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Exar Corporation 48720 Kato Road, Fremont CA, 94538 • 510-668-7017 • www.exar.com

SP385E_100_030811

Charge Pump

The charge pump is a Exar–patented design

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

compared to older less–efficient designs. The

charge pump still requires four external capaci-

tors, but uses a four–phase voltage shifting

technique to attain symmetrical 10V power

supplies. There is a free–running oscillator

that controls the four phases of the voltage

shifting. A description of each phase follows.

Phase 1

— V

the clock cycle, the positive side of capacitors

C1 and C2 are initially charged to +5V. Cl+

is then switched to ground and the charge in

C1– is transferred to C2–. Since C2+ is con-

nected to +5V, the voltage potential across

capacitor C2 is now 10V.

Phase 2

— V

nects the negative terminal of C2 to the V

SS

storage capacitor and the positive terminal of

C2 to ground, and transfers the generated

–l0V to C3. Simultaneously, the positive side of

capacitor C 1 is switched to +5V and the nega-

tive side is connected to ground.

Phase 3

— V

the clock is identical to the first phase — the

charge transferred in C1 produces –5V in the

negative terminal of C1, which is applied to the

negative side of capacitor C2. Since C2+ is at

+5V, the voltage potential across C2 is l0V.

Phase 4

— V

clock connects the negative terminal of C2

to ground, and transfers the generated l0V

across C2 to C4, the V

Again, simultaneously with this, the positive

side of capacitor C1 is switched to +5V and

the negative side is connected to ground, and

the cycle begins again.

Since both V+ and V– are separately gener-

ated from V

V– will be symmetrical. Older charge pump

approaches that generate V– from V+ will

show a decrease in the magnitude of V– com-

pared to V+ due to the inherent inefficiencies

in the design.

The clock rate for the charge pump typically

operates at 15kHz. The external capacitors

can be as low as 0.1µF with a 16V breakdown

voltage rating.

Figure 1. Charge Pump Waveform

+10V

a) C2+

GND

GND

b) C2–

–10V