oscillator as needed to maintain output voltage regula-

tion. This control scheme reduces supply current at

light loads and permits the use of small capacitors.

The functional diagram shown in Figure 1 indicates the

two phases of MAX868 operation: charge phase (

Φ1)

and discharge phase (

Φ2). In charge phase, the

switches on the left-hand side close, and the switches

on the right-hand side open. In the discharge phase,

the inverse occurs.

Figure 2 illustrates that in charge phase, both flying

capacitors are charged in parallel. The load is serviced

entirely by the charge stored in the output capacitor.

Figure 3 demonstrates the series connection of the fly-

ing capacitors in the discharge phase. The series com-

bination of the flying capacitors, when connected to the

output capacitor, transfers charge to the output in order

to maintain output voltage regulation. In normal opera-

tion, the MAX868 operates predominantly in charge

phase, switching to discharge phase only as needed to

maintain a regulated output.

Regulated, Adjustable -2x

Inverting Charge Pump

_______________________________________________________________________________________

5

Pin Description

Active-Low Shutdown Input. Connect SHDN to GND to put the MAX868 in shutdown mode and reduce sup-

ply current to 0.1µA. Connect to IN for normal operation. OUT is actively pulled to GND in shutdown.

SHDN

9

Feedback Input. Connect FB to a resistor divider for a regulated output voltage. Connect to IN to generate

FB

10

Positive Terminal of Flying Capacitor C1

C1+

5

Negative Terminal of Flying Capacitor C2

C2-

6

Supply-Voltage Input. Input voltage range is 1.8V to 5.5V.

IN

7

Positive Terminal of Flying Capacitor C2

C2+

8

Power Ground

PGND

4

Negative Terminal of Flying Capacitor C1

C1-

3

PIN

Charge-Pump Output

OUT

2

Analog Ground

GND

1

FUNCTION

NAME

OUT

C2-

C1+

C1-

FB

Φ1

Φ2

OSCILLATOR

C2+

IN

SHDN

Figure 1. Functional Diagram