SP6660EB datasheet(15/22 Pages) EXAR

15 / 22 page

SP6660DS/11

SP6660 200mA Charge Pump Inverter or Doubler

Optimizing Capacitor Selection

Refer to Figure 24 for the total output resistance

for various capacitance values and oscillator

frequencies. The reservoir and charge pump

capacitor values are equal. The capacitance

values required to maintain comparable ripple

and output resistance typically diminish

proportionately as the pump frequency of the

SP6660 increases.

The test conditions for the curves of Figure 24

are the same as for Figures 2 to 20 for the circuits

in Figures 1 and 2; additional conditions are as

follows:

C1 = C2 = 0.2

Ω ESR capacitors

OUT = 4.2Ω

The flat portion of the curves shown at a 5.2

Ω

effective output resistance is a result of the

SP6660's 5.25

Ω output resistance where

5.2

Ω = R

OUT(SP6660) + (4 x ESRC1) + ESRC2.

Instead of the typical 5.2

Ω, R

OUT = 4.2Ω is used

because the typical specification includes the

effect of the ESRs of the capacitors used in the

test circuit in Figures 1 and 2.

Refer to Figures 17, 18, 19 and 20 for the output

currents using 0.33

µF to 220µF capacitors.

Output currents are plotted for 3.0V and 4.5V

inputs taking into consideration a 10% to 20%

loss in the input voltage. The SP6660 5.2

Ω

series resistance limits increases in output current

vs. capacitance for values much higher than

µF. Larger values may still be useful to reduce

ripple.

Designing a Multiple of the SP6660

Negative Inverted Output Voltage

The SP6660 can be cascaded to allow a designer

to provide a multiple of the negative inverted

output voltage of a single SP6660 device. The

approximate total output resistance, R

TOT,of the

cascaded SP6660 devices is equal to the sum of

the individual SP6660 output resistance values,

OUT. The output voltage, VTOT, is a multiple of

the number of cascaded SP6660 devices and the

output voltage of an individual SP6660 device,

OUT. Refer to Figure 25 for the circuit cascading

SP6660 devices. Note that the capacitance value

of C1 for the charge pump and C2 at V

OUT is

multiplied respectively to the number of cascaded

SP6660 devices.

Connecting the SP6660 in Parallel

SP6660 devices can be connected in parallel

to reduce the total output resistance. The

approximate total output resistance, R

TOT, of the

multiple devices connected in parallel is equal

to the output resistance of an individual SP6660

device divided by the total number of devices

connected. Refer to Figure 26 for the circuit

connecting multiple SP6660 devices in parallel.

Note that only the charge pump capacitor value

of C1 is multiplied respectively by the number

of SP6660 connected in parallel. A single

capacitor C2 at the output voltage V

OUT of the

"nth" device connected in parallel serves all

devices connected.

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