4

Date: 5/20/04

SP682 Micro Power Inverting Charge Pump

© Copyright 2004 Sipex Corporation

Table 1. R

Table 2. C3 .vs. V

RIPPLE

SHUTDOWN FEATURE

The SP682 charge pump includes a shutdown

feature (pin 8) which disables the charge pump

when the V

activate the shutdown mode. If shutdown is not

needed, it can be left open where an internal

pull-down resistor will always keep the charge

pump active.

Typical input current for the

shutdown pin is 3

µA. The shutdown feature is

another option for conserving power in portable

applications, reducing current to only 1

µA.

PARALLEL DEVICES

Multiple SP682 charge pumps can be connected

in parallel. However, the effective output resis-

tance now is the output resistance of a single

device divided by the number of devices. Con-

necting multiple pumps allows the user to save

on the storage capacitor.

The charge pump

capacitors still must be separate for each device.

APPLICATIONS INFORMATION

The SP682 charge pump produces a doubled,

inverted voltage from the V

can serve in many applications where a negative

–5V to –10V output is needed. Typical applica-

tions include powering analog switches, and

biasing LCD displays and panels.

The ESR of the charge pump capacitors also

determine the output resistance. Assuming that

switch resistances are approximately equal, the

output resistance can be derived as shown be-

low:

R

+ 1 / (f

R

+5V using 3.3

µF capacitors. The total internal

switch resistance (16*R

90

Ω. The table below shows the comparison of

R

C1, C2 (

ROUT (

0.05

4085

0.10

2084

0.47

510

1.00

285

3.30

140

4.70

125

10.00

105

22.00

94

The output voltage ripple is also affected by the

capacitors, specifically C

Larger values

will reduce the output ripple for a given output

current load of current. The formula represen-

tation is:

V

To minimize the output ripple, the C

capacitor can be increased to over 10

µF whereas

the pump capacitors can range from 1

µF to 5µF.

Table 2 shows the typical V

C

C

V

0.50

1020

1.00

520

3.30

172

4.70

120

10.00

70

22.00

43

THEORY OF OPERATION