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

SP207E_00_072309

The Exar charge-pump generates V+ and V-

independently from V

condition V+ and V- will be symmetrical.

Older charge pump approaches generate

V+ and then use part of that stored charge

to generate V-. Because of inherent losses,

the magnitude of V- will be smaller than V+

on these older designs.

Underlightlyloadedconditionstheintelligent

pump oscillator maximizes efficiency by

running only as needed to maintain V+ and

V-. Since interface transceivers often spend

muchoftheirtimeatidle,thispower-efficient

innovation can greatly reduce total power

consumption. This improvement is made

possiblebytheindependentphasesequence

of the Exar charge-pump design.

The clock rate for the charge pump typically

operates at greater than 5kHz, allowing

thepumptorunefficientlywithsmall0.1µF

capacitors.Efficientoperationdependson

rapidly charging and discharging C

C

close to the IC and have low ESR (equivalent

series resistance). Low cost surface mount

ceramic capacitors (such as are widely used

for power-supply decoupling) are ideal for

use on the charge pump.

However the charge pumps are designed

to be able to function properly with a wide

range of capacitor styles and values. If

polarized capacitors are used, the positive

and negative terminals should be connected

as shown.

Figure 4. Charge Pump — Phase 4

+10V

+

+

+

+

–

–

–

–

Voltage potential across any of the capaci-

tors will never exceed 2 x V

capacitors with working v

oltages as low as

0V rating may be used with a nominal V

CC

supply. C

than V

adequate. The reference terminal of the

V

V

minimum 6V working voltage is required.

Higherworkingvoltagesand/orcapacitance

values may be advised if operating at higher

V

capacitors age.

+10V

a) C

2

+

gND

gND

b) C

2

–

–10V

Figure 5. Typical waveforms seen on ca-

pacitor C2 when all drivers are at maximum

load.