MAX868 Просмотр технического описания (PDF) - Maxim Integrated

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MAX868

Regulated, Adjustable -2x Inverting Charge Pump

Maxim Integrated 

Regulated, Adjustable -2x

Inverting Charge Pump

Pin Description

PIN NAME

FUNCTION

1

GND Analog Ground

2

OUT Charge-Pump Output

3

C1-

Negative Terminal of Flying Capacitor C1

4

PGND Power Ground

5

C1+

Positive Terminal of Flying Capacitor C1

6

C2-

Negative Terminal of Flying Capacitor C2

7

IN

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

8

C2+

Positive Terminal of Flying Capacitor C2

9

SHDN

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.

10

FB

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

an unregulated -2 x VIN output voltage.

Detailed Description

The MAX868 inverting charge pump uses pulse-

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

ulated negative output voltage up to -2 x VIN. PFM

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.

IN

SHDN

C2+

C2-

C1+

C1-

Φ1 Φ2

OSCILLATOR

Figure 1. Functional Diagram

OUT

FB

COUT

VREF

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