3

Date: 5/20/04

SP682 Micro Power Inverting Charge Pump

© Copyright 2004 Sipex Corporation

about 12kHz (20kHz maximum) which con-

serves power as opposed to higher frequency

which operation typically draws more power

from V

specified are 3.3

µF but the absolute minimum

should be 1

µF.

EFFICIENCY INFORMATION

A charge pump theoretically produces a doubled

voltage at 100% efficiency. However in the real

world, there is a small voltage drop on the output

which reduces the output efficiency. The SP682

can usually run 99.9% efficient without driving

a load. While driving a 1k

Ω load, the SP682

remains over 90% efficiency.

Output Voltage Efficiency = V

V

V

Power Loss = I

The efficiency changes as the external charge

pump capacitors are varied. Larger capacitor

values will strengthen the output and reduce

output ripple. Although smaller capacitors will

cost less and save board space, lower values will

reduce the output drive capability and also in-

crease the output ripple.

Figure 3. Charge Pump Phase 2

Figure 2. Charge Pump Phase 1

The SP682's charge pump design is a simpli-

fied version of Sipex's original patented charge

pump design (5,306,954) except that it only

generates a negative output. The charge pump

utilizes external capacitors to store the charge.

Figure 1 shows the waveform found on the

negative side of capacitor C2. There is a free–

running oscillator, running at 12kHz, that con-

trols the two phases of the voltage shifting. A

description of each phase follows.

Phase 1

V

the clock cycle, the positive side of capacitors

C

then switched to ground and the charge on C

1

–

is transferred to C

2

2

+5V, the voltage potential across capacitor C

2

is now 10V.

Phase 2

V

nects the negative terminal of C

storage capacitor and the positive terminal of

C

to C

Simultaneously, the positive side of

capacitor C

tive side is connected to ground.

The oscillator frequency or clock rate for the

charge pump is designed for low power opera-

tion. The oscillator operates at a frequency of

Figure 1. Charge Pump Waveform

–5V

–5V

+5V

++

–

–

+

–

–10V

++

–

–

+

–

THEORY OF OPERATION