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

SP2EH_23EH_0_060

8

Charge–Pump

The charge pump is a

Exar-patenteddesign

(5,306,954) and uses a unique approach

compared to older less-efficient designs.

The charge pumps still requires four external

capacitors, but uses a four-phase voltage

shifting technique to attain symmetrical 0V

power supplies. Figure 3a shows the wave-

form found on the positive side of capacitor

C2, and Figure 3b shows the negative side

of capacitor C2. There is a free-running

oscillator that controls the four phases of

the voltage shifting. A description of each

phaseisasfollows:

+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.

Phase 1

— V

theclockcycle,thepositivesideofcapacitors

C

+

is

then switched to ground and charge on C



–

is

transferred to C

2

–

. Since C

2

+

is connected to

+5V, the voltage potential across capacitor

C

Phase 2

— V

nects the negative terminal of C

storage capacitor and the positive terminal

of C

–l0V to C

of capacitor C

negative side is connected to ground.

Phase 3

— V

of the clock is identical to the first phase

— the charge transferred in C

–5V in the negative terminal of C

is applied to the negative side of capacitor

C

+

is at +5V, the voltage potential

across C

Phase 4

— V

clock connects the negative terminal of C

2

to ground and transfers the generated l0V

across C

Again, simultaneously with this, the positive

side of capacitor C

the negative side is connected to ground,

and the cycle begins again.

erated from V

V+ will show a decrease in the magnitude

inefficienciesinthedesign.

The clock rate for the charge pump typically

operates at 5kHz. The external capacitors

must be a minimum of 0.µF with a 6V

breakdown rating.