5

Exar Corporation 48720 Kato Road, Fremont CA, 94538 • (50)668-7000 • Fax (50)668-707 • www.exar.com

SP232A_00_05308

DESCRiPTion

Phase 4

Vdd transfer. The fourth phase connects the

negative terminal of C2 to ground and the posi-

tive terminal of C2 to the Vdd storage capacitor .

This transfers the doubled (V+) voltage onto C3.

Meanwhile, capacitor C is charged from Vcc to

prepare it for its next phase.

Under lightly loaded conditions, the intelligent

pump oscillator maximizes efficiency by running

only as needed to maintain V+ and V- voltage

levels. Since interface transceivers spend most

of their time at idle, this power-efficient innova-

tion can greatly reduce total power consump-

tion. This improvement is made possible by

the independent phase sequences of the Exar

charge pump design.

The clock rate of the charge pump typically op-

erates greater than 70kHz allowing the pump to

operate efficiently with small 0.1uF capacitors.

Efficient operation depends on rapid charging

and discharging of C and C2. Therefore, the

capacitors should be mounted as close as pos-

sible to the IC and have a low ESR (equivalent

series resistance). Inexpensive, surface mount

ceramic capacitors are ideal for use on charge

pump. If polarized capacitors are used the posi-

tive and negative terminals should be connect-

ed as shown on the typical operating circuit. A

diagram of the 4 individual phases is shown in

Figure .

C

2

+

-

V

CC

C

3

C

4

C

1

+

+

+

Phases 1 and 3: Store/Double.

Double charge from C1 onto C2.

C2 is now charged to -2xVcc

+

C

2

+

-

V

CC

C

3

C

4

C

1

+

+

+

+

C

2

+

-

V

CC

C

3

C

4

C

1

+

+

+

Phase 2 – Vss transfer from C2 to C4.

Meanwhile C1 is charged to Vcc

+

Vss

transfer from C2 to C3.

Meanwhile C1 is charged to Vcc

V

DD

V-

V+

V-

V+

V+

Patented 5,306,954

Figure . Charge pump phases